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Runtime and compile-time metaprogramming

The Groovy language supports two flavors of metaprogramming: runtime metaprogramming and compile-time metaprogramming. The first one allows altering the class model and the behavior of a program at runtime, while the second only occurs at compile-time. Both have pros and cons, that we will detail in this section.

1. Runtime metaprogramming

With runtime metaprogramming we can postpone to runtime the decision to intercept, inject and even synthesize methods of classes and interfaces. For a deep understanding of Groovy MOP we need to understand Groovy objects and Groovy’s method handling. In Groovy we work with three kinds of objects: POJO, POGO and Groovy Interceptors. Groovy allows metaprogramming for all types of objects but in different manner.

  • POJO - A regular Java object, whose class can be written in Java or any other language for the JVM.

  • POGO - A Groovy object, whose class is written in Groovy. It extends java.lang.Object and implements the groovy.lang.GroovyObject interface by default.

  • Groovy Interceptor - A Groovy object that implements the groovy.lang.GroovyInterceptable interface and has method-interception capability, which we’ll discuss in the GroovyInterceptable section.

For every method call Groovy checks whether the object is a POJO or a POGO. For POJOs, Groovy fetches it’s MetaClass from the groovy.lang.MetaClassRegistry and delegates method invocation to it. For POGOs, Groovy takes more steps, as illustrated in the following figure:

GroovyInterceptions
Figure 1. Groovy interception mechanism

1.1. GroovyObject interface

groovy.lang.GroovyObject is the main interface in Groovy as the Object class is in Java. GroovyObject has a default implementation in the groovy.lang.GroovyObjectSupport class and it is responsible to transfer invocation to the groovy.lang.MetaClass object. The GroovyObject source looks like this:

package groovy.lang;

public interface GroovyObject {

    Object invokeMethod(String name, Object args);

    Object getProperty(String propertyName);

    void setProperty(String propertyName, Object newValue);

    MetaClass getMetaClass();

    void setMetaClass(MetaClass metaClass);
}

1.1.1. invokeMethod

According to the schema in Runtime Metaprogramming this method is called when the method you called is not present on a Groovy object. Here is a simple example using a overridden invokeMethod() method:

class SomeGroovyClass {

    def invokeMethod(String name, Object args) {
        return "called invokeMethod $name $args"
    }

    def test() {
        return 'method exists'
    }
}

def someGroovyClass = new SomeGroovyClass()

assert someGroovyClass.test() == 'method exists'
assert someGroovyClass.someMethod() == 'called invokeMethod someMethod []'

1.1.2. get/setProperty

Every read access to a property can be intercepted by overriding the getProperty() method of the current object. Here is a simple example:

class SomeGroovyClass {

    def property1 = 'ha'
    def field2 = 'ho'
    def field4 = 'hu'

    def getField1() {
        return 'getHa'
    }

    def getProperty(String name) {
        if (name != 'field3')
            return metaClass.getProperty(this, name) (1)
        else
            return 'field3'
    }
}

def someGroovyClass = new SomeGroovyClass()

assert someGroovyClass.field1 == 'getHa'
assert someGroovyClass.field2 == 'ho'
assert someGroovyClass.field3 == 'field3'
assert someGroovyClass.field4 == 'hu'
1 Forwards the request to the getter for all properties except field3.

You can intercept write access to properties by overriding the setProperty() method:

class POGO {

    String property

    void setProperty(String name, Object value) {
        this.@"$name" = 'overridden'
    }
}

def pogo = new POGO()
pogo.property = 'a'

assert pogo.property == 'overridden'

1.1.3. get/setMetaClass

You can a access an object’s metaClass or set your own MetaClass implementation for changing the default interception mechanism. For example you can write your own implementation of the MetaClass interface and assign to it to objects and accordingly change the interception mechanism:

// getMetaclass
someObject.metaClass

// setMetaClass
someObject.metaClass = new OwnMetaClassImplementation()
You can find an additional example in the GroovyInterceptable topic.

1.2. get/setAttribute

This functionality is related to the MetaClass implementation. In the default implementation you can access fields without invoking their getters and setters. The examples below demonstrate this approach:

class SomeGroovyClass {

    def field1 = 'ha'
    def field2 = 'ho'

    def getField1() {
        return 'getHa'
    }
}

def someGroovyClass = new SomeGroovyClass()

assert someGroovyClass.metaClass.getAttribute(someGroovyClass, 'field1') == 'ha'
assert someGroovyClass.metaClass.getAttribute(someGroovyClass, 'field2') == 'ho'
class POGO {

    private String field
    String property1

    void setProperty1(String property1) {
        this.property1 = "setProperty1"
    }
}

def pogo = new POGO()
pogo.metaClass.setAttribute(pogo, 'field', 'ha')
pogo.metaClass.setAttribute(pogo, 'property1', 'ho')

assert pogo.field == 'ha'
assert pogo.property1 == 'ho'

1.3. methodMissing

Groovy supports the concept of methodMissing. This method differs from invokeMethod in that it is only invoked in case of a failed method dispatch, when no method can be found for the given name and/or the given arguments:

class Foo {

   def methodMissing(String name, def args) {
        return "this is me"
   }
}

assert new Foo().someUnknownMethod(42l) == 'this is me'

Typically when using methodMissing it is possible to cache the result for the next time the same method is called.

For example consider dynamic finders in GORM. These are implemented in terms of methodMissing. The code resembles something like this:

class GORM {

   def dynamicMethods = [...] // an array of dynamic methods that use regex

   def methodMissing(String name, args) {
       def method = dynamicMethods.find { it.match(name) }
       if(method) {
          GORM.metaClass."$name" = { Object[] varArgs ->
             method.invoke(delegate, name, varArgs)
          }
          return method.invoke(delegate,name, args)
       }
       else throw new MissingMethodException(name, delegate, args)
   }
}

Notice how, if we find a method to invoke, we then dynamically register a new method on the fly using ExpandoMetaClass. This is so that the next time the same method is called it is more efficient. This way of using methodMissing does not have the overhead of invokeMethod and is not expensive from the second call on.

1.4. propertyMissing

Groovy supports the concept of propertyMissing for intercepting otherwise failing property resolution attempts. In the case of a getter method, propertyMissing takes a single String argument containing the property name:

class Foo {
   def propertyMissing(String name) { name }
}

assert new Foo().boo == 'boo'

The propertyMissing(String) method is only called when no getter method for the given property can be found by the Groovy runtime.

For setter methods a second propertyMissing definition can be added that takes an additional value argument:

class Foo {
   def storage = [:]
   def propertyMissing(String name, value) { storage[name] = value }
   def propertyMissing(String name) { storage[name] }
}

def f = new Foo()
f.foo = "bar"

assert f.foo == "bar"

As with methodMissing it is best practice to dynamically register new properties at runtime to improve the overall lookup performance.

methodMissing and propertyMissing methods that deal with static methods and properties can be added via the ExpandoMetaClass.

1.5. GroovyInterceptable

The groovy.lang.GroovyInterceptable interface is marker interface that extends GroovyObject and is used to notify the Groovy runtime that all methods should be intercepted through the method dispatcher mechanism of the Groovy runtime.

package groovy.lang;

public interface GroovyInterceptable extends GroovyObject {
}

When a Groovy object implements the GroovyInterceptable interface, it’s invokeMethod() is called for any method calls. Below you can see a simple example of a object of this type:

class Interception implements GroovyInterceptable {

    def definedMethod() { }

    def invokeMethod(String name, Object args) {
        'invokedMethod'
    }
}

The next piece of code is a test which shows that both calls to existing and non-existing methods will return the same value.

class InterceptableTest extends GroovyTestCase {

    void testCheckInterception() {
        def interception = new Interception()

        assert interception.definedMethod() == 'invokedMethod'
        assert interception.someMethod() == 'invokedMethod'
    }
}
We cannot use default groovy methods like println because these methods are injected into all groovy objects so they will be intercepted too.

If we want to intercept all methods call but do not want to implement the GroovyInterceptable interface we can implement invokeMethod() on an object’s MetaClass. This approach works for both POGOs and POJOs, as shown by this example:

class InterceptionThroughMetaClassTest extends GroovyTestCase {

    void testPOJOMetaClassInterception() {
        String invoking = 'ha'
        invoking.metaClass.invokeMethod = { String name, Object args ->
            'invoked'
        }

        assert invoking.length() == 'invoked'
        assert invoking.someMethod() == 'invoked'
    }

    void testPOGOMetaClassInterception() {
        Entity entity = new Entity('Hello')
        entity.metaClass.invokeMethod = { String name, Object args ->
            'invoked'
        }

        assert entity.build(new Object()) == 'invoked'
        assert entity.someMethod() == 'invoked'
    }
}
Additional information about MetaClass can be found in the MetaClasses section.

1.6. Categories

There are situations where it is useful if a class not under control had additional methods. In order to enable this capability, Groovy implements a feature borrowed from Objective-C, called Categories.

Categories are implemented with so-called category classes. A category class is special in that it needs to meet certain pre-defined rules for defining extension methods.

There are a few categories that are included in the system for adding functionality to classes that make them more usable within the Groovy environment:

Category classes aren’t enabled by default. To use the methods defined in a category class it is necessary to apply the scoped use method that is provided by the GDK and available from inside every Groovy object instance:

use(TimeCategory)  {
    println 1.minute.from.now       (1)
    println 10.hours.ago

    def someDate = new Date()       (2)
    println someDate - 3.months
}
1 TimeCategory adds methods to Integer
2 TimeCategory adds methods to Date

The use method takes the category class as its first parameter and a closure code block as second parameter. Inside the Closure access to the category methods is available. As can be seen in the example above even JDK classes like java.lang.Integer or java.util.Date can be enriched with user-defined methods.

A category needs not to be directly exposed to the user code, the following will also do:

class JPACategory{
  // Let's enhance JPA EntityManager without getting into the JSR committee
  static void persistAll(EntityManager em , Object[] entities) { //add an interface to save all
    entities?.each { em.persist(it) }
  }
}

def transactionContext = {
  EntityManager em, Closure c ->
  def tx = em.transaction
  try {
    tx.begin()
    use(JPACategory) {
      c()
    }
    tx.commit()
  } catch (e) {
    tx.rollback()
  } finally {
    //cleanup your resource here
  }
}

// user code, they always forget to close resource in exception, some even forget to commit, let's not rely on them.
EntityManager em; //probably injected
transactionContext (em) {
 em.persistAll(obj1, obj2, obj3)
 // let's do some logics here to make the example sensible
 em.persistAll(obj2, obj4, obj6)
}

When we have a look at the groovy.time.TimeCategory class we see that the extension methods are all declared as static methods. In fact, this is one of the requirements that must be met by category classes for its methods to be successfully added to a class inside the use code block:

public class TimeCategory {

    public static Date plus(final Date date, final BaseDuration duration) {
        return duration.plus(date);
    }

    public static Date minus(final Date date, final BaseDuration duration) {
        final Calendar cal = Calendar.getInstance();

        cal.setTime(date);
        cal.add(Calendar.YEAR, -duration.getYears());
        cal.add(Calendar.MONTH, -duration.getMonths());
        cal.add(Calendar.DAY_OF_YEAR, -duration.getDays());
        cal.add(Calendar.HOUR_OF_DAY, -duration.getHours());
        cal.add(Calendar.MINUTE, -duration.getMinutes());
        cal.add(Calendar.SECOND, -duration.getSeconds());
        cal.add(Calendar.MILLISECOND, -duration.getMillis());

        return cal.getTime();
    }

    // ...

Another requirement is the first argument of the static method must define the type the method is attached to once being activated. The other arguments are the normal arguments the method will take as parameters.

Because of the parameter and static method convention, category method definitions may be a bit less intuitive than normal method definitions. As an alternative Groovy comes with a @Category annotation that transforms annotated classes into category classes at compile-time.

class Distance {
    def number
    String toString() { "${number}m" }
}

@Category(Number)
class NumberCategory {
    Distance getMeters() {
        new Distance(number: this)
    }
}

use (NumberCategory)  {
    assert 42.meters.toString() == '42m'
}

Applying the @Category annotation has the advantage of being able to use instance methods without the target type as a first parameter. The target type class is given as an argument to the annotation instead.

There is a distinct section on @Category in the compile-time metaprogramming section.

1.7. Metaclasses

(TBD)

1.7.1. Custom metaclasses

(TBD)

Delegating metaclass

(TBD)

Magic package (Maksym Stavytskyi)

(TBD)

1.7.2. Per instance metaclass

(TBD)

1.7.3. ExpandoMetaClass

Groovy comes with a special MetaClass the so-called ExpandoMetaClass. It is special in that it allows for dynamically adding or changing methods, constructors, properties and even static methods by using a neat closure syntax.

Applying those modifications can be especially useful in mocking or stubbing scenarios as shown in the Testing Guide.

Every java.lang.Class is supplied by Groovy with a special metaClass property that will give you a reference to an ExpandoMetaClass instance. This instance can then be used to add methods or change the behaviour of already existing ones.

By default ExpandoMetaClass doesn’t do inheritance. To enable this you must call ExpandoMetaClass#enableGlobally() before your app starts such as in the main method or servlet bootstrap.

The following sections go into detail on how ExpandoMetaClass can be used in various scenarios.

Methods

Once the ExpandoMetaClass is accessed by calling the metaClass property, methods can added by using either the left shift << or the = operator.

Note that the left shift operator is used to append a new method. If a public method with the same name and parameter types is declared by the class or interface, including those inherited from superclasses and superinterfaces but excluding those added to the metaClass at runtime, an exception will be thrown. If you want to replace a method declared by the class or interface you can use the = operator.

The operators are applied on a non-existent property of metaClass passing an instance of a Closure code block.

class Book {
   String title
}

Book.metaClass.titleInUpperCase << {-> title.toUpperCase() }

def b = new Book(title:"The Stand")

assert "THE STAND" == b.titleInUpperCase()

The example above shows how a new method can be added to a class by accessing the metaClass property and using the << or = operator to assign a Closure code block. The Closure parameters are interpreted as method parameters. Parameterless methods can be added by using the {→ …​} syntax.

Properties

ExpandoMetaClass supports two mechanisms for adding or overriding properties.

Firstly, it has support for declaring a mutable property by simply assigning a value to a property of metaClass:

class Book {
   String title
}

Book.metaClass.author = "Stephen King"
def b = new Book()

assert "Stephen King" == b.author

Another way is to add getter and/or setter methods by using the standard mechanisms for adding instance methods.

class Book {
  String title
}
Book.metaClass.getAuthor << {-> "Stephen King" }

def b = new Book()

assert "Stephen King" == b.author

In the source code example above the property is dictated by the closure and is a read-only property. It is feasible to add an equivalent setter method but then the property value needs to be stored for later usage. This could be done as shown in the following example.

class Book {
  String title
}

def properties = Collections.synchronizedMap([:])

Book.metaClass.setAuthor = { String value ->
   properties[System.identityHashCode(delegate) + "author"] = value
}
Book.metaClass.getAuthor = {->
   properties[System.identityHashCode(delegate) + "author"]
}

This is not the only technique however. For example in a servlet container one way might be to store the values in the currently executing request as request attributes (as is done in some cases in Grails).

Constructors

Constructors can be added by using a special constructor property. Either the << or = operator can be used to assign a Closure code block. The Closure arguments will become the constructor arguments when the code is executed at runtime.

class Book {
    String title
}
Book.metaClass.constructor << { String title -> new Book(title:title) }

def book = new Book('Groovy in Action - 2nd Edition')
assert book.title == 'Groovy in Action - 2nd Edition'
Be careful when adding constructors however, as it is very easy to get into stack overflow troubles.
Static Methods

Static methods can be added using the same technique as instance methods with the addition of the static qualifier before the method name.

class Book {
   String title
}

Book.metaClass.static.create << { String title -> new Book(title:title) }

def b = Book.create("The Stand")
Borrowing Methods

With ExpandoMetaClass it is possible to use Groovy’s method pointer syntax to borrow methods from other classes.

class Person {
    String name
}
class MortgageLender {
   def borrowMoney() {
      "buy house"
   }
}

def lender = new MortgageLender()

Person.metaClass.buyHouse = lender.&borrowMoney

def p = new Person()

assert "buy house" == p.buyHouse()
Dynamic Method Names

Since Groovy allows you to use Strings as property names this in turns allows you to dynamically create method and property names at runtime. To create a method with a dynamic name simply use the language feature of reference property names as strings.

class Person {
   String name = "Fred"
}

def methodName = "Bob"

Person.metaClass."changeNameTo${methodName}" = {-> delegate.name = "Bob" }

def p = new Person()

assert "Fred" == p.name

p.changeNameToBob()

assert "Bob" == p.name

The same concept can be applied to static methods and properties.

One application of dynamic method names can be found in the Grails web application framework. The concept of "dynamic codecs" is implemented by using dynamic method names.

HTMLCodec Class
class HTMLCodec {
    static encode = { theTarget ->
        HtmlUtils.htmlEscape(theTarget.toString())
    }

    static decode = { theTarget ->
    	HtmlUtils.htmlUnescape(theTarget.toString())
    }
}

The example above shows a codec implementation. Grails comes with various codec implementations each defined in a single class. At runtime there will be multiple codec classes in the application classpath. At application startup the framework adds a encodeXXX and a decodeXXX method to certain meta-classes where XXX is the first part of the codec class name (e.g. encodeHTML). This mechanism is in the following shown in some Groovy pseudo-code:

def codecs = classes.findAll { it.name.endsWith('Codec') }

codecs.each { codec ->
    Object.metaClass."encodeAs${codec.name-'Codec'}" = { codec.newInstance().encode(delegate) }
    Object.metaClass."decodeFrom${codec.name-'Codec'}" = { codec.newInstance().decode(delegate) }
}


def html = '<html><body>hello</body></html>'

assert '<html><body>hello</body></html>' == html.encodeAsHTML()
Runtime Discovery

At runtime it is often useful to know what other methods or properties exist at the time the method is executed. ExpandoMetaClass provides the following methods as of this writing:

  • getMetaMethod

  • hasMetaMethod

  • getMetaProperty

  • hasMetaProperty

Why can’t you just use reflection? Well because Groovy is different, it has the methods that are "real" methods and methods that are available only at runtime. These are sometimes (but not always) represented as MetaMethods. The MetaMethods tell you what methods are available at runtime, thus your code can adapt.

This is of particular use when overriding invokeMethod, getProperty and/or setProperty.

GroovyObject Methods

Another feature of ExpandoMetaClass is that it allows to override the methods invokeMethod, getProperty and setProperty, all of them can be found in the groovy.lang.GroovyObject class.

The following example shows how to override invokeMethod:

class Stuff {
   def invokeMe() { "foo" }
}

Stuff.metaClass.invokeMethod = { String name, args ->
   def metaMethod = Stuff.metaClass.getMetaMethod(name, args)
   def result
   if(metaMethod) result = metaMethod.invoke(delegate,args)
   else {
      result = "bar"
   }
   result
}

def stf = new Stuff()

assert "foo" == stf.invokeMe()
assert "bar" == stf.doStuff()

The first step in the Closure code is to lookup the MetaMethod for the given name and arguments. If the method can be found everything is fine and it is delegated to. If not, a dummy value is returned.

A MetaMethod is a method that is known to exist on the MetaClass whether added at runtime or at compile-time.

The same logic can be used to override setProperty or getProperty.

class Person {
   String name = "Fred"
}

Person.metaClass.getProperty = { String name ->
   def metaProperty = Person.metaClass.getMetaProperty(name)
   def result
   if(metaProperty) result = metaProperty.getProperty(delegate)
   else {
      result = "Flintstone"
   }
   result
}

def p = new Person()

assert "Fred" == p.name
assert "Flintstone" == p.other

The important thing to note here is that instead of a MetaMethod a MetaProperty instance is looked up. If that exists the getProperty method of the MetaProperty is called, passing the delegate.

Overriding Static invokeMethod

ExpandoMetaClass even allows for overriding static method with a special invokeMethod syntax.

class Stuff {
   static invokeMe() { "foo" }
}

Stuff.metaClass.'static'.invokeMethod = { String name, args ->
   def metaMethod = Stuff.metaClass.getStaticMetaMethod(name, args)
   def result
   if(metaMethod) result = metaMethod.invoke(delegate,args)
   else {
      result = "bar"
   }
   result
}

assert "foo" == Stuff.invokeMe()
assert "bar" == Stuff.doStuff()

The logic that is used for overriding the static method is the same as we’ve seen before for overriding instance methods. The only difference is the access to the metaClass.static property and the call to getStaticMethodName for retrieving the static MetaMethod instance.

Extending Interfaces

It is possible to add methods onto interfaces with ExpandoMetaClass. To do this however, it must be enabled globally using the ExpandoMetaClass.enableGlobally() method before application start-up.

List.metaClass.sizeDoubled = {-> delegate.size() * 2 }

def list = []

list << 1
list << 2

assert 4 == list.sizeDoubled()

1.8. Extension modules

1.8.1. Extending existing classes

An extension module allows you to add new methods to existing classes, including classes which are precompiled, like classes from the JDK. Those new methods, unlike those defined through a metaclass or using a category, are available globally. For example, when you write:

Standard extension method
def file = new File(...)
def contents = file.getText('utf-8')

The getText method doesn’t exist on the File class. However, Groovy knows it because it is defined in a special class, ResourceGroovyMethods:

ResourceGroovyMethods.java
public static String getText(File file, String charset) throws IOException {
 return IOGroovyMethods.getText(newReader(file, charset));
}

You may notice that the extension method is defined using a static method in a helper class (where various extension methods are defined). The first argument of the getText method corresponds to the receiver, while additional parameters correspond to the arguments of the extension method. So here, we are defining a method called getText on the File class (because the first argument is of type File), which takes a single argument as a parameter (the encoding String).

The process of creating an extension module is simple:

  • write an extension class like above

  • write a module descriptor file

Then you have to make the extension module visible to Groovy, which is as simple as having the extension module classes and descriptor available on classpath. This means that you have the choice:

  • either provide the classes and module descriptor directly on classpath

  • or bundle your extension module into a jar for reusability

An extension module may add two kind of methods to a class:

  • instance methods (to be called on an instance of a class)

  • static methods (to be called on the class itself)

1.8.2. Instance methods

To add an instance method to an existing class, you need to create an extension class. For example, let’s say you want to add a maxRetries method on Integer which accepts a closure and executes it at most n times until no exception is thrown. To do that, you only need to write the following:

MaxRetriesExtension.groovy
class MaxRetriesExtension {                                     (1)
    static void maxRetries(Integer self, Closure code) {        (2)
        int retries = 0
        Throwable e
        while (retries<self) {
            try {
                code.call()
                break
            } catch (Throwable err) {
                e = err
                retries++
            }
        }
        if (retries==0 && e) {
            throw e
        }
    }
}
1 The extension class
2 First argument of the static method corresponds to the receiver of the message, that is to say the extended instance

Then, after having declared your extension class, you can call it this way:

int i=0
5.maxRetries {
    i++
}
assert i == 1
i=0
try {
    5.maxRetries {
        throw new RuntimeException("oops")
    }
} catch (RuntimeException e) {
    assert i == 5
}

1.8.3. Static methods

It is also possible to add static methods to a class. In that case, the static method needs to be defined in its own file. Static and instance extension methods cannot be present in the same class.

StaticStringExtension.groovy
class StaticStringExtension {                                       (1)
    static String greeting(String self) {                           (2)
        'Hello, world!'
    }
}
1 The static extension class
2 First argument of the static method corresponds to the class being extended and is unused

In which case you can call it directly on the String class:

assert String.greeting() == 'Hello, world!'

1.8.4. Module descriptor

For Groovy to be able to load your extension methods, you must declare your extension helper classes. You must create a file named org.codehaus.groovy.runtime.ExtensionModule into the META-INF/services directory:

org.codehaus.groovy.runtime.ExtensionModule
moduleName=Test module for specifications
moduleVersion=1.0-test
extensionClasses=support.MaxRetriesExtension
staticExtensionClasses=support.StaticStringExtension

The module descriptor requires 4 keys:

  • moduleName : the name of your module

  • moduleVersion: the version of your module. Note that version number is only used to check that you don’t load the same module in two different versions.

  • extensionClasses: the list of extension helper classes for instance methods. You can provide several classes, given that they are comma separated.

  • staticExtensionClasses: the list of extension helper classes for static methods. You can provide several classes, given that they are comma separated.

Note that it is not required for a module to define both static helpers and instance helpers, and that you may add several classes to a single module. You can also extend different classes in a single module without problem. It is even possible to use different classes in a single extension class, but it is recommended to group extension methods into classes by feature set.

1.8.5. Extension modules and classpath

It’s worth noting that you can’t use an extension which is compiled at the same time as code using it. That means that to use an extension, it has to be available on classpath, as compiled classes, before the code using it gets compiled. Usually, this means that you can’t have the test classes in the same source unit as the extension class itself. Since in general, test sources are separated from normal sources and executed in another step of the build, this is not an issue.

1.8.6. Compatibility with type checking

Unlike categories, extension modules are compatible with type checking: if they are found on classpath, then the type checker is aware of the extension methods and will not complain when you call them. It is also compatible with static compilation.

2. Compile-time metaprogramming

Compile-time metaprogramming in Groovy allows code generation at compile-time. Those transformations are altering the Abstract Syntax Tree (AST) of a program, which is why in Groovy we call it AST transformations. AST transformations allow you to hook into the compilation process, modify the AST and continue the compilation process to generate regular bytecode. Compared to runtime metaprogramming, this has the advantage of making the changes visible in the class file itself (that is to say, in the bytecode). Making it visible in the bytecode is important for example if you want the transformations to be part of the class contract (implementing interfaces, extending abstract classes, …​) or even if you need your class to be callable from Java (or other JVM languages). For example, an AST transformation can add methods to a class. If you do it with runtime metaprogramming, the new method would only be visible from Groovy. If you do the same using compile-time metaprogramming, the method would be visible from Java too. Last but not least, performance would likely be better with compile-time metaprogramming (because no initialization phase is required).

In this section, we will start with explaining the various compile-time transformations that are bundled with the Groovy distribution. In a subsequent section, we will describe how you can implement your own AST transformations and what are the disadvantages of this technique.

2.1. Available AST transformations

Groovy comes with various AST transformations covering different needs: reducing boilerplate (code generation), implementing design patterns (delegation, …​), logging, declarative concurrency, cloning, safer scripting, tweaking the compilation, implementing Swing patterns, testing and eventually managing dependencies. If none of those AST transformations cover your needs, you can still implement your own, as show in section Developing your own AST transformations.

AST transformations can be separated into two categories:

  • global AST transformations are applied transparently, globally, as soon as they are found on compile classpath

  • local AST transformations are applied by annotating the source code with markers. Unlike global AST transformations, local AST transformations may support parameters.

Groovy doesn’t ship with any global AST transformation, but you can find a list of local AST transformations available for you to use in your code here:

2.1.1. Code generation transformations

This category of transformation includes AST transformations which help removing boilerplate code. This is typically code that you have to write but that does not carry any useful information. By autogenerating this boilerplate code, the code you have to write is left clean and concise and the chance of introducing an error by getting such boilerplate code incorrect is reduced.

@groovy.transform.ToString

The @ToString AST transformation generates a human readable toString representation of the class. For example, annotating the Person class like below will automatically generate the toString method for you:

import groovy.transform.ToString

@ToString
class Person {
    String firstName
    String lastName
}

With this definition, then the following assertion passes, meaning that a toString method taking the field values from the class and printing them out has been generated:

def p = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p.toString() == 'Person(Jack, Nicholson)'

The @ToString annotation accepts several parameters which are summarized in the following table:

Attribute Default value Description Example

includeNames

false

Whether to include names of properties in generated toString.

@ToString(includeNames=true)
class Person {
    String firstName
    String lastName
}

def p = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p.toString() == 'Person(firstName:Jack, lastName:Nicholson)'

excludes

Empty list

List of properties to exclude from toString

@ToString(excludes=['firstName'])
class Person {
    String firstName
    String lastName
}

def p = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p.toString() == 'Person(Nicholson)'

includes

Empty list

List of fields to include in toString

@ToString(includes=['lastName'])
class Person {
    String firstName
    String lastName
}

def p = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p.toString() == 'Person(Nicholson)'

includeSuper

False

Should superclass be included in toString

@ToString
class Id { long id }

@ToString(includeSuper=true)
class Person extends Id {
    String firstName
    String lastName
}

def p = new Person(id:1, firstName: 'Jack', lastName: 'Nicholson')
assert p.toString() == 'Person(Jack, Nicholson, Id(1))'

includeSuperProperties

False

Should super properties be included in toString

class Person {
    String name
}

@ToString(includeSuperProperties = true, includeNames = true)
class BandMember extends Person {
    String bandName
}

def bono = new BandMember(name:'Bono', bandName: 'U2').toString()

assert bono.toString() == 'BandMember(bandName:U2, name:Bono)'

includeFields

False

Should fields be included in toString, in addition to properties

@ToString(includeFields=true)
class Person {
    String firstName
    String lastName
    private int age
    void test() {
       age = 42
    }
}

def p = new Person(firstName: 'Jack', lastName: 'Nicholson')
p.test()
assert p.toString() == 'Person(Jack, Nicholson, 42)'

ignoreNulls

False

Should properties/fields with null value be displayed

@ToString(ignoreNulls=true)
class Person {
    String firstName
    String lastName
}

def p = new Person(firstName: 'Jack')
assert p.toString() == 'Person(Jack)'

includePackage

True

Use fully qualified class name instead of simple name in toString

@ToString(includePackage=true)
class Person {
    String firstName
    String lastName
}

def p = new Person(firstName: 'Jack', lastName:'Nicholson')
assert p.toString() == 'acme.Person(Jack, Nicholson)'

cache

False

Cache the toString string. Should only be set to true if the class is immutable.

@ToString(cache=true)
class Person {
    String firstName
    String lastName
}

def p = new Person(firstName: 'Jack', lastName:'Nicholson')
def s1 = p.toString()
def s2 = p.toString()
assert s1 == s2
assert s1 == 'Person(Jack, Nicholson)'
assert s1.is(s2) // same instance
@groovy.transform.EqualsAndHashCode

The @EqualsAndHashCode AST transformation aims at generating equals and hashCode methods for you. The generated hashcode follows the best practices as described in Effective Java by Josh Bloch:

import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode
class Person {
    String firstName
    String lastName
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
def p2 = new Person(firstName: 'Jack', lastName: 'Nicholson')

assert p1==p2
assert p1.hashCode() == p2.hashCode()

There are several options available to tweak the behavior of @EqualsAndHashCode:

Attribute Default value Description Example

excludes

Empty list

List of properties to exclude from equals/hashCode

import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode(excludes=['firstName'])
class Person {
    String firstName
    String lastName
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
def p2 = new Person(firstName: 'Bob', lastName: 'Nicholson')

assert p1==p2
assert p1.hashCode() == p2.hashCode()

includes

Empty list

List of fields to include in equals/hashCode

import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode(includes=['lastName'])
class Person {
    String firstName
    String lastName
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
def p2 = new Person(firstName: 'Bob', lastName: 'Nicholson')

assert p1==p2
assert p1.hashCode() == p2.hashCode()

callSuper

False

Whether to include super in equals and hashCode calculations

import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode
class Living {
    String race
}

@EqualsAndHashCode(callSuper=true)
class Person extends Living {
    String firstName
    String lastName
}

def p1 = new Person(race:'Human', firstName: 'Jack', lastName: 'Nicholson')
def p2 = new Person(race: 'Human beeing', firstName: 'Jack', lastName: 'Nicholson')

assert p1!=p2
assert p1.hashCode() != p2.hashCode()

includeFields

False

Should fields be included in equals/hashCode, in addition to properties

import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode(includeFields=true)
class Person {
    private String firstName

    Person(String firstName) {
        this.firstName = firstName
    }
}

def p1 = new Person('Jack')
def p2 = new Person('Jack')
def p3 = new Person('Bob')

assert p1 == p2
assert p1 != p3

cache

False

Cache the hashCode computation. Should only be set to true if the class is immutable.

import groovy.transform.EqualsAndHashCode
import groovy.transform.Immutable

@Immutable
class SlowHashCode {
    int hashCode() {
        sleep 100
        127
    }
}

@EqualsAndHashCode(cache=true)
@Immutable
class Person {
    SlowHashCode slowHashCode = new SlowHashCode()
}

def p = new Person()
p.hashCode()

def start = System.currentTimeMillis()
p.hashCode()
assert System.currentTimeMillis() - start < 100

useCanEqual

True

Should equals call canEqual helper method.

@groovy.transform.TupleConstructor

The @TupleConstructor annotation aims at eliminating boilerplate code by generating constructors for you. A tuple constructor is created for each property, with default values (using the Java default values). For example, the following code will generate 3 constructors:

import groovy.transform.TupleConstructor

@TupleConstructor
class Person {
    String firstName
    String lastName
}

// traditional map-style constructor
def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
// generated tuple constructor
def p2 = new Person('Jack', 'Nicholson')
// generated tuple constructor with default value for second property
def p3 = new Person('Jack')

The first constructor is a no-arg constructor which allows the traditional map-style construction. It is worth noting that if the first property (or field) has type LinkedHashMap or if there is a single Map, AbstractMap or HashMap property (or field), then the map-style mapping is not available.

The other constructors are generated by taking the properties in the order they are defined. Groovy will generate as many constructors as they are properties (or fields, depending on the options).

The @TupleConstructor AST transformation accepts several configuration options:

Attribute Default value Description Example

excludes

Empty list

List of properties to exclude from tuple constructor generation

import groovy.transform.TupleConstructor

@TupleConstructor(excludes=['lastName'])
class Person {
    String firstName
    String lastName
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
def p2 = new Person('Jack')
try {
    // will fail because the second property is excluded
    def p3 = new Person('Jack', 'Nicholson')
} catch (e) {
    assert e.message.contains ('Could not find matching constructor')
}

includes

Empty list

List of fields to include in tuple constructor generation

import groovy.transform.TupleConstructor

@TupleConstructor(includes=['firstName'])
class Person {
    String firstName
    String lastName
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
def p2 = new Person('Jack')
try {
    // will fail because the second property is not included
    def p3 = new Person('Jack', 'Nicholson')
} catch (e) {
    assert e.message.contains ('Could not find matching constructor')
}

includeFields

False

Should fields be included in tuple constructor generation, in addition to properties

import groovy.transform.TupleConstructor

@TupleConstructor(includeFields=true)
class Person {
    String firstName
    String lastName
    private String occupation
    public String toString() {
        "$firstName $lastName: $occupation"
    }
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson', occupation: 'Actor')
def p2 = new Person('Jack', 'Nicholson', 'Actor')

assert p1.firstName == p2.firstName
assert p1.lastName == p2.lastName
assert p1.toString() == 'Jack Nicholson: Actor'
assert p1.toString() == p2.toString()

includeProperties

True

Should properties be included in tuple constructor generation

import groovy.transform.TupleConstructor

@TupleConstructor(includeProperties=false)
class Person {
    String firstName
    String lastName
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')

try {
    def p2 = new Person('Jack', 'Nicholson')
} catch(e) {
    // will fail because properties are not included
}

includeSuperFields

False

Should fields from super classes be included in tuple constructor generation

import groovy.transform.TupleConstructor

class Base {
    protected String occupation
    public String occupation() { this.occupation }
}

@TupleConstructor(includeSuperFields=true)
class Person extends Base {
    String firstName
    String lastName
    public String toString() {
        "$firstName $lastName: ${occupation()}"
    }
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson', occupation: 'Actor')

def p2 = new Person('Actor', 'Jack', 'Nicholson')

assert p1.firstName == p2.firstName
assert p1.lastName == p2.lastName
assert p1.toString() == 'Jack Nicholson: Actor'
assert p2.toString() == p1.toString()

includeSuperProperties

True

Should properties from super classes be included in tuple constructor generation

import groovy.transform.TupleConstructor

class Base {
    String occupation
}

@TupleConstructor(includeSuperProperties=true)
class Person extends Base {
    String firstName
    String lastName
    public String toString() {
        "$firstName $lastName: $occupation"
    }
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')

def p2 = new Person('Actor', 'Jack', 'Nicholson')

assert p1.firstName == p2.firstName
assert p1.lastName == p2.lastName
assert p1.toString() == 'Jack Nicholson: null'
assert p2.toString() == 'Jack Nicholson: Actor'

callSuper

False

Should super properties be called within a call to the parent constructor rather than set as properties

import groovy.transform.TupleConstructor

class Base {
    String occupation
    Base() {}
    Base(String job) { occupation = job?.toLowerCase() }
}

@TupleConstructor(includeSuperProperties = true, callSuper=true)
class Person extends Base {
    String firstName
    String lastName
    public String toString() {
        "$firstName $lastName: $occupation"
    }
}

def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')

def p2 = new Person('ACTOR', 'Jack', 'Nicholson')

assert p1.firstName == p2.firstName
assert p1.lastName == p2.lastName
assert p1.toString() == 'Jack Nicholson: null'
assert p2.toString() == 'Jack Nicholson: actor'

force

False

By default, the transformation will do nothing if a constructor is already defined. Setting this property to true, the constructor will be generated and it’s your responsibility to ensure that no duplicate constructor is defined

See javadocs

@groovy.transform.Canonical

The @Canonical AST transformation combines the effects of the @ToString, @EqualsAndHashCode and @TupleConstructor annotations:

import groovy.transform.Canonical

@Canonical
class Person {
    String firstName
    String lastName
}
def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p1.toString() == 'Person(Jack, Nicholson)' // Effect of @ToString

def p2 = new Person('Jack','Nicholson') // Effect of @TupleConstructor
assert p2.toString() == 'Person(Jack, Nicholson)'

assert p1==p2 // Effect of @EqualsAndHashCode
assert p1.hashCode()==p2.hashCode() // Effect of @EqualsAndHashCode

A similar immutable class can be generated using the @Immutable AST transformation instead. The @Canonical AST transformation supports several configuration options:

Attribute Default value Description Example

excludes

Empty list

List of properties to exclude from tuple constructor generation

import groovy.transform.Canonical

@Canonical(excludes=['lastName'])
class Person {
    String firstName
    String lastName
}
def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p1.toString() == 'Person(Jack)' // Effect of @ToString

def p2 = new Person('Jack') // Effect of @TupleConstructor
assert p2.toString() == 'Person(Jack)'

assert p1==p2 // Effect of @EqualsAndHashCode
assert p1.hashCode()==p2.hashCode() // Effect of @EqualsAndHashCode

includes

Empty list

List of fields to include in tuple constructor generation

import groovy.transform.Canonical

@Canonical(includes=['firstName'])
class Person {
    String firstName
    String lastName
}
def p1 = new Person(firstName: 'Jack', lastName: 'Nicholson')
assert p1.toString() == 'Person(Jack)' // Effect of @ToString

def p2 = new Person('Jack') // Effect of @TupleConstructor
assert p2.toString() == 'Person(Jack)'

assert p1==p2 // Effect of @EqualsAndHashCode
assert p1.hashCode()==p2.hashCode() // Effect of @EqualsAndHashCode
@groovy.transform.InheritConstructors

The @InheritConstructor AST transformation aims at generating constructors matching super constructors for you. This is in particular useful when overriding exception classes:

import groovy.transform.InheritConstructors

@InheritConstructors
class CustomException extends Exception {}

// all those are generated constructors
new CustomException()
new CustomException("A custom message")
new CustomException("A custom message", new RuntimeException())
new CustomException(new RuntimeException())

// Java 7 only
// new CustomException("A custom message", new RuntimeException(), false, true)

The @InheritConstructor AST transformation supports the following configuration options:

Attribute Default value Description Example

constructorAnnotations

False

Whether to carry over annotations from the constructor during copying

@Retention(RetentionPolicy.RUNTIME)
@Target([ElementType.CONSTRUCTOR])
public @interface ConsAnno {}

class Base {
  @ConsAnno Base() {}
}

@InheritConstructors(constructorAnnotations=true)
class Child extends Base {}

assert Child.constructors[0].annotations[0].annotationType().name == 'ConsAnno'

parameterAnnotations

False

Whether to carry over annotations from the constructor parameters when copying the constructor

@Retention(RetentionPolicy.RUNTIME)
@Target([ElementType.PARAMETER])
public @interface ParamAnno {}

class Base {
  Base(@ParamAnno String name) {}
}

@InheritConstructors(parameterAnnotations=true)
class Child extends Base {}

assert Child.constructors[0].parameterAnnotations[0][0].annotationType().name == 'ParamAnno'
@groovy.lang.Category

The @Category AST transformation simplifies the creation of Groovy categories. Historically, a Groovy category was written like this:

class TripleCategory {
    public static Integer triple(Integer self) {
        3*self
    }
}
use (TripleCategory) {
    assert 9 == 3.triple()
}

The @Category transformation lets you write the same using an instance-style class, rather than a static class style. This removes the need for having the first argument of each method being the receiver. The category can be written like this:

@Category(Integer)
class TripleCategory {
    public Integer triple() { 3*this }
}
use (TripleCategory) {
    assert 9 == 3.triple()
}

Note that the mixed in class can be referenced using this instead. It’s also worth noting that using instance fields in a category class is inherently unsafe: categories are not stateful (like traits).

@groovy.transform.IndexedProperty

The @IndexedProperty annotation aims at generating indexed getters/setters for properties of list/array types. This is in particular useful if you want to use a Groovy class from Java. While Groovy supports GPath to access properties, this is not available from Java. The @IndexedProperty annotation will generate indexed properties of the following form:

class SomeBean {
    @IndexedProperty String[] someArray = new String[2]
    @IndexedProperty List someList = []
}

def bean = new SomeBean()
bean.setSomeArray(0, 'value')
bean.setSomeList(0, 123)

assert bean.someArray[0] == 'value'
assert bean.someList == [123]
@groovy.lang.Lazy

The @Lazy AST transformation implements lazy initialization of fields. For example, the following code:

class SomeBean {
    @Lazy LinkedList myField
}

will produce the following code:

List $myField
List getMyField() {
    if ($myField!=null) { return $myField }
    else {
        $myField = new LinkedList()
        return $myField
    }
}

The default value which is used to initialize the field is the default constructor of the declaration type. It is possible to define a default value by using a closure on the right hand side of the property assignment, as in the following example:

class SomeBean {
    @Lazy LinkedList myField = { ['a','b','c']}()
}

In that case, the generated code looks like the following:

List $myField
List getMyField() {
    if ($myField!=null) { return $myField }
    else {
        $myField = { ['a','b','c']}()
        return $myField
    }
}

If the field is declared volatile then initialization will be synchronized using the double-checked locking pattern.

Using the soft=true parameter, the helper field will use a SoftReference instead, providing a simple way to implement caching. In that case, if the garbage collector decides to collect the reference, initialization will occur the next time the field is accessed.

@groovy.lang.Newify

The @Newify AST transformation is used to bring alternative syntaxes to construct objects:

  • Using the Python style:

@Newify([Tree,Leaf])
class TreeBuilder {
    Tree tree = Tree(Leaf('A'),Leaf('B'),Tree(Leaf('C')))
}
  • or using the Ruby style:

@Newify([Tree,Leaf])
class TreeBuilder {
    Tree tree = Tree.new(Leaf.new('A'),Leaf.new('B'),Tree.new(Leaf.new('C')))
}

The Ruby version can be disabled by setting the auto flag to false.

@groovy.transform.Sortable

The @Sortable AST transformation is used to help write classes that are Comparable and easily sorted by numerous properties. It is easy to use as shown in the following example where we annotate the Person class:

import groovy.transform.Sortable

@Sortable class Person {
    String first
    String last
    Integer born
}

The generated class has the following properties:

  • it implements the Comparable interface

  • it contains a compareTo method with an implementation based on the natural ordering of the first, last and born properties

  • it has three methods returning comparators: comparatorByFirst, comparatorByLast and comparatorByBorn.

The generated compareTo method will look like this:

public int compareTo(java.lang.Object obj) {
    if (this.is(obj)) {
        return 0
    }
    if (!(obj instanceof Person)) {
        return -1
    }
    java.lang.Integer value = this.first <=> obj.first
    if (value != 0) {
        return value
    }
    value = this.last <=> obj.last
    if (value != 0) {
        return value
    }
    value = this.born <=> obj.born
    if (value != 0) {
        return value
    }
    return 0
}

As an example of the generated comparators, the comparatorByFirst comparator will have a compare method that looks like this:

public int compare(java.lang.Object arg0, java.lang.Object arg1) {
    if (arg0 == arg1) {
        return 0
    }
    if (arg0 != null && arg1 == null) {
        return -1
    }
    if (arg0 == null && arg1 != null) {
        return 1
    }
    return arg0.first <=> arg1.first
}

The Person class can be used wherever a Comparable is expected and the generated comparators wherever a Comparator is expected as shown by these examples:

def people = [
    new Person(first: 'Johnny', last: 'Depp', born: 1963),
    new Person(first: 'Keira', last: 'Knightley', born: 1985),
    new Person(first: 'Geoffrey', last: 'Rush', born: 1951),
    new Person(first: 'Orlando', last: 'Bloom', born: 1977)
]

assert people[0] > people[2]
assert people.sort()*.last == ['Rush', 'Depp', 'Knightley', 'Bloom']
assert people.sort(false, Person.comparatorByFirst())*.first == ['Geoffrey', 'Johnny', 'Keira', 'Orlando']
assert people.sort(false, Person.comparatorByLast())*.last == ['Bloom', 'Depp', 'Knightley', 'Rush']
assert people.sort(false, Person.comparatorByBorn())*.last == ['Rush', 'Depp', 'Bloom', 'Knightley']

Normally, all properties are used in the generated compareTo method in the priority order in which they are defined. You can include or exclude certain properties from the generated compareTo method by giving a list of property names in the includes or excludes annotation attributes. If using includes, the order of the property names given will determine the priority of properties when comparing. To illustrate, consider the following Person class definition:

@Sortable(includes='first,born') class Person {
    String last
    int born
    String first
}

It will have two comparator methods comparatorByFirst and comparatorByBorn and the generated compareTo method will look like this:

public int compareTo(java.lang.Object obj) {
    if (this.is(obj)) {
        return 0
    }
    if (!(obj instanceof Person)) {
        return -1
    }
    java.lang.Integer value = this.first <=> obj.first
    if (value != 0) {
        return value
    }
    value = this.born <=> obj.born
    if (value != 0) {
        return value
    }
    return 0
}

This Person class can be used as follows:

def people = [
    new Person(first: 'Ben', last: 'Affleck', born: 1972),
    new Person(first: 'Ben', last: 'Stiller', born: 1965)
]

assert people.sort()*.last == ['Stiller', 'Affleck']
@groovy.transform.builder.Builder

The @Builder AST transformation is used to help write classes that can be created using fluent api calls. The transform supports multiple building strategies to cover a range of cases and there are a number of configuration options to customize the building process. If you’re an AST hacker, you can also define your own strategy class. The following table lists the available strategies that are bundled with Groovy and the configuration options each strategy supports.

Strategy

Description

builderClassName

builderMethodName

buildMethodName

prefix

includes/excludes

SimpleStrategy

chained setters

n/a

n/a

n/a

yes, default "set"

yes

ExternalStrategy

explicit builder class, class being built untouched

n/a

n/a

yes, default "build"

yes, default ""

yes

DefaultStrategy

creates a nested helper class

yes, default <TypeName>Builder

yes, default "builder"

yes, default "build"

yes, default ""

yes

InitializerStrategy

creates a nested helper class providing type-safe fluent creation

yes, default <TypeName>Initializer

yes, default "createInitializer"

yes, default "create" but usually only used internally

yes, default ""

yes

SimpleStrategy

To use the SimpleStrategy, annotate your Groovy class using the @Builder annotation, and specify the strategy as shown in this example:

import groovy.transform.builder.*

@Builder(builderStrategy=SimpleStrategy)
class Person {
    String first
    String last
    Integer born
}

Then, just call the setters in a chained fashion as shown here:

def p1 = new Person().setFirst('Johnny').setLast('Depp').setBorn(1963)
assert "$p1.first $p1.last" == 'Johnny Depp'

For each property, a generated setter will be created which looks like this:

public Person setFirst(java.lang.String first) {
    this.first = first
    return this
}

You can specify a prefix as shown in this example:

import groovy.transform.builder.*

@Builder(builderStrategy=SimpleStrategy, prefix="")
class Person {
    String first
    String last
    Integer born
}

And calling the chained setters would look like this:

def p = new Person().first('Johnny').last('Depp').born(1963)
assert "$p.first $p.last" == 'Johnny Depp'

You can use the SimpleStrategy in conjunction with @Canonical. If your @Builder annotation doesn’t have explicit includes or excludes annotation attributes but your @Canonical annotation does, the ones from @Canonical will be re-used for @Builder.

The annotation attributes builderClassName, buildMethodName, builderMethodName and forClass are not supported for this strategy.

Groovy already has built-in building mechanisms. Don’t rush to using @Builder if the built-in mechanisms meet your needs. Some examples:
def p2 = new Person(first: 'Keira', last: 'Knightley', born: 1985)
def p3 = new Person().with {
    first = 'Geoffrey'
    last = 'Rush'
    born = 1951
}
ExternalStrategy

To use the ExternalStrategy, create and annotate a Groovy builder class using the @Builder annotation, specify the class the builder is for using forClass and indicate use of the ExternalStrategy. Suppose you have the following class you would like a builder for:

class Person {
    String first
    String last
    int born
}

you explicitly create and use your builder class as follows:

import groovy.transform.builder.*

@Builder(builderStrategy=ExternalStrategy, forClass=Person)
class PersonBuilder { }

def p = new PersonBuilder().first('Johnny').last('Depp').born(1963).build()
assert "$p.first $p.last" == 'Johnny Depp'

Note that the (normally empty) builder class you provide will be filled in with appropriate setters and a build method. The generated build method will look something like:

public Person build() {
    Person _thePerson = new Person()
    _thePerson.first = first
    _thePerson.last = last
    _thePerson.born = born
    return _thePerson
}

The class you are creating the builder for can be any Java or Groovy class following the normal JavaBean conventions, e.g. a no-arg constructor and setters for the properties. Here is an example using a Java class:

import groovy.transform.builder.*

@Builder(builderStrategy=ExternalStrategy, forClass=javax.swing.DefaultButtonModel)
class ButtonModelBuilder {}

def model = new ButtonModelBuilder().enabled(true).pressed(true).armed(true).rollover(true).selected(true).build()
assert model.isArmed()
assert model.isPressed()
assert model.isEnabled()
assert model.isSelected()
assert model.isRollover()

The generated builder can be customised using the prefix, includes, excludes and buildMethodName annotation attributes. Here is an example illustrating various customisations:

import groovy.transform.builder.*
import groovy.transform.Canonical

@Canonical
class Person {
    String first
    String last
    int born
}

@Builder(builderStrategy=ExternalStrategy, forClass=Person, includes=['first', 'last'], buildMethodName='create', prefix='with')
class PersonBuilder { }

def p = new PersonBuilder().withFirst('Johnny').withLast('Depp').create()
assert "$p.first $p.last" == 'Johnny Depp'

The builderMethodName and builderClassName annotation attributes for @Builder aren’t applicable for this strategy.

You can use the ExternalStrategy in conjunction with @Canonical. If your @Builder annotation doesn’t have explicit includes or excludes annotation attributes but the @Canonical annotation of the class you are creating the builder for does, the ones from @Canonical will be re-used for @Builder.

DefaultStrategy

To use the DefaultStrategy, annotate your Groovy class using the @Builder annotation as shown in this example:

import groovy.transform.builder.Builder

@Builder
class Person {
    String firstName
    String lastName
    int age
}

def person = Person.builder().firstName("Robert").lastName("Lewandowski").age(21).build()
assert person.firstName == "Robert"
assert person.lastName == "Lewandowski"
assert person.age == 21

If you want, you can customize various aspects of the building process using the builderClassName, buildMethodName, builderMethodName, prefix, includes and excludes annotation attributes, some of which are used in the example here:

import groovy.transform.builder.Builder

@Builder(buildMethodName='make', builderMethodName='maker', prefix='with', excludes='age')
class Person {
    String firstName
    String lastName
    int age
}

def p = Person.maker().withFirstName("Robert").withLastName("Lewandowski").make()
assert "$p.firstName $p.lastName" == "Robert Lewandowski"

This strategy also supports annotating static methods and constructors. In this case, the static method or constructor parameters become the properties to use for building purposes and in the case of static methods, the return type of the method becomes the target class being built. If you have more than one @Builder annotation used within a class (at either the class, method or constructor positions) then it is up to you to ensure that the generated helper classes and factory methods have unique names (i.e. no more than one can use the default name values). Here is an example highlighting method and constructor usage (and also illustrating the renaming required for unique names).

import groovy.transform.builder.*
import groovy.transform.*

@ToString
@Builder
class Person {
  String first, last
  int born

  Person(){}

  @Builder(builderClassName='MovieBuilder', builderMethodName='byRoleBuilder')
  Person(String roleName) {
     if (roleName == 'Jack Sparrow') {
         this.first = 'Johnny'; this.last = 'Depp'; this.born = 1963
     }
  }

  @Builder(builderClassName='NameBuilder', builderMethodName='nameBuilder', prefix='having', buildMethodName='fullName')
  static String join(String first, String last) {
      first + ' ' + last
  }

  @Builder(builderClassName='SplitBuilder', builderMethodName='splitBuilder')
  static Person split(String name, int year) {
      def parts = name.split(' ')
      new Person(first: parts[0], last: parts[1], born: year)
  }
}

assert Person.splitBuilder().name("Johnny Depp").year(1963).build().toString() == 'Person(Johnny, Depp, 1963)'
assert Person.byRoleBuilder().roleName("Jack Sparrow").build().toString() == 'Person(Johnny, Depp, 1963)'
assert Person.nameBuilder().havingFirst('Johnny').havingLast('Depp').fullName() == 'Johnny Depp'
assert Person.builder().first("Johnny").last('Depp').born(1963).build().toString() == 'Person(Johnny, Depp, 1963)'

The forClass annotation attribute is not supported for this strategy.

InitializerStrategy

To use the InitializerStrategy, annotate your Groovy class using the @Builder annotation, and specify the strategy as shown in this example:

import groovy.transform.builder.*
import groovy.transform.*

@ToString
@Builder(builderStrategy=InitializerStrategy)
class Person {
    String firstName
    String lastName
    int age
}

Your class will be locked down to have a single public constructor taking a "fully set" initializer. It will also have a factory method to create the initializer. These are used as follows:

@CompileStatic
def firstLastAge() {
    assert new Person(Person.createInitializer().firstName("John").lastName("Smith").age(21)).toString() == 'Person(John, Smith, 21)'
}
firstLastAge()

Any attempt to use the initializer which doesn’t involve setting all the properties (though order is not important) will result in a compilation error. If you don’t need this level of strictness, you don’t need to use @CompileStatic.

You can use the InitializerStrategy in conjunction with @Canonical and @Immutable. If your @Builder annotation doesn’t have explicit includes or excludes annotation attributes but your @Canonical annotation does, the ones from @Canonical will be re-used for @Builder. Here is an example using @Builder with @Immutable:

import groovy.transform.builder.*
import groovy.transform.*

@Builder(builderStrategy=InitializerStrategy)
@Immutable
class Person {
    String first
    String last
    int born
}

@CompileStatic
def createFirstLastBorn() {
  def p = new Person(Person.createInitializer().first('Johnny').last('Depp').born(1963))
  assert "$p.first $p.last $p.born" == 'Johnny Depp 1963'
}

createFirstLastBorn()

This strategy also supports annotating static methods and constructors. In this case, the static method or constructor parameters become the properties to use for building purposes and in the case of static methods, the return type of the method becomes the target class being built. If you have more than one @Builder annotation used within a class (at either the class, method or constructor positions) then it is up to you to ensure that the generated helper classes and factory methods have unique names (i.e. no more than one can use the default name values). For an example of method and constructor usage but using the DefaultStrategy strategy, consult that strategy’s documentation.

The annotation attribute forClass is not supported for this strategy.

2.1.2. Class design annotations

This category of annotations are aimed at simplifying the implementation of well-known design patterns (delegation, singleton, …​) by using a declarative style.

@groovy.lang.Delegate

The @Delegate AST transformation aims at implementing the delegation design pattern. In the following class:

class Event {
    @Delegate Date when
    String title
}

The when field is annotated with @Delegate, meaning that the Event class will delegate calls to Date methods to the when field. In this case, the generated code looks like this:

class Event {
    Date when
    String title
    boolean before(Date other) {
        when.before(other)
    }
    // ...
}

Then you can call the before method, for example, directly on the Event class:

def ev = new Event(title:'Groovy keynote', when: Date.parse('yyyy/MM/dd', '2013/09/10'))
def now = new Date()
assert ev.before(now)

The behavior of the @Delegate AST transformation can be changed using the following parameters:

Attribute Default value Description Example

interfaces

True

Should the interfaces implemented by the field be implemented by the class too

interface Greeter { void sayHello() }
class MyGreeter implements Greeter { void sayHello() { println 'Hello!'} }

class DelegatingGreeter { // no explicit interface
    @Delegate MyGreeter greeter = new MyGreeter()
}
def greeter = new DelegatingGreeter()
assert greeter instanceof Greeter // interface was added transparently

deprecated

false

If true, also delegates methods annotated with @Deprecated

class WithDeprecation {
    @Deprecated
    void foo() {}
}
class WithoutDeprecation {
    @Deprecated
    void bar() {}
}
class Delegating {
    @Delegate(deprecated=true) WithDeprecation with = new WithDeprecation()
    @Delegate WithoutDeprecation without = new WithoutDeprecation()
}
def d = new Delegating()
d.foo() // passes thanks to deprecated=true
d.bar() // fails because of @Deprecated

methodAnnotations

False

Whether to carry over annotations from the methods of the delegate to your delegating method.

class WithAnnotations {
    @Transactional
    void method() {
    }
}
class DelegatingWithoutAnnotations {
    @Delegate WithAnnotations delegate
}
class DelegatingWithAnnotations {
    @Delegate(methodAnnotations = true) WithAnnotations delegate
}
def d1 = new DelegatingWithoutAnnotations()
def d2 = new DelegatingWithAnnotations()
assert d1.class.getDeclaredMethod('method').annotations.length==0
assert d2.class.getDeclaredMethod('method').annotations.length==1

parameterAnnotations

False

Whether to carry over annotations from the method parameters of the delegate to your delegating method.

class WithAnnotations {
    void method(@NotNull String str) {
    }
}
class DelegatingWithoutAnnotations {
    @Delegate WithAnnotations delegate
}
class DelegatingWithAnnotations {
    @Delegate(parameterAnnotations = true) WithAnnotations delegate
}
def d1 = new DelegatingWithoutAnnotations()
def d2 = new DelegatingWithAnnotations()
assert d1.class.getDeclaredMethod('method',String).parameterAnnotations[0].length==0
assert d2.class.getDeclaredMethod('method',String).parameterAnnotations[0].length==1

excludes

Empty array

A list of methods to be excluded from delegation. For more fine-grained control, see also excludeTypes.

class Worker {
    void task1() {}
    void task2() {}
}
class Delegating {
    @Delegate(excludes=['task2']) Worker worker = new Worker()
}
def d = new Delegating()
d.task1() // passes
d.task2() // fails because method is excluded

includes

Empty array

A list of methods to be included in delegation. For more fine-grained control, see also includeTypes.

class Worker {
    void task1() {}
    void task2() {}
}
class Delegating {
    @Delegate(includes=['task1']) Worker worker = new Worker()
}
def d = new Delegating()
d.task1() // passes
d.task2() // fails because method is not included

excludeTypes

Empty array

A list of interfaces containing method signatures to be excluded from delegation

interface AppendStringSelector {
    StringBuilder append(String str)
}
class UpperStringBuilder {
    @Delegate(excludeTypes=AppendStringSelector)
    StringBuilder sb1 = new StringBuilder()

    @Delegate(includeTypes=AppendStringSelector)
    StringBuilder sb2 = new StringBuilder()

    String toString() { sb1.toString() + sb2.toString().toUpperCase() }
}
def usb = new UpperStringBuilder()
usb.append(3.5d)
usb.append('hello')
usb.append(true)
assert usb.toString() == '3.5trueHELLO'

includeTypes

Empty array

A list of interfaces containing method signatures to be included in delegation

interface AppendBooleanSelector {
    StringBuilder append(boolean b)
}
interface AppendFloatSelector {
    StringBuilder append(float b)
}
class NumberBooleanBuilder {
    @Delegate(includeTypes=AppendBooleanSelector, interfaces=false)
    StringBuilder nums = new StringBuilder()
    @Delegate(includeTypes=[AppendFloatSelector], interfaces=false)
    StringBuilder bools = new StringBuilder()
    String result() { "${nums.toString()}
@groovy.transform.Immutable

The @Immutable AST transformation simplifies the creation of immutable classes, that is to say classes for which members are deemed immutable. For that, all you have to do is annotating the class like in the following example:

import groovy.transform.Immutable

@Immutable
class Point {
    int x
    int y
}

Immutable classes generated with @Immutable are automatically made final. For a class to be immutable, you have to make sure that properties are of an immutable type (primitive or boxed types), of a known-immutable type or another class annotated with @Immutable. The effect of applying @Immutable to a class are pretty similar to those of applying the @Canonical AST transformation, but with an immutable class: automatic generation of toString, equals and hashCode methods for example, but trying to modify a property would throw a ReadOnlyPropertyException in that case.

Since @Immutable relies on a predefined list of known immutable classes (like java.net.URI or java.lang.String and fails if you use a type which is not in that list, you are allowed to instruct the transformation that some types are deemed immutable thanks to the following parameters:

Attribute Default value Description Example

knownImmutableClasses

Empty list

A list of classes which are deemed immutable.

import groovy.transform.Immutable
import groovy.transform.TupleConstructor

@TupleConstructor
final class Point {
    final int x
    final int y
    public String toString() { "($x,$y)" }
}

@Immutable(knownImmutableClasses=[Point])
class Triangle {
    Point a,b,c
}

knownImmutables

Empty list

A list of property names which are deemed immutable.

import groovy.transform.Immutable
import groovy.transform.TupleConstructor

@TupleConstructor
final class Point {
    final int x
    final int y
    public String toString() { "($x,$y)" }
}

@Immutable(knownImmutables=['a','b','c'])
class Triangle {
    Point a,b,c
}

copyWith

false

A boolean whether to generate a copyWith( Map ) method.

import groovy.transform.Immutable

@Immutable( copyWith=true )
class User {
    String  name
    Integer age
}

def bob   = new User( 'bob', 43 )
def alice = bob.copyWith( name:'alice' )
assert alice.name == 'alice'
assert alice.age  == 43
@groovy.transform.Memoized

The @Memoized AST transformations simplifies the implementation of caching by allowing the result of method calls to be cached just by annotating the method with @Memoized. Let’s imagine the following method:

long longComputation(int seed) {
    // slow computation
    Thread.sleep(100*seed)
    System.nanoTime()
}

This emulates a long computation, based on the actual parameters of the method. Without @Memoized, each method call would take several seconds plus it would return a random result:

def x = longComputation(1)
def y = longComputation(1)
assert x!=y

Adding @Memoized changes the semantics of the method by adding caching, based on the parameters:

@Memoized
long longComputation(int seed) {
    // slow computation
    Thread.sleep(100*seed)
    System.nanoTime()
}

def x = longComputation(1) // returns after 100 milliseconds
def y = longComputation(1) // returns immediatly
def z = longComputation(2) // returns after 200 milliseconds
assert x==y
assert x!=z

The size of the cache can be configured using two optional parameters:

  • protectedCacheSize: the number of results which are guaranteed not to be cleared after garbage collection

  • maxCacheSize: the maximum number of results that can be kept in memory

By default, the size of the cache is unlimited and no cache result is protected from garbage collection. Setting a protectedCacheSize>0 would create an unlimited cache with some results protected. Setting maxCacheSize>0 would create a limited cache but without any protection from garbage protection. Setting both would create a limited, protected cache.

@groovy.lang.Singleton

The @Singleton annotation can be used to implement the singleton design pattern on a class. The singleton instance is defined eagerly by default, using class initialization, or lazily, in which case the field is initialized using double checked locking.

@Singleton
class GreetingService {
    String greeting(String name) { "Hello, $name!" }
}
assert GreetingService.instance.greeting('Bob') == 'Hello, Bob!'

By default, the singleton is created eagerly when the class is initialized and available through the instance property. It is possible to change the name of the singleton using the property parameter:

@Singleton(property='theOne')
class GreetingService {
    String greeting(String name) { "Hello, $name!" }
}

assert GreetingService.theOne.greeting('Bob') == 'Hello, Bob!'

And it is also possible to make initialization lazy using the lazy parameter:

class Collaborator {
    public static boolean init = false
}
@Singleton(lazy=true,strict=false)
class GreetingService {
    static void init() {}
    GreetingService() {
        Collaborator.init = true
    }
    String greeting(String name) { "Hello, $name!" }
}
GreetingService.init() // make sure class is initialized
assert Collaborator.init == false
GreetingService.instance
assert Collaborator.init == true
assert GreetingService.instance.greeting('Bob') == 'Hello, Bob!'

In this example, we also set the strict parameter to false, which allows us to define our own constructor.

@groovy.transform.Mixin

Deprecated. Consider using traits instead.

2.1.3. Logging improvements

Groovy provides AST transformation that helps integrating with the most widely used logging frameworks. It’s worth noting that annotating a class with one of those annotations doesn’t prevent you from adding the appropriate logging framework on classpath.

All transformations work in a similar way:

  • add static final log field corresponding to the logger

  • wrap all calls to log.level() into the appropriate log.isLevelEnabled guard, depending on the underlying framework

Those transformations support two parameters:

  • value (default log) corresponds to the name of the logger field

  • category (defaults to the class name) is the name of the logger category

@groovy.util.logging.Log

The first logging AST transformation available is the @Log annotation which relies on the JDK logging framework. Writing:

@groovy.util.logging.Log
class Greeter {
    void greet() {
        log.info 'Called greeter'
        println 'Hello, world!'
    }
}

is equivalent to writing:

import java.util.logging.Level
import java.util.logging.Logger

class Greeter {
    private static final Logger log = Logger.getLogger(Greeter.name)
    void greet() {
        if (log.isLoggable(Level.INFO)) {
            log.info 'Called greeter'
        }
        println 'Hello, world!'
    }
}
@groovy.util.logging.Commons

Groovy supports the Apache Commons Logging framework using to the @Commons annotation. Writing:

@groovy.util.logging.Commons
class Greeter {
    void greet() {
        log.debug 'Called greeter'
        println 'Hello, world!'
    }
}

is equivalent to writing:

import org.apache.commons.logging.LogFactory
import org.apache.commons.logging.Log

class Greeter {
    private static final Log log = LogFactory.getLog(Greeter)
    void greet() {
        if (log.isDebugEnabled()) {
            log.debug 'Called greeter'
        }
        println 'Hello, world!'
    }
}
@groovy.util.logging.Log4j

Groovy supports the Apache Log4j 1.x framework using to the @Log4j annotation. Writing:

@groovy.util.logging.Log4j
class Greeter {
    void greet() {
        log.debug 'Called greeter'
        println 'Hello, world!'
    }
}

is equivalent to writing:

import org.apache.log4j.Logger

class Greeter {
    private static final Logger log = Logger.getLogger(Greeter)
    void greet() {
        if (log.isDebugEnabled()) {
            log.debug 'Called greeter'
        }
        println 'Hello, world!'
    }
}
@groovy.util.logging.Log4j2

Groovy supports the Apache Log4j 2.x framework using to the @Log4j2 annotation. Writing:

@groovy.util.logging.Log4j2
class Greeter {
    void greet() {
        log.debug 'Called greeter'
        println 'Hello, world!'
    }
}

is equivalent to writing:

import org.apache.logging.log4j.LogManager
import org.apache.logging.log4j.Logger

class Greeter {
    private static final Logger log = LogManager.getLogger(Greeter)
    void greet() {
        if (log.isDebugEnabled()) {
            log.debug 'Called greeter'
        }
        println 'Hello, world!'
    }
}
@groovy.util.logging.Slf4j

Groovy supports the Simple Logging Facade for Java (SLF4J) framework using to the @Slf4j annotation. Writing:

@groovy.util.logging.Slf4j
class Greeter {
    void greet() {
        log.debug 'Called greeter'
        println 'Hello, world!'
    }
}

is equivalent to writing:

import org.slf4j.LoggerFactory
import org.slf4j.Logger

class Greeter {
    private static final Logger log = LoggerFactory.getLogger(Greeter)
    void greet() {
        if (log.isDebugEnabled()) {
            log.debug 'Called greeter'
        }
        println 'Hello, world!'
    }
}

2.1.4. Declarative concurrency

The Groovy language provides a set of annotations aimed at simplifying common concurrency patterns in a declarative approach.

@groovy.transform.Synchronized

The @Synchronized AST transformations works in a similar way to the synchronized keyword but locks on different objects for safer concurrency. It can be applied on any method or static method:

import groovy.transform.Synchronized

import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit

class Counter {
    int cpt
    @Synchronized
    int incrementAndGet() {
        cpt++
    }
    int get() {
        cpt
    }
}

Writing this is equivalent to creating a lock object and wrapping the whole method into a synchronized block:

class Counter {
    int cpt
    private final Object $lock = new Object()

    int incrementAndGet() {
        synchronized($lock) {
            cpt++
        }
    }
    int get() {
        cpt
    }

}

By default, @Synchronized creates a field named $lock (or $LOCK for a static method) but you can make it use any field you want by specifying the value attribute, like in the following example:

import groovy.transform.Synchronized

import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit

class Counter {
    int cpt
    private final Object myLock = new Object()

    @Synchronized('myLock')
    int incrementAndGet() {
        cpt++
    }
    int get() {
        cpt
    }
}
@groovy.transform.WithReadLock and @groovy.transform.WithWriteLock

The @WithReadLock AST transformation works in conjunction with the @WithWriteLock transformation to provide read/write synchronization using the ReentrantReadWriteLock facility that the JDK provides. The annotation can be added to a method or a static method. It will transparently create a $reentrantLock final field (or $REENTRANTLOCK for a static method) and proper synchronization code will be added. For example, the following code:

import groovy.transform.WithReadLock
import groovy.transform.WithWriteLock

class Counters {
    public final Map<String,Integer> map = [:].withDefault { 0 }

    @WithReadLock
    int get(String id) {
        map.get(id)
    }

    @WithWriteLock
    void add(String id, int num) {
        Thread.sleep(200) // emulate long computation
        map.put(id, map.get(id)+num)
    }
}

is equivalent to this:

import groovy.transform.WithReadLock as WithReadLock
import groovy.transform.WithWriteLock as WithWriteLock

public class Counters {

    private final Map<String, Integer> map
    private final java.util.concurrent.locks.ReentrantReadWriteLock $reentrantlock

    public int get(java.lang.String id) {
        $reentrantlock.readLock().lock()
        try {
            map.get(id)
        }
        finally {
            $reentrantlock.readLock().unlock()
        }
    }

    public void add(java.lang.String id, int num) {
        $reentrantlock.writeLock().lock()
        try {
            java.lang.Thread.sleep(200)
            map.put(id, map.get(id) + num )
        }
        finally {
            $reentrantlock.writeLock().unlock()
        }
    }
}

Both @WithReadLock and @WithWriteLock support specifying an alternative lock object. In that case, the referenced field must be declared by the user, like in the following alternative:

import groovy.transform.WithReadLock
import groovy.transform.WithWriteLock

import java.util.concurrent.locks.ReentrantReadWriteLock

class Counters {
    public final Map<String,Integer> map = [:].withDefault { 0 }
    private final ReentrantReadWriteLock customLock = new ReentrantReadWriteLock()

    @WithReadLock('customLock')
    int get(String id) {
        map.get(id)
    }

    @WithWriteLock('customLock')
    void add(String id, int num) {
        Thread.sleep(200) // emulate long computation
        map.put(id, map.get(id)+num)
    }
}

For details

2.1.5. Easier cloning and externalizing

Groovy provides two annotations aimed at facilitating the implementation of Cloneable and Externalizable interfaces, respectively named @AutoClone and @AutoExternalize.

@groovy.transform.AutoClone

The @AutoClone annotation is aimed at implementing the @java.lang.Cloneable interface using various strategies, thanks to the style parameter:

  • the default AutoCloneStyle.CLONE strategy calls super.clone() first then clone() on each cloneable property

  • the AutoCloneStyle.SIMPLE strategy uses a regular constructor call and copies properties from the source to the clone

  • the AutoCloneStyle.COPY_CONSTRUCTOR strategy creates and uses a copy constructor

  • the AutoCloneStyle.SERIALIZATION strategy uses serialization (or externalization) to clone the object

Each of those strategies have pros and cons which are discussed in the Javadoc for groovy.transform.AutoClone and groovy.transform.AutoCloneStyle .

For example, the following example:

import groovy.transform.AutoClone

@AutoClone
class Book {
    String isbn
    String title
    List<String> authors
    Date publicationDate
}

is equivalent to this:

class Book implements Cloneable {
    String isbn
    String title
    List<String> authors
    Date publicationDate

    public Book clone() throws CloneNotSupportedException {
        Book result = super.clone()
        result.authors = authors instanceof Cloneable ? (List) authors.clone() : authors
        result.publicationDate = publicationDate.clone()
        result
    }
}

Note that the String properties aren’t explicitly handled because Strings are immutable and the clone() method from Object will copy the String references. The same would apply to primitive fields and most of the concrete subclasses of java.lang.Number.

In addition to cloning styles, @AutoClone supports multiple options:

Attribute Default value Description Example

excludes

Empty list

A list of property or field names that need to be excluded from cloning. A string consisting of a comma-separated field/property names is also allowed. See groovy.transform.AutoClone#excludes for details

import groovy.transform.AutoClone
import groovy.transform.AutoCloneStyle

@AutoClone(style=AutoCloneStyle.SIMPLE,excludes='authors')
class Book {
    String isbn
    String title
    List authors
    Date publicationDate
}

includeFields

false

By default, only properties are cloned. Setting this flag to true will also clone fields.

import groovy.transform.AutoClone
import groovy.transform.AutoCloneStyle

@AutoClone(style=AutoCloneStyle.SIMPLE,includeFields=true)
class Book {
    String isbn
    String title
    List authors
    protected Date publicationDate
}
@groovy.transform.AutoExternalize

The @AutoExternalize AST transformation will assist in the creation of java.io.Externalizable classes. It will automatically add the interface to the class and generate the writeExternal and readExternal methods. For example, this code:

import groovy.transform.AutoExternalize

@AutoExternalize
class Book {
    String isbn
    String title
    float price
}

will be converted into:

class Book implements java.io.Externalizable {
    String isbn
    String title
    float price

    void writeExternal(ObjectOutput out) throws IOException {
        out.writeObject(isbn)
        out.writeObject(title)
        out.writeFloat( price )
    }

    public void readExternal(ObjectInput oin) {
        isbn = (String) oin.readObject()
        title = (String) oin.readObject()
        price = oin.readFloat()
    }

}

The @AutoExternalize annotation supports two parameters which will let you slightly customize its behavior:

Attribute Default value Description Example

excludes

Empty list

A list of property or field names that need to be excluded from externalizing. A string consisting of a comma-separated field/property names is also allowed. See groovy.transform.AutoExternalize#excludes for details

import groovy.transform.AutoExternalize

@AutoExternalize(excludes='price')
class Book {
    String isbn
    String title
    float price
}

includeFields

false

By default, only properties are externalized. Setting this flag to true will also clone fields.

import groovy.transform.AutoExternalize

@AutoExternalize(includeFields=true)
class Book {
    String isbn
    String title
    protected float price
}

2.1.6. Safer scripting

The Groovy language makes it easy to execute user scripts at runtime (for example using groovy.lang.GroovyShell), but how do you make sure that a script won’t eat all CPU (infinite loops) or that concurrent scripts won’t slowly consume all available threads of a thread pool? Groovy provides several annotations which are aimed towards safer scripting, generating code which will for example allow you to interrupt execution automatically.

@groovy.transform.ThreadInterrupt

One complicated situation in the JVM world is when a thread can’t be stopped. The Thread#stop method exists but is deprecated (and isn’t reliable) so your only chance relies in Thread#interrupt. Calling the latter will set the interrupt flag on the thread, but it will not stop the execution of the thread. This is problematic because it’s the responsibility of the code executing in the thread to check the interrupt flag and properly exit. This makes sense when you, as a developer, know that the code you are executing is meant to be run in an independent thread, but in general, you don’t know it. It’s even worse with user scripts, who might not even know what a thread is (think of DSLs).

@ThreadInterrupt simplifies this by adding thread interruption checks at critical places in the code:

  • loops (for, while)

  • first instruction of a method

  • first instruction of a closure body

Let’s imagine the following user script:

while (true) {
    i++
}

This is an obvious infinite loop. If this code executes in its own thread, interrupting wouldn’t help: if you join on the thread, then the calling code would be able to continue, but the thread would still be alive, running in background without any ability for you to stop it, slowly causing thread starvation.

One possibility to work around this is to set up your shell this way:

def config = new CompilerConfiguration()
config.addCompilationCustomizers(
        new ASTTransformationCustomizer(ThreadInterrupt)
)
def binding = new Binding(i:0)
def shell = new GroovyShell(binding,config)

The shell is then configured to automatically apply the @ThreadInterrupt AST transformations on all scripts. This allows you to execute user scripts this way:

def t = Thread.start {
    shell.evaluate(userCode)
}
t.join(1000) // give at most 1000ms for the script to complete
if (t.alive) {
    t.interrupt()
}

The transformation automatically modified user code like this:

while (true) {
    if (Thread.currentThread().interrupted) {
        throw new InterruptedException('The current thread has been interrupted.')
    }
    i++
}

The check which is introduced inside the loop guarantees that if the interrupt flag is set on the current thread, an exception will be thrown, interrupting the execution of the thread.

@ThreadInterrupt supports multiple options that will let you further customize the behavior of the transformation:

Attribute Default value Description Example

thrown

java.lang.InterruptedException

Specifies the type of exception which is thrown if the thread is interrupted.

class BadException extends Exception {
    BadException(String message) { super(message) }
}

def config = new CompilerConfiguration()
config.addCompilationCustomizers(
        new ASTTransformationCustomizer(thrown:BadException, ThreadInterrupt)
)
def binding = new Binding(i:0)
def shell = new GroovyShell(this.class.classLoader,binding,config)

def userCode = """
try {
    while (true) {
        i++
    }
} catch (BadException e) {
    i = -1
}
"""

def t = Thread.start {
    shell.evaluate(userCode)
}
t.join(1000) // give at most 1s for the script to complete
assert binding.i > 0
if (t.alive) {
    t.interrupt()
}
Thread.sleep(500)
assert binding.i == -1'''

checkOnMethodStart

true

Should an interruption check be inserted at the beginning of each method body. See groovy.transform.ThreadInterrupt for details.

@ThreadInterrupt(checkOnMethodStart=false)

applyToAllClasses

true

Should the transformation be applied on all classes of the same source unit (in the same source file). See groovy.transform.ThreadInterrupt for details.

@ThreadInterrupt(applyToAllClasses=false)
class A { ... } // interrupt checks added
class B { ... } // no interrupt checks

applyToAllMembers

true

Should the transformation be applied on all members of class. See groovy.transform.ThreadInterrupt for details.

class A {
    @ThreadInterrupt(applyToAllMembers=false)
    void method1() { ... } // interrupt checked added
    void method2() { ... } // no interrupt checks
}
@groovy.transform.TimedInterrupt

The @TimedInterrupt AST transformation tries to solve a slightly different problem from @groovy.transform.ThreadInterrupt: instead of checking the interrupt flag of the thread, it will automatically throw an exception if the thread has been running for too long.

This annotation does not spawn a monitoring thread. Instead, it works in a similar manner as @ThreadInterrupt by placing checks at appropriate places in the code. This means that if you have a thread blocked by I/O, it will not be interrupted.

Imagine the following user code:

def fib(int n) { n<2?n:fib(n-1)+fib(n-2) }

result = fib(600)

The implementation of the famous Fibonacci number computation here is far from optimized. If it is called with a high n value, it can take minutes to answer. With @TimedInterrupt, you can choose how long a script is allowed to run. The following setup code will allow the user script to run for 1 second at max:

def config = new CompilerConfiguration()
config.addCompilationCustomizers(
        new ASTTransformationCustomizer(value:1, TimedInterrupt)
)
def binding = new Binding(result:0)
def shell = new GroovyShell(this.class.classLoader, binding,config)

This code is equivalent to annotating a class with @TimedInterrupt like this:

@TimedInterrupt(value=1, unit=TimeUnit.SECONDS)
class MyClass {
    def fib(int n) {
        n<2?n:fib(n-1)+fib(n-2)
    }
}

@TimedInterrupt supports multiple options that will let you further customize the behavior of the transformation:

Attribute Default value Description Example

value

Long.MAX_VALUE

Used in combination with unit to specify after how long execution times out.

@TimedInterrupt(value=500L, unit= TimeUnit.MILLISECONDS, applyToAllClasses = false)
class Slow {
    def fib(n) { n<2?n:fib(n-1)+fib(n-2) }
}
def result
def t = Thread.start {
    result = new Slow().fib(500)
}
t.join(5000)
assert result == null
assert !t.alive

unit

TimeUnit.SECONDS

Used in combination with value to specify after how long execution times out.

@TimedInterrupt(value=500L, unit= TimeUnit.MILLISECONDS, applyToAllClasses = false)
class Slow {
    def fib(n) { n<2?n:fib(n-1)+fib(n-2) }
}
def result
def t = Thread.start {
    result = new Slow().fib(500)
}
t.join(5000)
assert result == null
assert !t.alive

thrown

java.util.concurrent.TimeoutException

Specifies the type of exception which is thrown if timeout is reached.

@TimedInterrupt(thrown=TooLongException, applyToAllClasses = false, value=1L)
class Slow {
    def fib(n) { Thread.sleep(100); n<2?n:fib(n-1)+fib(n-2) }
}
def result
def t = Thread.start {
    try {
        result = new Slow().fib(50)
    } catch (TooLongException e) {
        result = -1
    }
}
t.join(5000)
assert result == -1

checkOnMethodStart

true

Should an interruption check be inserted at the beginning of each method body. See groovy.transform.TimedInterrupt for details.

@TimedInterrupt(checkOnMethodStart=false)

applyToAllClasses

true

Should the transformation be applied on all classes of the same source unit (in the same source file). See groovy.transform.TimedInterrupt for details.

@TimedInterrupt(applyToAllClasses=false)
class A { ... } // interrupt checks added
class B { ... } // no interrupt checks

applyToAllMembers

true

Should the transformation be applied on all members of class. See groovy.transform.TimedInterrupt for details.

class A {
    @TimedInterrupt(applyToAllMembers=false)
    void method1() { ... } // interrupt checked added
    void method2() { ... } // no interrupt checks
}
@TimedInterrupt is currently not compatible with static methods!
@groovy.transform.ConditionalInterrupt

The last annotation for safer scripting is the base annotation when you want to interrupt a script using a custom strategy. In particular, this is the annotation of choice if you want to use resource management (limit the number of calls to an API, …​). In the following example, user code is using an infinite loop, but @ConditionalInterrupt will allow us to check a quota manager and interrupt automatically the script:

@ConditionalInterrupt({Quotas.disallow('user')})
class UserCode {
    void doSomething() {
        int i=0
        while (true) {
            println "Consuming resources ${++i}"
        }
    }
}

The quota checking is very basic here, but it can be any code:

class Quotas {
    static def quotas = [:].withDefault { 10 }
    static boolean disallow(String userName) {
        println "Checking quota for $userName"
        (quotas[userName]--)<0
    }
}

We can make sure @ConditionalInterrupt works properly using this test code:

assert Quotas.quotas['user'] == 10
def t = Thread.start {
    new UserCode().doSomething()
}
t.join(5000)
assert !t.alive
assert Quotas.quotas['user'] < 0

Of course, in practice, it is unlikely that @ConditionalInterrupt will be itself added by hand on user code. It can be injected in a similar manner as the example shown in the ThreadInterrupt section, using the org.codehaus.groovy.control.customizers.ASTTransformationCustomizer :

def config = new CompilerConfiguration()
def checkExpression = new ClosureExpression(
        Parameter.EMPTY_ARRAY,
        new ExpressionStatement(
                new MethodCallExpression(new ClassExpression(ClassHelper.make(Quotas)), 'disallow', new ConstantExpression('user'))
        )
)
config.addCompilationCustomizers(
        new ASTTransformationCustomizer(value: checkExpression, ConditionalInterrupt)
)

def shell = new GroovyShell(this.class.classLoader,new Binding(),config)

def userCode = """
        int i=0
        while (true) {
            println "Consuming resources \\${++i}"
        }
"""

assert Quotas.quotas['user'] == 10
def t = Thread.start {
    shell.evaluate(userCode)
}
t.join(5000)
assert !t.alive
assert Quotas.quotas['user'] < 0

@ConditionalInterrupt supports multiple options that will let you further customize the behavior of the transformation:

Attribute Default value Description Example

value

The closure which will be called to check if execution is allowed. If the closure returns false, execution is allowed. If it returns true, then an exception will be thrown.

@ConditionalInterrupt({ ... })

thrown

java.lang.InterruptedException

Specifies the type of exception which is thrown if execution should be aborted.

config.addCompilationCustomizers(
        new ASTTransformationCustomizer(thrown: QuotaExceededException,value: checkExpression, ConditionalInterrupt)
)
assert Quotas.quotas['user'] == 10
def t = Thread.start {
    try {
        shell.evaluate(userCode)
    } catch (QuotaExceededException) {
        Quotas.quotas['user'] = 'Quota exceeded'
    }
}
t.join(5000)
assert !t.alive
assert Quotas.quotas['user'] == 'Quota exceeded'

checkOnMethodStart

true

Should an interruption check be inserted at the beginning of each method body. See groovy.transform.ConditionalInterrupt for details.

@ConditionalInterrupt(checkOnMethodStart=false)

applyToAllClasses

true

Should the transformation be applied on all classes of the same source unit (in the same source file). See groovy.transform.ConditionalInterrupt for details.

@ConditionalInterrupt(applyToAllClasses=false)
class A { ... } // interrupt checks added
class B { ... } // no interrupt checks

applyToAllMembers

true

Should the transformation be applied on all members of class. See groovy.transform.ConditionalInterrupt for details.

class A {
    @ConditionalInterrupt(applyToAllMembers=false)
    void method1() { ... } // interrupt checked added
    void method2() { ... } // no interrupt checks
}

2.1.7. Compiler directives

This category of AST transformations groups annotations which have a direct impact on the semantics of the code, rather than focusing on code generation. With that regards, they can be seen as compiler directives that either change the behavior of a program at compile time or runtime.

@groovy.transform.Field

The @Field annotation only makes sense in the context of a script and aims at solving a common scoping error with scripts. The following example will for example fail at runtime:

def x

String line() {
    "="*x
}

x=3
assert "===" == line()
x=5
assert "=====" == line()

The error that is thrown may be difficult to interpret: groovy.lang.MissingPropertyException: No such property: x. The reason is that scripts are compiled to classes and the script body is itself compiled as a single run() method. Methods which are defined in the scripts are independent, so the code above is equivalent to this:

class MyScript extends Script {

    String line() {
        "="*x
    }

    public def run() {
        def x
        x=3
        assert "===" == line()
        x=5
        assert "=====" == line()
    }
}

So def x is effectively interpreted as a local variable, outside of the scope of the line method. The @Field AST transformation aims at fixing this by changing the scope of the variable to a field of the enclosing script:

@Field def x

String line() {
    "="*x
}

x=3
assert "===" == line()
x=5
assert "=====" == line()

The resulting, equivalent, code is now:

class MyScript extends Script {

    def x

    String line() {
        "="*x
    }

    public def run() {
        x=3
        assert "===" == line()
        x=5
        assert "=====" == line()
    }
}
@groovy.transform.PackageScope

By default, Groovy visibility rules imply that if you create a field without specifying a modifier, then the field is interpreted as a property:

class Person {
    String name // this is a property
}

Should you want to create a package private field instead of a property (private field+getter/setter), then annotate your field with @PackageScope:

class Person {
    @PackageScope String name // not a property anymore
}

The @PackageScope annotation can also be used for classes, methods and constructors. In addition, by specifying a list of PackageScopeTarget values as the annotation attribute at the class level, all members within that class that don’t have an explicit modifier and match the provided PackageScopeTarget will remain package protected. For example to apply to fields within a class use the following annotation:

import static groovy.transform.PackageScopeTarget.FIELDS
@PackageScope(FIELDS)
class Person {
  String name     // not a property, package protected
  Date dob        // not a property, package protected
  private int age // explicit modifier, so won't be touched
}

The @PackageScope annotation is seldom used as part of normal Groovy conventions but is sometimes useful for factory methods that should be visible internally within a package or for methods or constructors provided for testing purposes, or when integrating with third-party libraries which require such visibility conventions.

@groovy.transform.AnnotationCollector

@AnnotationCollector allows the creation of meta-annotation, which are described in a dedicated section.

@groovy.transform.TypeChecked

@TypeChecked activates compile-time type checking on your Groovy code. See section on type checking for details.

@groovy.transform.CompileStatic

@CompileStatic activates static compilation on your Groovy code. See section on type checking for details.

@groovy.transform.CompileDynamic

@CompileDynamic disables static compilation on parts of your Groovy code. See section on type checking for details.

@groovy.lang.DelegatesTo

@DelegatesTo is not, technically speaking, an AST transformation. It is aimed at documenting code and helping the compiler in case you are using type checking or static compilation. The annotation is described thoroughly in the DSL section of this guide.

2.1.8. Swing patterns

@groovy.beans.Bindable

@Bindable is an AST transformation that transforms a regular property into a bound property (according to the JavaBeans specification). The @Bindable annotation can be placed on a property or a class. To convert all properties of a class into bound properties, on can annotate the class like in this example:

import groovy.beans.Bindable

@Bindable
class Person {
    String name
    int age
}

This is equivalent to writing this:

import java.beans.PropertyChangeListener
import java.beans.PropertyChangeSupport

class Person {
    final private PropertyChangeSupport this$propertyChangeSupport

    String name
    int age

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        this$propertyChangeSupport.addPropertyChangeListener(listener)
    }

    public void addPropertyChangeListener(String name, PropertyChangeListener listener) {
        this$propertyChangeSupport.addPropertyChangeListener(name, listener)
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        this$propertyChangeSupport.removePropertyChangeListener(listener)
    }

    public void removePropertyChangeListener(String name, PropertyChangeListener listener) {
        this$propertyChangeSupport.removePropertyChangeListener(name, listener)
    }

    public void firePropertyChange(String name, Object oldValue, Object newValue) {
        this$propertyChangeSupport.firePropertyChange(name, oldValue, newValue)
    }

    public PropertyChangeListener[] getPropertyChangeListeners() {
        return this$propertyChangeSupport.getPropertyChangeListeners()
    }

    public PropertyChangeListener[] getPropertyChangeListeners(String name) {
        return this$propertyChangeSupport.getPropertyChangeListeners(name)
    }
}

@Bindable therefore removes a lot of boilerplate from your class, dramatically increasing readability. If the annotation is put on a single property, only that property is bound:

import groovy.beans.Bindable

class Person {
    String name
    @Bindable int age
}
@groovy.beans.ListenerList

The @ListenerList AST transformation generates code for adding, removing and getting the list of listeners to a class, just by annotating a collection property:

import java.awt.event.ActionListener
import groovy.beans.ListenerList

class Component {
    @ListenerList
    List<ActionListener> listeners;
}

The transform will generate the appropriate add/remove methods based on the generic type of the list. In addition, it will also create fireXXX methods based on the public methods declared on the class:

import java.awt.event.ActionEvent
import java.awt.event.ActionListener as ActionListener
import groovy.beans.ListenerList as ListenerList

public class Component {

    @ListenerList
    private List<ActionListener> listeners

    public void addActionListener(ActionListener listener) {
        if ( listener == null) {
            return
        }
        if ( listeners == null) {
            listeners = []
        }
        listeners.add(listener)
    }

    public void removeActionListener(ActionListener listener) {
        if ( listener == null) {
            return
        }
        if ( listeners == null) {
            listeners = []
        }
        listeners.remove(listener)
    }

    public ActionListener[] getActionListeners() {
        Object __result = []
        if ( listeners != null) {
            __result.addAll(listeners)
        }
        return (( __result ) as ActionListener[])
    }

    public void fireActionPerformed(ActionEvent param0) {
        if ( listeners != null) {
            ArrayList<ActionListener> __list = new ArrayList<ActionListener>(listeners)
            for (def listener : __list ) {
                listener.actionPerformed(param0)
            }
        }
    }
}

@Bindable supports multiple options that will let you further customize the behavior of the transformation:

Attribute Default value Description Example

name

Generic type name

By default, the suffix which will be appended to add/remove/…​ methods is the simple class name of the generic type of the list.

class Component {
    @ListenerList(name='item')
    List<ActionListener> listeners;
}

synchronize

false

If set to true, generated methods will be synchronized

class Component {
    @ListenerList(synchronize = true)
    List<ActionListener> listeners;
}
@groovy.beans.Vetoable

The @Vetoable annotation works in a similar manner to @Bindable but generates constrained property according to the JavaBeans specification, instead of bound properties. The annotation can be placed on a class, meaning that all properties will be converted to constrained properties, or on a single property. For example, annotating this class with @Vetoable:

import groovy.beans.Vetoable

import java.beans.PropertyVetoException
import java.beans.VetoableChangeListener

@Vetoable
class Person {
    String name
    int age
}

is equivalent to writing this:

public class Person {

    private String name
    private int age
    final private java.beans.VetoableChangeSupport this$vetoableChangeSupport

    public void addVetoableChangeListener(VetoableChangeListener listener) {
        this$vetoableChangeSupport.addVetoableChangeListener(listener)
    }

    public void addVetoableChangeListener(String name, VetoableChangeListener listener) {
        this$vetoableChangeSupport.addVetoableChangeListener(name, listener)
    }

    public void removeVetoableChangeListener(VetoableChangeListener listener) {
        this$vetoableChangeSupport.removeVetoableChangeListener(listener)
    }

    public void removeVetoableChangeListener(String name, VetoableChangeListener listener) {
        this$vetoableChangeSupport.removeVetoableChangeListener(name, listener)
    }

    public void fireVetoableChange(String name, Object oldValue, Object newValue) throws PropertyVetoException {
        this$vetoableChangeSupport.fireVetoableChange(name, oldValue, newValue)
    }

    public VetoableChangeListener[] getVetoableChangeListeners() {
        return this$vetoableChangeSupport.getVetoableChangeListeners()
    }

    public VetoableChangeListener[] getVetoableChangeListeners(String name) {
        return this$vetoableChangeSupport.getVetoableChangeListeners(name)
    }

    public void setName(String value) throws PropertyVetoException {
        this.fireVetoableChange('name', name, value)
        name = value
    }

    public void setAge(int value) throws PropertyVetoException {
        this.fireVetoableChange('age', age, value)
        age = value
    }
}

If the annotation is put on a single property, only that property is made vetoable:

import groovy.beans.Vetoable

class Person {
    String name
    @Vetoable int age
}

2.1.9. Test assistance

@groovy.lang.NotYetImplemented

@NotYetImplemented is used to invert the result of a JUnit 3/4 test case. It is in particular useful if a feature is not yet implemented but the test is. In that case, it is expected that the test fails. Marking it with @NotYetImplemented will inverse the result of the test, like in this example:

import groovy.transform.NotYetImplemented

class Maths {
    static int fib(int n) {
        // todo: implement later
    }
}

class MathsTest extends GroovyTestCase {
    @NotYetImplemented
    void testFib() {
        def dataTable = [
                1:1,
                2:1,
                3:2,
                4:3,
                5:5,
                6:8,
                7:13
        ]
        dataTable.each { i, r ->
            assert Maths.fib(i) == r
        }
    }
}

Another advantage of using this technique is that you can write test cases for bugs before knowing how to fix them. If some time in the future, a modification in the code fixes a bug by side effect, you’ll be notified because a test which was expected to fail passed.

@groovy.transform.ASTTest

@ASTTest is a special AST transformation meant to help debugging other AST transformations or the Groovy compiler itself. It will let the developer "explore" the AST during compilation and perform assertions on the AST rather than on the result of compilation. This means that this AST transformations gives access to the AST before the bytecode is produced. @ASTTest can be placed on any annotable node and requires two parameters:

  • phase: sets at which phase at which @ASTTest will be triggered. The test code will work on the AST tree at the end of this phase.

  • value: the code which will be executed once the phase is reached, on the annotated node

Compile phase has to be chosen from one of org.codehaus.groovy.control.CompilePhase . However, since it is not possible to annotate a node twice with the same annotation, you will not be able to use @ASTTest on the same node at two distinct compile phases.

value is a closure expression which has access to a special variable node corresponding to the annotated node, and a helper lookup method which will be discussed here. For example, you can annotate a class node like this:

import groovy.transform.ASTTest
import org.codehaus.groovy.ast.ClassNode
import static org.codehaus.groovy.control.CompilePhase.*

@ASTTest(phase=CONVERSION, value={   (1)
    assert node instanceof ClassNode (2)
    assert node.name == 'Person'     (3)
})
class Person {

}
1 we’re checking the state of the Abstract Syntax Tree after the CONVERSION phase
2 node refers to the AST node which is annotated by @ASTTest
3 it can be used to perform assertions at compile time

One interesting feature of @ASTTest is that if an assertion fails, then compilation will fail. Now imagine that we want to check the behavior of an AST transformation at compile time. We will take @PackageScope here, and we will want to verify that a property annotated with @PackageScope becomes a package private field. For this, we have to know at which phase the transform runs, which can be found in org.codehaus.groovy.transform.PackageScopeASTTransformation : semantic analysis. Then a test can be written like this:

import groovy.transform.ASTTest
import groovy.transform.PackageScope

import static org.codehaus.groovy.control.CompilePhase.*

@ASTTest(phase=SEMANTIC_ANALYSIS, value= {
    def nameNode = node.properties.find { it.name == 'name' }
    def ageNode = node.properties.find { it.name == 'age' }
    assert nameNode
    assert ageNode == null // shouldn't be a property anymore
    def ageField = node.getDeclaredField 'age'
    assert ageField.modifiers == 0
})
class Person {
    String name
    @PackageScope int age
}

The @ASTTest annotation can only be placed wherever the grammar allows it. Sometimes, you would like to test the contents of an AST node which is not annotable. In this case, @ASTTest provides a convenient lookup method which will search the AST for nodes which are labelled with a special token:

def list = lookup('anchor') (1)
Statement stmt = list[0] (2)
1 returns the list of AST nodes which label is 'anchor'
2 it is always necessary to choose which element to process since lookup always returns a list

Imagine, for example, that you want to test the declared type of a for loop variable. Then you can do it like this:

import groovy.transform.ASTTest
import groovy.transform.PackageScope
import org.codehaus.groovy.ast.ClassHelper
import org.codehaus.groovy.ast.expr.DeclarationExpression
import org.codehaus.groovy.ast.stmt.ForStatement

import static org.codehaus.groovy.control.CompilePhase.*

class Something {
    @ASTTest(phase=SEMANTIC_ANALYSIS, value= {
        def forLoop = lookup('anchor')[0]
        assert forLoop instanceof ForStatement
        def decl = forLoop.collectionExpression.expressions[0]
        assert decl instanceof DeclarationExpression
        assert decl.variableExpression.name == 'i'
        assert decl.variableExpression.originType == ClassHelper.int_TYPE
    })
    void someMethod() {
        int x = 1;
        int y = 10;
        anchor: for (int i=0; i<x+y; i++) {
            println "$i"
        }
    }
}

@ASTTest also exposes those variables inside the test closure:

  • node corresponds to the annotated node, as usual

  • compilationUnit gives access to the current org.codehaus.groovy.control.CompilationUnit

  • compilePhase returns the current compile phase (org.codehaus.groovy.control.CompilePhase)

The latter is interesting if you don’t specify the phase attribute. In that case, the closure will be executed after each compile phase after (and including) SEMANTIC_ANALYSIS. The context of the transformation is kept after each phase, giving you a chance to check what changed between two phases.

As an example, here is how you could dump the list of AST transformations registered on a class node:

import groovy.transform.ASTTest
import groovy.transform.CompileStatic
import groovy.transform.Immutable
import org.codehaus.groovy.ast.ClassNode
import org.codehaus.groovy.control.CompilePhase

@ASTTest(value={
    System.err.println "Compile phase: $compilePhase"
    ClassNode cn = node
    System.err.println "Global AST xforms: ${compilationUnit?.ASTTransformationsContext?.globalTransformNames}"
    CompilePhase.values().each {
        def transforms = cn.getTransforms(it)
        if (transforms) {
            System.err.println "Ast xforms for phase $it:"
            transforms.each { map ->
                System.err.println(map)
            }
        }
    }
})
@CompileStatic
@Immutable
class Foo {
}

And here is how you can memorize variables for testing between two phases:

import groovy.transform.ASTTest
import groovy.transform.ToString
import org.codehaus.groovy.ast.ClassNode
import org.codehaus.groovy.control.CompilePhase

@ASTTest(value={
    if (compilePhase==CompilePhase.INSTRUCTION_SELECTION) {             (1)
        println "toString() was added at phase: ${added}"
        assert added == CompilePhase.CANONICALIZATION                   (2)
    } else {
        if (node.getDeclaredMethods('toString') && added==null) {       (3)
            added = compilePhase                                        (4)
        }
    }
})
@ToString
class Foo {
    String name
}
1 if the current compile phase is instruction selection
2 then we want to make sure toString was added at CANONICALIZATION
3 otherwise, if toString exists and that the variable from the context, added is null
4 then it means that this compile phase is the one where toString was added

2.1.10. Grape handling

@groovy.lang.Grab
@groovy.lang.GrabConfig
@groovy.lang.GrabExclude
@groovy.lang.GrabResolver
@groovy.lang.Grapes

Grape is a dependency management engine embedded into Groovy, relying on several annotations which are described thoroughly in this section of the guide.

2.2. Developing AST transformations

There are two kinds of transformations: global and local transformations.

  • Global transformations are applied to by the compiler on the code being compiled, wherever the transformation apply. Compiled classes that implement global transformations are in a JAR added to the classpath of the compiler and contain service locator file META-INF/services/org.codehaus.groovy.transform.ASTTransformation with a line with the name of the transformation class. The transformation class must have a no-args constructor and implement the org.codehaus.groovy.transform.ASTTransformation interface. It will be run against every source in the compilation, so be sure to not create transformations which scan all the AST in an expansive and time-consuming manner, to keep the compiler fast.

  • Local transformations are transformations applied locally by annotating code elements you want to transform. For this, we reuse the annotation notation, and those annotations should implement org.codehaus.groovy.transform.ASTTransformation. The compiler will discover them and apply the transformation on these code elements.

2.2.1. Compilation phases guide

Groovy AST transformations must be performed in one of the nine defined compilation phases (org.codehaus.groovy.control.CompilePhase).

Global transformations may be applied in any phase, but local transformations may only be applied in the semantic analysis phase or later. Briefly, the compiler phases are:

  • Initialization: source files are opened and environment configured

  • Parsing: the grammar is used to to produce tree of tokens representing the source code

  • Conversion: An abstract syntax tree (AST) is created from token trees.

  • Semantic Analysis: Performs consistency and validity checks that the grammar can’t check for, and resolves classes.

  • Canonicalization: Complete building the AST

  • Instruction Selection: instruction set is chosen, for example Java 6 or Java 7 bytecode level

  • Class Generation: creates the bytecode of the class in memory

  • Output: write the binary output to the file system

  • Finalization: Perform any last cleanup

Generally speaking, there is more type information available later in the phases. If your transformation is concerned with reading the AST, then a later phase where information is more plentiful might be a good choice. If your transformation is concerned with writing AST, then an earlier phase where the tree is more sparse might be more convenient.

2.2.2. Local transformations

Local AST transformations are relative to the context they are applied to. In most cases, the context is defined by an annotation that will define the scope of the transform. For example, annotating a field would mean that the transformation applies to the field, while annotating the class would mean that the transformation applies to the whole class.

As a naive and simple example, consider wanting to write a @WithLogging transformation that would add console messages at the start and end of a method invocation. So the following "Hello World" example would actually print "Hello World" along with a start and stop message:

Poor man’s aspect oriented programming
@WithLogging
def greet() {
    println "Hello World"
}

greet()

A local AST transformation is an easy way to do this. It requires two things:

An ASTTransformation is a callback that gives you access to the org.codehaus.groovy.control.SourceUnit, through which you can get a reference to the org.codehaus.groovy.ast.ModuleNode (AST).

The AST (Abstract Syntax Tree) is a tree structure consisting mostly of org.codehaus.groovy.ast.expr.Expression (expressions) or org.codehaus.groovy.ast.expr.Statement (statements). An easy way to learn about the AST is to explore it in a debugger. Once you have the AST, you can analyze it to find out information about the code or rewrite it to add new functionality.

The local transformation annotation is the simple part. Here is the @WithLogging one:

import org.codehaus.groovy.transform.GroovyASTTransformationClass

import java.lang.annotation.ElementType
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy
import java.lang.annotation.Target

@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.METHOD])
@GroovyASTTransformationClass(["gep.WithLoggingASTTransformation"])
public @interface WithLogging {
}

The annotation retention can be SOURCE because you won’t need the annotation past that. The element type here is METHOD, the @WithLogging because the annotation applies to methods.

But the most important part is the @GroovyASTTransformationClass annotation. This links the @WithLogging annotation to the ASTTransformation class you will write. gep.WithLoggingASTTransformation is the fully qualified class name of the ASTTransformation we are going to write. This line wires the annotation to the transformation.

With this in place, the Groovy compiler is going to invoke gep.WithLoggingASTTransformation every time an @WithLogging is found in a source unit. Any breakpoint set within LoggingASTTransformation will now be hit within the IDE when running the sample script.

The ASTTransformation class is a little more complex. Here is the very simple, and very naive, transformation to add a method start and stop message for @WithLogging:

@CompileStatic                                                                  (1)
@GroovyASTTransformation(phase=CompilePhase.SEMANTIC_ANALYSIS)                  (2)
class WithLoggingASTTransformation implements ASTTransformation {               (3)

    @Override
    void visit(ASTNode[] nodes, SourceUnit sourceUnit) {                        (4)
        MethodNode method = (MethodNode) nodes[1]                               (5)

        def startMessage = createPrintlnAst("Starting $method.name")            (6)
        def endMessage = createPrintlnAst("Ending $method.name")                (7)

        def existingStatements = ((BlockStatement)method.code).statements       (8)
        existingStatements.add(0, startMessage)                                 (9)
        existingStatements.add(endMessage)                                      (10)

    }

    private static Statement createPrintlnAst(String message) {                 (11)
        new ExpressionStatement(
            new MethodCallExpression(
                new VariableExpression("this"),
                new ConstantExpression("println"),
                new ArgumentListExpression(
                    new ConstantExpression(message)
                )
            )
        )
    }
}
1 even if not mandatory, if you write an AST transformation in Groovy, it is highly recommended to use CompileStatic because it will improve performance of the compiler.
2 annotate with org.codehaus.groovy.transform.GroovyASTTransformation to tell at which compilation phase the transform needs to run. Here, it’s at the semantic analysis phase.
3 implement the ASTTransformation interface
4 which only has a single visit method
5 the nodes parameter is a 2 AST node array, for which the first one is the annotation node (@WithLogging) and the second one is the annotated node (the method node)
6 create a statement that will print a message when we enter the method
7 create a statement that will print a message when we exit the method
8 get the method body, which in this case is a BlockStatement
9 add the enter method message before the first statement of existing code
10 append the exit method message after the last statement of existing code
11 creates an ExpressionStatement wrapping a MethodCallExpression corresponding to this.println("message")

It is important to notice that for the brevity of this example, we didn’t make the necessary checks, such as checking that the annotated node is really a MethodNode, or that the method body is an instance of BlockStatement. This exercise is left to the reader.

Note the creation of the new println statements in the createPrintlnAst(String) method. Creating AST for code is not always simple. In this case we need to construct a new method call, passing in the receiver/variable, the name of the method, and an argument list. When creating AST, it might be helpful to write the code you’re trying to create in a Groovy file and then inspect the AST of that code in the debugger to learn what to create. Then write a function like createPrintlnAst using what you learned through the debugger.

In the end:

@WithLogging
def greet() {
    println "Hello World"
}

greet()

Produces:

Starting greet
Hello World
Ending greet
It is important to note that an AST transformation participates directly in the compilation process. A common error by beginners is to have the AST transformation code in the same source tree as a class that uses the transformation. Being in the same source tree in general means that they are compiled at the same time. Since the transformation itself is going to be compiled in phases and that each compile phase processes all files of the same source unit before going to the next one, there’s a direct consequence: the transformation will not be compiled before the class that uses it! In conclusion, AST transformations need to be precompiled before you can use them. In general, it is as easy as having them in a separate source tree.

2.2.3. Global transformations

Global AST transformation are similar to local one with a major difference: they do not need an annotation, meaning that they are applied globally, that is to say on each class being compiled. It is therefore very important to limit their use to last resort, because it can have a significant impact on the compiler performance.

Following the example of the local AST transformation, imagine that we would like to trace all methods, and not only those which are annotated with @WithLogging. Basically, we need this code to behave the same as the one annotated with @WithLogging before:

def greet() {
    println "Hello World"
}

greet()

To make this work, there are two steps:

  1. create the org.codehaus.groovy.transform.ASTTransformation descriptor inside the META-INF/services directory

  2. create the ASTTransformation implementation

The descriptor file is required and must be found on classpath. It will contain a single line:

META-INF/services/org.codehaus.groovy.transform.ASTTransformation
gep.WithLoggingASTTransformation

The code for the transformation looks similar to the local case, but instead of using the ASTNode[] parameter, we need to use the SourceUnit instead:

gep/WithLoggingASTTransformation.groovy
@CompileStatic                                                                  (1)
@GroovyASTTransformation(phase=CompilePhase.SEMANTIC_ANALYSIS)                  (2)
class WithLoggingASTTransformation implements ASTTransformation {               (3)

    @Override
    void visit(ASTNode[] nodes, SourceUnit sourceUnit) {                        (4)
        def methods = sourceUnit.AST.methods                                    (5)
        methods.each { method ->                                                (6)
            def startMessage = createPrintlnAst("Starting $method.name")        (7)
            def endMessage = createPrintlnAst("Ending $method.name")            (8)

            def existingStatements = ((BlockStatement)method.code).statements   (9)
            existingStatements.add(0, startMessage)                             (10)
            existingStatements.add(endMessage)                                  (11)
        }
    }

    private static Statement createPrintlnAst(String message) {                 (12)
        new ExpressionStatement(
            new MethodCallExpression(
                new VariableExpression("this"),
                new ConstantExpression("println"),
                new ArgumentListExpression(
                    new ConstantExpression(message)
                )
            )
        )
    }
}
1 even if not mandatory, if you write an AST transformation in Groovy, it is highly recommended to use CompileStatic because it will improve performance of the compiler.
2 annotate with org.codehaus.groovy.transform.GroovyASTTransformation to tell at which compilation phase the transform needs to run. Here, it’s at the semantic analysis phase.
3 implement the ASTTransformation interface
4 which only has a single visit method
5 the sourceUnit parameter gives access to the source being compiled, so we get the AST of the current source and retrieve the list of methods from this file
6 we iterate on each method from the source file
7 create a statement that will print a message when we enter the method
8 create a statement that will print a message when we exit the method
9 get the method body, which in this case is a BlockStatement
10 add the enter method message before the first statement of existing code
11 append the exit method message after the last statement of existing code
12 creates an ExpressionStatement wrapping a MethodCallExpression corresponding to this.println("message")

2.2.4. AST API guide

AbstractASTTransformation

While you have seen that you can directly implement the ASTTransformation interface, in almost all cases you will not do this but extend the org.codehaus.groovy.transform.AbstractASTTransformation class. This class provides several utility methods that make AST transformations easier to write. Almost all AST transformations included in Groovy extend this class.

ClassCodeExpressionTransformer

It is a common use case to be able to transform an expression into another. Groovy provides a class which makes it very easy to do this: org.codehaus.groovy.ast.ClassCodeExpressionTransformer

To illustrate this, let’s create a @Shout transformation that will transform all String constants in method call arguments into their uppercase version. For example:

@Shout
def greet() {
    println "Hello World"
}

greet()

should print:

HELLO WORLD

Then the code for the transformation can use the ClassCodeExpressionTransformer to make this easier:

@CompileStatic
@GroovyASTTransformation(phase=CompilePhase.SEMANTIC_ANALYSIS)
class ShoutASTTransformation implements ASTTransformation {

    @Override
    void visit(ASTNode[] nodes, SourceUnit sourceUnit) {
        ClassCodeExpressionTransformer trn = new ClassCodeExpressionTransformer() {         (1)
            private boolean inArgList = false
            @Override
            protected SourceUnit getSourceUnit() {
                sourceUnit                                                                  (2)
            }

            @Override
            Expression transform(final Expression exp) {
                if (exp instanceof ArgumentListExpression) {
                    inArgList = true
                } else if (inArgList &&
                    exp instanceof ConstantExpression && exp.value instanceof String) {
                    return new ConstantExpression(exp.value.toUpperCase())                  (3)
                }
                def trn = super.transform(exp)
                inArgList = false
                trn
            }
        }
        trn.visitMethod((MethodNode)nodes[1])                                               (4)
    }
}
1 Internally the transformation creates a ClassCodeExpressionTransformer
2 The transformer needs to return the source unit
3 if a constant expression of type string is detected inside an argument list, transform it into its upper case version
4 call the transformer on the method being annotated
AST Nodes
Writing an AST transformation requires a deep knowledge of the internal Groovy API. In particular it requires knowledge about the AST classes. Since those classes are internal, there are chances that the API will change in the future, meaning that your transformations could break. Despite that warning, the AST has been very stable over time and such a thing rarely happens.

Classes of the Abstract Syntax Tree belong to the org.codehaus.groovy.ast package. It is recommended to the reader to use the Groovy Console, in particular the AST browser tool, to gain knowledge about those classes. However, a good resource for learning is the AST Builder test suite.

2.2.5. Testing AST transformations

Separating source trees

This section is about good practices with regards to testing AST transformations. Previous sections highlighted the fact that to be able to execute an AST transformation, it has to be precompiled. It might sound obvious but a lot of people get caught on this, trying to use an AST transformation in the same source tree as where it is defined.

The first tip for testing AST transformation is therefore to separate test sources from the sources of the transform. Again, this is nothing but best practices, but you must make sure that your build too does actually compile them separately. This is the case by default with both Apache Maven and Gradle.

Debugging AST transformations

It is very handy to be able to put a breakpoint in an AST transformation, so that you can debug your code in the IDE. However, you might be surprised to see that your IDE doesn’t stop on the breakpoint. The reason is actually simple: if your IDE uses the Groovy compiler to compile the unit tests for your AST transformation, then the compilation is triggered from the IDE, but the process which will compile the files doesn’t have debugging options. It’s only when the test case is executed that the debugging options are set on the virtual machine. In short: it is too late, the class has been compiled already, and your transformation is already applied.

A very easy workaround is to use the GroovyTestCase class which provides an assertScript method. This means that instead of writing this in a test case:

static class Subject {
    @MyTransformToDebug
    void methodToBeTested() {}
}

void testMyTransform() {
    def c = new Subject()
    c.methodToBeTested()
}

You should write:

void testMyTransformWithBreakpoint() {
    assertScript '''
        import metaprogramming.MyTransformToDebug

        class Subject {
            @MyTransformToDebug
            void methodToBeTested() {}
        }
        def c = new Subject()
        c.methodToBeTested()
    '''
}

The difference is that when you use assertScript, the code in the assertScript block is compiled when the unit test is executed. That is to say that this time, the Subject class will be compiled with debugging active, and the breakpoint is going to be hit.

ASTTest

Last but not least, testing an AST transformation is also about testing the state of the AST during compilation. Groovy provides a tool named @ASTTest for this: it is an annotation that will let you add assertions on an abstract syntax tree. Please check the documentation for ASTTest for more details.

2.2.6. External references

If you are interested in a step-by-step tutorial about writing AST transformations, you can follow this workshop.