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This is an example of how to replace a MetaClass to adjust the default behavior. Each groovy object has a metaClass that is used to manage the dynamic nature of the language. This class intercepts calls to groovy objects to ensure that the appropriate grooviness can be added. For example, when an object is constructed, the MetaClass's inovkeConstructor()is called.  One feature of the invokeConstructor allows us to create groovy objects using a map argument to set the properties of the object (new X([prop1: value1, prop2: value2])).

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This technique installs the meta class at runtime using the InvokerHelper to gain access to the registry which allows us to change the meta class instance that is in use. Note that this is "at runtime" so instances created or used The behaviour for objects created before the change are also impacted. You will have to answer for yourself whether this is a good idea for your particular problemdepends on whether they are Java or Groovy objects.  Groovy objects "cache" their metaclass, so once they're created, changing the registry has no effect.  However, Java objects don't have a cache, so accesses to them always start with a trip to the registry, meaning any registry changes affect existing and new instances alike.

This sample code overrides the invokeMethod method to augment the behavior but there are other options that you can choose from like set and getAttribute, invokeStaticMethod and invokeConstructor. The complete list can be found in the Groovy's source release in "src/main/groovy/lang/DelegatingMetaClass.java".

Code Block
import org.codehaus.groovy.runtime.InvokerHelper

class DelegatingMetaClassInvokeHelperTest extends GroovyTestCase
{
    void testReplaceMetaClass()
    {
        /*
         * Constructing first instance before meta class replacment
         * is made.
         */
        def firstInstance = "first"
        assertEquals "first", firstInstance.toString()

        def myMetaClass = new MyDelegatingMetaClass(String.class)
        def invoker = InvokerHelper.instance
        invoker.metaRegistry.setMetaClass(String.class, myMetaClass)

        /*
         * Constructing second instance after meta class replacment
         * is made.
         */
        def secondInstance = "second"

        /*
         * Since we are replacing a meta class at the class level
         * we are changing the behavior of the first and second
         * instance of the string.
         */
        assertEquals "changed first", firstInstance.toString()
        assertEquals "changed second", secondInstance.toString()
    }
}

class MyDelegatingMetaClass extends groovy.lang.DelegatingMetaClass
{
    MyDelegatingMetaClass(final Class aclass)
    {
        super(aclass);
        initialize()
    }

    public Object invokeMethod(Object a_object, String a_methodName, Object[] a_arguments)
    {
        return "changed ${super.invokeMethod(a_object, a_methodName, a_arguments)}"
    }
}

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