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Release notes for Groovy 2.6

(Material on this page is still under development!)

Parrot Parser

With Groovy 2.6, you can enable the Parrot parser which supports additional syntax options and language features.

Enabling the parser

The plan for Groovy 3.0, is to have the new parser enabled by default. For Groovy 2.6, you enable the new parser using a system property. Use -Dgroovy.antlr4=true (set via JAVA_OPTS if needed).

If building from source, you need to provide Gradle with -PuseAntlr4 in order to include the classes which support the new parser.

do/while loop

Java’s class do/while loop is now supported. Example:

// classic Java-style do..while loop
def count = 5
def fact = 1
do {
    fact *= count--
} while(count > 1)
assert fact == 120

Enhanced classic Java-style for loop

The more elaborate form of Java’s classic for loop with comma-separate expressions is now supported. Example:

def facts = []
def count = 5
for (int fact = 1, i = 1; i <= count; i++, fact *= i) {
    facts << fact
}
assert facts == [1, 2, 6, 24, 120]

Multi-assignment in combination with for loop

Groovy has supported multi-assignment statements since Groovy 1.6:

// multi-assignment with types
def (String x, int y) = ['foo', 42]
assert "$x $y" == 'foo 42'

These can now appear in for loops:

// multi-assignment goes loopy
def baNums = []
for (def (String u, int v) = ['bar', 42]; v < 45; u++, v++) {
    baNums << "$u $v"
}
assert baNums == ['bar 42', 'bas 43', 'bat 44']

Java-style array initialization

Groovy has always supported literal list/array definitions using square brackets and has avoided Java-style curly braces so as not to conflict with closure definitions. In the case where the curly braces come immediately after an array type declaration however, there is no ambiguity with closure definitions, so the Java style is now also supported.

Examples:

def primes = new int[] {2, 3, 5, 7, 11}
assert primes.size() == 5 && primes.sum() == 28
assert primes.class.name == '[I'

def pets = new String[] {'cat', 'dog'}
assert pets.size() == 2 && pets.sum() == 'catdog'
assert pets.class.name == '[Ljava.lang.String;'

// traditional Groovy alternative still supported
String[] groovyBooks = [ 'Groovy in Action', 'Making Java Groovy' ]
assert groovyBooks.every{ it.contains('Groovy') }

Java-style Lambda syntax

The Java syntax for lambda expressions is now supported.

Note
Experimental Status: While this feature will remain, we regard its current implementation as experimental. Currently, lambda expressions are turned into equivalent Groovy closures. An advantage of this approach is that it works even when using JDK7 (i.e. using Groovy 2.6 with Parrot parser enabled on a version 7 JRE) but it doesn’t easily enable the same level of type inference and checking that Java 8 offers even when using @TypeChecked or @CompileStatic and might be much less performant in a range of scenarios. We are currently exploring supporting native lambda expression either instead of, or in addition to, conversion to closures. If we change the implementation, most users would experience no behavioral differences in their programs though a native implementation would likely be faster but only work on JDK8+. Our goal is to clarify this aspect of the language before reaching release candidate status.

Examples:

import static java.util.stream.Collectors.toList

(1..10).forEach((it) -> { println it })

assert (1..10).stream()
        .filter((it) -> it % 2 == 0)
        .map((it) -> it * 2)
        .collect(toList()) == [4, 8, 12, 16, 20]

The normal variants are supported and Groovy adds additional features such as default parameter values:

// general form
def add = (int x, int y) -> { def z = y; return x + z }
assert add(3, 4) == 7

// curly braces are optional for a single expression
def sub = (int x, int y) -> x - y
assert sub(4, 3) == 1

// parameter types are optional
def mult = (x, y) -> x * y
assert mult(3, 4) == 12

// no parentheses required for a single parameter with no type
def isEven = n -> n % 2 == 0
assert isEven(6)
assert !isEven(7)

// no arguments case
def theAnswer = () -> 42
assert theAnswer() == 42

// any statement requires braces
def checkMath = () -> { assert 1 + 1 == 2 }
checkMath()

// example showing default parameter values (no Java equivalent)
def addWithDefault = (int x, int y = 100) -> x + y
assert addWithDefault(1, 200) == 201
assert addWithDefault(1) == 101

Method references

The Java 8 method reference syntax using the double colon syntax is now supported.

Note
Experimental Status: While this feature will remain, we regard its current implementation as experimental. Currently, a method reference is turned into a Groovy method closure. This means that it even works when using JDK7 (i.e. using Groovy 2.6 with Parrot parser enabled on a version 7 JRE). We are currently exploring supporting native method references either instead of, or in addition to, method closures. If we change the implementation, we would expect no change in logical behavior though the new implementation may have different performance characteristics in some circumstances and the native implementation would only work on JDK8+. Our goal is to clarify this aspect of the language before reaching release candidate status.

The following examples illustrate referencing both static and instance methods of a class:

import java.util.stream.Stream
import static java.util.stream.Collectors.toList

// class::staticMethod
assert ['1', '2', '3'] ==
        Stream.of(1, 2, 3)
                .map(String::valueOf)
                .collect(toList())

// class::instanceMethod
assert ['A', 'B', 'C'] ==
        ['a', 'b', 'c'].stream()
                .map(String::toUpperCase)
                .collect(toList())

The following examples illustrate referencing methods of instance variables:

// instance::instanceMethod
def sizeAlphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'::length
assert sizeAlphabet() == 26

// instance::staticMethod
def hexer = 42::toHexString
assert hexer(127) == '7f'

The following examples illustrate referencing constructors:

// normal constructor
def r = Random::new
assert r().nextInt(10) in 0..9

// array constructor refs are handy when working with various Java libraries, e.g. streams
assert [1, 2, 3].stream().toArray().class.name == '[Ljava.lang.Object;'
assert [1, 2, 3].stream().toArray(Integer[]::new).class.name == '[Ljava.lang.Integer;'

// works with multi-dimensional arrays too
def make2d = String[][]::new
def tictac = make2d(3, 3)
tictac[0] = ['X', 'O', 'X']
tictac[1] = ['X', 'X', 'O']
tictac[2] = ['O', 'X', 'O']
assert tictac*.join().join('\n') == '''
XOX
XXO
OXO
'''.trim()

// also useful for your own classes
import groovy.transform.Canonical
import java.util.stream.Collectors

@Canonical
class Animal {
    String kind
}

def a = Animal::new
assert a('lion').kind == 'lion'

def c = Animal
assert c::new('cat').kind == 'cat'

def pets = ['cat', 'dog'].stream().map(Animal::new)
def names = pets.map(Animal::toString).collect(Collectors.joining( "," ))
assert names == 'Animal(cat),Animal(dog)'

!in and !instanceof operators

When wanting the negated form, rather than having to bracket expressions containing the in and instanceof infix operators and placing the exclamation operator in front of the brackets, an inline variant is now also supported. Examples:

/* assert !(45 instanceof Date) // old form */
assert 45 !instanceof Date

assert 4 !in [1, 3, 5, 7]

Elvis assignment operator

Groovy introduced the Elvis operator Example:

import groovy.transform.ToString

@ToString
class Element {
    String name
    int atomicNumber
}

def he = new Element(name: 'Helium')
he.with {
    name = name ?: 'Hydrogen'   // existing Elvis operator
    atomicNumber ?= 2           // new Elvis assignment shorthand
}
assert he.toString() == 'Element(Helium, 2)'

Identity comparison operators

Both === and !== are supported which are the same as calling the is() method.

import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode
class Creature { String type }

def cat = new Creature(type: 'cat')
def copyCat = cat
def lion = new Creature(type: 'cat')

assert cat.equals(lion) // Java logical equality
assert cat == lion      // Groovy shorthand operator

assert cat.is(copyCat)  // Groovy identity
assert cat === copyCat  // operator shorthand
assert cat !== lion     // negated operator shorthand

Safe indexing

String[] array = ['a', 'b']
assert 'b' == array?[1]      // get using normal array index
array?[1] = 'c'              // set using normal array index
assert 'c' == array?[1]

array = null
assert null == array?[1]     // return null for all index values
array?[1] = 'c'              // quietly ignore attempt to set value
assert array == null

ARM Try with resources

Groovy often provides better alternatives to Java 7’s try-with-resources statement for Automatic Resource Management (ARM). That syntax is now supported for Java programmers migrating to Groovy and still wanting to use the old style:

class FromResource extends ByteArrayInputStream {
    boolean closed = false

    @Override
    void close() throws IOException {
        super.close()
        closed = true
    }

    FromResource(String input) {
        super(input.bytes)
    }
}

class ToResource extends ByteArrayOutputStream {
    boolean closed = false

    @Override
    void close() throws IOException {
        super.close()
        closed = true
    }
}

try(
        FromResource from = new FromResource("ARM was here!")
        ToResource to = new ToResource()
) {
    to << from
} finally {
    assert from.closed
    assert to.closed
    assert to.toString() == 'ARM was here!'
}

Nested code blocks

An infrequently used structure within Java is the anonymous code block. It’s generally not encouraged as it’s often a sign that refactoring the related code into a method is in order. But it’s sometimes useful to restrict scoping and is now available in Groovy:

{
    def a = 1
    a++
    assert 2 == a
}
try {
    a++ // not defined at this point
} catch(MissingPropertyException ex) {
    println ex.message
}
{
    {
        // inner nesting is another scope
        def a = 'banana'
        assert a.size() == 6
    }
    def a = 1
    assert a == 1
}

Be aware though that in Groovy having a code block looking structure after any method call will be seen as an attempt to pass a closure as the last parameter in the method call. This happens even after a new line. So it’s safe to start an anonymous code block after any other block (e.g. an if-then-else statement or another anonymous code block). Anywhere else and you might need to terminate the previous statement with a semicolon. In which case, see the note above about refactoring your code! :-)

Interface default methods

Java 8 supports adding default implementations to interfaces. Groovy now supports this too:

interface Greetable {
    String target()

    default String salutation() {
        'Greetings'
    }

    default String greet() {
        "${salutation()}, ${target()}"
    }
}

class Greetee implements Greetable {
    String name
    @Override
    String target() { name }
}

def daniel = new Greetee(name: 'Daniel')
assert 'Greetings, Daniel' == "${daniel.salutation()}, ${daniel.target()}"
assert 'Greetings, Daniel' == daniel.greet()
Note
Experimental Status: While this feature will remain, we regard its current implementation as experimental. Currently, interface default methods are implemented using traits. This means that they work even when using JDK7 (i.e. using Groovy 2.6 with Parrot parser enabled on a version 7 JRE). We are currently exploring supporting native default methods in interfaces instead of, or in addition to, the trait implementation. If we change the implementation, we would expect no change in logical behavior though the new implementation may have different performance characteristics in some circumstances and the native implementation would only work on JDK8+. Our goal is to clarify this aspect of the language before reaching release candidate status.

Miscellaneous improvements

Embedded GroovyDoc

You can now embed Groovydoc in various ways.

It can be attached to the AST as node metadata and available during compilation for use by for instance AST transformations or other tools. This is enabled using the groovy.attach.groovydoc system property.

It can also be embedded into the class file via an annotation and available at runtime for access via reflection or via other tools. This is enabled using the 'groovy.attach.runtime.groovydoc' system property to globally enable the attachment of all Groovydoc or by using a special @Groovydoc tag/annotation at the start of any Groovydoc comment. You normally don’t deal with the annotation directly - the compiler handles proper placement of the annotation and an extension method allows easy access to the Groovydoc comment value. Here is a typical example:

/** fee fi */
class Foo {
    /** @Groovydoc fo fum */
    def bar() { }
}
// next line assumes groovy.attach.runtime.groovydoc system property is true
assert Foo.class.groovydoc.content.contains('fee fi')
def bar = Foo.methods.find{ it.name == 'bar' }
// next line always true due to explicit @Groovydoc
assert bar.groovydoc.content.contains('@Groovydoc fo fum')

JSR308 improvements (work in progress)

Groovy has been improving JSR-308 support over recent versions. As part of implementing the new grammar, additional support has been added.

JDK requirements

Groovy 2.6 requires JDK8+ to build and JDK7 is the minimum version of the JRE that we support.

More information

You can browse all the tickets closed for Groovy 2.6 in JIRA.