Cédric Champeau's blog: Coding a Groovy closure in pure Java

19 April 2010

Today I just wanted to share a simple trick that’s not well documented in Groovy : in a project which mixes Java and Groovy code, I’ve faced a problem where I wanted to use a Groovy method in Java which took a Closure as a parameter. Here’s how to do it. Actually, it’s quite simple :

Groovy

class Operation {
    def op(list, action) {
        list.each { action(it) }
    }
}

Java

public class MyJavaClass {
    private Operation groovy = new Operation();

    private void shouldBeUsedAsClosure(Object o) {
        System.out.println(o);
    }

    public void demonstrateClosureUsage() {
        // creates a closure which delegates calls to this Java object
        MethodClosure cl = new MethodClosure(this, "shouldBeUsedAsClosure");
        groovy.op(Arrays.asList("hello", "world"), cl);
    }

    public static void main(String... args) {
         MyJavaClass java = new MyJavaClass();
        java.demonstrateClosureUsage();
    }
}

This example is straighforward, however, this allows you to write your custom ``closures'' directly in Java. This is somehow interesting whenever performance matters and that critical portions of code need to be coded in Java.

Note that you can have multiple methods with the same name, Groovy will automatically dispatch the call to the appropriate delegate :

public class MyJavaClass {
    private Operation groovy = new Operation();

    private void shouldBeUsedAsClosure(String str) {
        System.out.println(str.toUpperCase());
    }

    private void shouldBeUsedAsClosure(int x) {
        System.out.println(x*x);
    }

    public void demonstrateClosureUsage() {
        // creates a closure which delegates calls to this Java object
        MethodClosure cl = new MethodClosure(this, "shouldBeUsedAsClosure");
        groovy.op(Arrays.asList("hello", "world","this","should","print","4 : ",2), cl);
    }

    public static void main(String... args) {
         MyJavaClass java = new MyJavaClass();
        java.demonstrateClosureUsage();
    }
}

This is an elegant way to avoid multiple instanceOf tests to check the type of the closure arguments. Last but not least, Groovy also provides a way to call static methods as closures, passing the class instead of an instance as the owner parameter :

   MethodClosure cl = new MethodClosure(MyJavaClass.class, "staticMethod");