Release notes for Groovy 5.0

Groovy 5 builds upon existing features of earlier versions of Groovy. In addition, it incorporates numerous new features and streamlines various legacy aspects of the Groovy codebase.

WARNING: Material on this page is still under development! We are currently working on alpha versions of Groovy 5.0 with a goal of gathering feedback on the language changes from our community. In addition, early versions assist other projects and tool vendors within the Groovy ecosystem to begin assessing the impact of moving to/supporting Groovy 5.0. Caution should be exercised if using new features as the details may change before final release.
We don’t recommend using alpha versions or incubating features for production systems. We don’t regard alpha versions as being feature-complete, so caution should be exercised before undertaking any large scale ports to Groovy 5. Having said that, we don’t expect porting to Groovy 5 from Groovy 4 should involve much effort.

New features

Support for var with multi-assignment

The var keyword can be used in combination with multi-assignment:

var (x, y) = [1, 2]
assert x == 1 && y == 2

Scripting alternate forms

JEP 445 (a preview feature for JDK 21) allows certain Java executable classes to have a shortened form, potentially leaving out the class declaration and having a simplified main method declaration. Such classes are still more verbose than Groovy scripts, but we have provided support for JEP 445 compatible classes in Groovy 5 to ease support for Java developers using Groovy and provide a few nice benefits to Groovy developers along the way.

Let’s recap the story so far. First, a traditional Java class:

public class HelloWorld {                       // Java
    public static void main(String[] args) {
        System.out.println("Hello, World!");

Next, the Groovy equivalent:

println 'Hello, World!'

What is being proposed for JEP 445 in Java:

void main() {                                   // Java future
    System.out.println("Hello, World!");

What Groovy will also support:

void main() {
    println 'Hello, World!'

Obviously, this is longer than the traditional Groovy one-liner script but has the advantage that we can place annotations on the main method. TYPE targeted annotations on the main method will be moved to the generated script class. METHOD targeted annotations remain on the method.

Classes created with an instance main method (like above) are JEP 445 compatible classes and will need to be invoked from the JDK with JEP 445 capability enabled (currently requires enabling preview in JDK21) or using the Groovy runner which now supports such classes from JDK11+.

For backwards compatibility, classes created with a static main method are promoted to have the normal public static void main signature. They can be run like normal Java or Groovy classes.

There are variants in Java and Groovy to also have arguments available:

static main(args) {
    println 'Groovy world!'

JEP 445 compatible classes may also contain other field and method definitions as shown here:

def main() {
    assert upper(foo) + lower(bar) == 'FOObar'

def upper(s) { s.toUpperCase() }

def lower = String::toLowerCase
def (foo, bar) = ['Foo', 'Bar']

But they can’t contain any other "uncontained" statements, otherwise they are treated like a normal Groovy script. Another important distinction for JEP 445 compatible classes is that fields (like lower, foo, and bar in the above example) don’t need to be annotated with @Field.

An additional form is also supported which involves overwriting the run method in a script. This provides an alternate form to the earlier shown main variants. The difference is that rather than producing a JEP 445 compatible class, Groovy produces a script class which extends the Script class in the normal way and has access to the normal script binding and context. The use case is again where you might want to supply annotations, e.g.:

def run() {
    var mapper = new ObjectMapper()
    assert mapper.writeValueAsString(this) == '{"pets":["cat","dog"]}'

public pets = ['cat', 'dog']

An Optional Type Checker for Format Strings

The format methods in java.util.Formatter, and other similar methods, support formatted printing in the style of C’s printf method with a format string and zero or more arguments.

Let’s consider an example which produces a string comprised of three terms:

  • a floating-point representation (%f) of PI (with 2 decimal places of precision),

  • the hex representation (%X) of 15 (as two uppercase digits with a leading 0),

  • and the Boolean (%B) True (in uppercase).

The assertion checks our expectations:

assert String.format('%4.2f %02X %B', Math.PI, 15, true) == '3.14 0F TRUE'

This is a powerful method supporting numerous conversions and flags. If the developer supplies incorrect conversions or flags, they will receive one of numerous possible runtime errors. As examples, consider the following mistakes and resulting runtime exceptions:

  • supplying a String as the parameter for either of the first two arguments results in an IllegalFormatConversionException,

  • leaving out the last argument results in a MissingFormatArgumentException,

  • supplying the leading zero flag for the Boolean parameter results in a FlagsConversionMismatchException.

The goal of the FormatStringChecker is to eliminate a large number of such runtime errors. If the API call passes type checking, it will be guaranteed to succeed at runtime.

AST transform additions and improvements

There is a new OperatorRename AST transform. This is very useful when using third-party libraries which use different names to those used by Groovy’s operator overloading functionality. For example, using the Apache Commons Numbers Fraction library:

def testAddOfTwoFractions() {
    var half = Fraction.of(1, 2)
    var third = Fraction.of(1, 3)
    assert half.add(third) == Fraction.of(5, 6)  // old style still works
    assert half + third == Fraction.of(5, 6)     // fraction '+' operator!

This transform is quite handy when using various matrix packages. Such packages often align with Groovy’s operator overloading conventions for many, but usually not all, operators. For instance, you might like to rename:

  • mult to multiply if using Ejml

  • add to plus if using Commons Math matrices

  • sub to minus if using Nd4j matrices

Extension method additions and improvements

Additional primitive array extensions

There are over 160 additional extension methods on primitive arrays. Methods like any, chop, each, eachWithIndex, equals, every, first, head, init, join, last, max, maxBy, maxComparing, min, minBy, minComparing, reverse, reverseEach, tail, and transpose now have variants for primitive arrays like int[], long[], double[], etc. And methods like flatten are added for int[][],long[][], etc.

Some examples:

int[] nums = -3..2
assert nums.any{ it > 1 }
       && nums.every(n -> n < 4)
       && nums.join(' ') == '-3 -2 -1 0 1 2'
       && nums.head() == -3
       && nums.tail() == -2..2
       && nums.max() == 2
       && nums.min{ it.abs() } == 0
       && nums.maxComparing(Comparator.reverseOrder()) == -3
       && nums.reverse() == 2..-3

int[][] matrix = [[1,   2],
                  [10,  20],
                  [100, 200]]
assert matrix.transpose() == [[1, 10, 100],
                              [2, 20, 200]]

In some cases, the methods existed for a few of the primitive types but now work with more primitive types. In numerous cases, the functionality was only available by converting the array to a list first - which was easy but increased memory usage and decreased performance. For other cases, implementations now avoid un/boxing where possible. All up this means that Groovy now works better in data science scenarios allowing more streamlined and performant code.

Additional File and Path extensions

There are some additional extension methods for File objects:

def myscript = new File('MyScript.groovy')
assert myscript     // Groovy truth: true if the file exists
assert myscript.extension == 'groovy'
assert myscript.baseName == 'MyScript'

And similar methods for Path objects:

def mypic = path.resolve('MyFigure.png')
assert mypic       // Groovy truth: true if the file exists
assert mypic.extension == 'png'
assert mypic.baseName == 'MyFigure'

Additional Collection extensions

We have added a flattenMany method which is a close cousin to the collectMany method. These are Groovy’s flatMap like methods.

var items = ["1", "2", "foo", "3", "bar"]
var toInt = s -> s.number ? Optional.of(s.toInteger()) : Optional.empty()
assert items.flattenMany(toInt) == [1, 2, 3]
assert items.flattenMany(String::toList) == ['1', '2', 'f', 'o', 'o', '3', 'b', 'a', 'r']
assert items.flattenMany{ it.split(/[aeiou]/) } == ['1', '2', 'f', '3', 'b', 'r']

assert ['01/02/99', '12/12/23'].flattenMany{ it.split('/') } ==
    ['01', '02', '99', '12', '12', '23']

If you are working solely with collections, using collectMany will offer improved type inference with type checked code. If you also want to flat other things like arrays and optionals, flattenMany gives some added flexibility.

There are additional variants of collectEntries for arrays, iterables and iterators with separate functions for transforming the keys and values. There are variants with and without collectors. There are also variants which transform just the key or value. The withCollectedKeys method collects key/value pairs for each item with the item as the value and the key being the item transformed by the supplied function. The withCollectedValues method collects key/value pairs for each item with the item as the key and the value being the item transformed by the supplied function.

def languages = ['Groovy', 'Java', 'Kotlin', 'Scala']

def collector = [clojure:7]
assert languages.collectEntries(collector, String::toLowerCase, String::size) ==
    [clojure:7, groovy:6, java:4, kotlin:6, scala:5]
assert languages.withCollectedKeys(s -> s.take(1)) ==
    [G:'Groovy', J:'Java', K:'Kotlin', S:'Scala']
assert languages.withCollectedValues(s -> s.size()) ==
    [Groovy:6, Java:4, Kotlin:6, Scala:5]

There are also equivalent variants for maps. The collectEntries method takes separate functions for transforming the keys and values. The collectKeys and collectValues variants take a single function for transforming just the keys and values respectively.

def lengths = [Groovy:6, Java:4, Kotlin:6, Scala:5]

assert lengths.collectEntries(String::toLowerCase, { it ** 2 }) ==
    [groovy:36, java:16, kotlin:36, scala:25]
assert lengths.collectKeys{ it[0] } == [G:6, J:4, K:6, S:5]
assert lengths.collectValues(Math.&pow.rcurry(2)) ==
    [Groovy:36.0, Java:16.0, Kotlin:36.0, Scala:25.0]
assert lengths.collectValues(Math.&pow.curry(2).memoize()) ==
    [Groovy:64.0, Java:16.0, Kotlin:64.0, Scala:32.0]

There are a number of new extensions for Sets including operator overload variants:

var a = [1, 2, 3] as Set
var b = [2, 3, 4] as Set
assert a.union(b) == [1, 2, 3, 4] as Set
assert a.intersect(b) == [2, 3] as Set
assert (a | b) == [1, 2, 3, 4] as Set
assert (a & b) == [2, 3] as Set
assert (a ^ b) == [1, 4] as Set

Set d = ['a', 'B', 'c']
Set e = ['A', 'b', 'D']
assert d.and(e, String.CASE_INSENSITIVE_ORDER) == ['a', 'B'] as Set
assert e.union(d, String.CASE_INSENSITIVE_ORDER) == ['A', 'b', 'D', 'c'] as Set

Checked collections

Java, being statically typed, tries hard to ensure type safety at compile time but provides some flexibility to work with objects whose type can only be checked at runtime. Because of type erasure, Java’s runtime checking is curtailed to some degree. It is not unusual for errant programs to fail, e.g. with a ClassCastException. The issue is the failure may occur a long way from the part of the code which caused the problem. The CheckedXXX classes within java.util.Collections provide a way to improve type safety and find such issues at the origin of the problem. When debugging errant code, one recommendation is to wrap your collections with the checked classes. Once the error is found and fixed, remove the wrapping code for better performance.

Code using Groovy’s dynamic nature can be even more lenient than Java code, so Groovy can benefit from these classes even more than Java, so we made them easy to use with an asChecked method added for the common collection types.

// assume type checking turned off
List<String> names = ['john', 'pete']
names << 'mary' // ok
names << 35     // danger! but unnoticed at this point
println names*.toUpperCase() // fails here

In this example, we could turn type checking on for immediate feedback but in general we might be using a library with less information available due to type erasure. In any case, we can use asChecked to fail early:

// assume type checking turned off
List<String> names = ['john', 'pete'].asChecked(String)
names << 'mary' // ok
names << 35     // boom! fails early

Other improvements

Underscore as a placeholder

The use of "_" (underscore) as a placeholder for unused parameters is earmarked for inclusion in future Java versions (see "Treatment of underscores" in JEP 302: Lambda Leftovers). This is available in Groovy 5. Some examples:

// unused components in multi-assignment
var (_, y, m, _, _, d) = Calendar.instance
println "Today is $y-${m+1}-$d" // Today is 2023-8-23

// unused lambda parameters
def c = (_, _, a, b) -> a + b
def d = (_, a, _, b) -> a + b
def e = (_, a, b, _) -> a + b
assert c(1000, 100, 10, 1) == 11
assert d(1000, 100, 10, 1) == 101
assert e(1000, 100, 10, 1) == 110

// unused closure parameters
def f = { a, _, _, b -> a + b }
def g = { a, _, b, _ -> a + b }
def h = { a, b, _, _ -> a + b }
assert f(1000, 100, 10, 1) == 1001
assert g(1000, 100, 10, 1) == 1010
assert h(1000, 100, 10, 1) == 1100

Miscellaneous enhancements

There is now a utility method to produce simple ascii-art barcharts. The following code:

['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'].each {
    println "\n${it.padRight(12)}${bar(it.size(), 0, 10, 10)}"

produces this image:


Ongoing work

Enhanced switch (under investigation)

Groovy has always had a very powerful switch statement. The statement could be made more powerful, e.g. support destructuring, and could be supported in contexts where expressions are expected.

As inspiration, Java has made, or is investigating future enhancements including switch expressions and other related enhancements: JEP 354: Switch Expressions (Second Preview) JEP 361: Switch Expressions JEP 405: Record Patterns & Array Patterns (Preview) JEP 406: Pattern Matching for switch (Preview) We should investigate these proposals both in terms of enhancing the existing Groovy switch but also in terms of deciding which syntax from Java we might like to support in the future.

Other languages like Python are also improving their switch statements: PEP 622 — Structural Pattern Matching. We should investigate whether any features of their design make sense for Groovy’s dynamic nature.

As an example of destructuring, instead of the following existing code:

def make3D(pt) {
    switch(pt) {
        case Point3D:
            return pt
        case Point2D:
            return new Point3D(pt.x, pt.y, 0)
        case List:
            def (x, y, z) = pt
            if (x == 0 && y == 0 && z == 0)
                throw new IllegalArgumentException("Origin not allowed")
            return new Point3D(x, y, z)

You could potentially use something like:

def make3D(pt) {
    switch(pt) {
        case Point3D:
            return pt
        case Point2D(x, y):
            return new Point3D(x, y, 0)
        case [0, 0, 0]:
            throw new IllegalArgumentException("Origin not allowed")
        case [x, y, z]:
            return new Point3D(x, y, z)

An example of guarded patterns being considered for Java:

static void testTriangle(Shape s) {
    switch (s) {
        case null ->
        case Triangle t when t.calculateArea() > 100 ->
            System.out.println("Large triangle");
        case Triangle t ->
            System.out.println("Small triangle");
        default ->

Another destructuring example:

int eval(Expr n) {
     return switch(n) {
         case IntExpr(int i) -> i;
         case NegExpr(Expr n) -> -eval(n);
         case AddExpr(Expr left, Expr right) -> eval(left) + eval(right);
         case MulExpr(Expr left, Expr right) -> eval(left) * eval(right);
         default -> throw new IllegalStateException();

We should consider the currently proposed nested record pattern when exploring our destructuring options, e.g.:

static void printColorOfUpperLeftPoint(Rectangle r) {
    if (r instanceof Rectangle(ColoredPoint(Point p, Color c), ColoredPoint lr)) {

Other Java-inspired enhancements

  • Module definitions written in Groovy (i.e. module-info.groovy) GROOVY-9273

Other breaking changes

  • Scripts containing a static main method and no statements outside that method have changed slightly for improved JEP 445 compatibility. The script class for such methods no longer extends Script and hence no longer has access to the script context or bindings. For many such scripts, access to the binding isn’t needed and there is now a simpler structure for those scripts. Scripts which need access to the binding should instead use a no-arg instance run method. (GROOVY-11118)

  • The getProperty method allows for getting properties that don’t exist within a class. Previously, static properties from an outer class were given priority over overrides by getProperty. This is in conflict with the priority given to outer classes in other places. (GROOVY-10985)

  • The minus operator for sets in Groovy was subject to an existing JDK bug in the JDK’s AbstractSet#removeAll method. The behavior now conforms with the behavior of the fix being proposed for that bug. If for some strange reason you rely on the buggy behavior, you can use the removeAll method directly rather than the minus operator (at least until it is fixed in the JDK). (GROOVY-10964)

  • Groovy 4 had a $getLookup method used to work around stricter JPMS access requirements. Groovy no longer needs this hook. This method is not normally visible or of use to typical Groovy users but if framework writers are making use of that hook, they should rework their code. (GROOVY-10931)

  • Groovy was incorrectly setting a null default value for annotations without a default value. If framework writers have made use of, or coded around the buggy behavior, they may need to rework their code. It might mean simplification by removing a workaround. (GROOVY-10862)

  • Some Groovy AST transform annotations, like @ToString were given RUNTIME retention even though Groovy itself and typical Groovy user behavior never needs access to that annotation at runtime. This was done with a view that perhaps some future tools or framework might be able to use that information in some useful way. We know of no such frameworks or tools, so we have changed the retention to SOURCE to give cleaner class files. (GROOVY-10862)

  • Groovy’s % operator is called the "remainder" operator. Informally, it is also known as the "mod" operator and indeed, for operator overloading purposes we have historically used the mod method. While this name is in part just a convention, it can cause some confusion, since for example, the BigInteger class has both remainder and mod methods and our behavior, like Java’s, follows the behavior of the remainder method. In Groovy 5, operator overloading for % is now handled by the remainder method. Fallback behavior is supported and workarounds exist for folks already using the mod method. (GROOVY-10800)

  • Improvements have been made to better align how method selection is performed between the dynamic Groovy runtime and with static compilation. (GROOVY-8788)

  • In earlier versions of Groovy, the compiler was lenient when finding duplicate imports or an import and a similarly-named class definition. While having duplicates was considered poor style, the compiler followed the lenient behavior of letting the last definition "win", ignoring earlier definitions. E.g. for two imports (Groovy 1-4):

    import java.util.Date
    import java.sql.Date
    println Date // => class java.sql.Date

    or an import and a class definition (Groovy 1-4):

    import java.sql.Date
    class Date { }
    println Date // => class Date

    or a regular import and an alias import (Groovy 1-4):

    import java.util.Date
    import java.util.Calendar as Date // don't do this!
    println Date // => class java.util.Calendar

    From Groovy 5, the compiler now follows Java behavior and gives an error in such cases (GROOVY-8254). A slightly more lenient approach is taken when using groovysh. For the groovysh repl, a newly entered import is deemed to override an old import with the same simple name, with the old import being discarded (GROOVY-11224).

  • Improvements have been made to improve consistency when accessing fields within Map-like classes. (GROOVY-6144, GROOVY-5001)

JDK requirements

Groovy 5 requires JDK16+ to build and JDK11 is the minimum version of the JRE that we support. Groovy 5 has been tested on JDK versions 11 through 20.

More information

You can browse all the tickets closed for Groovy 5.0 in JIRA.