Using the Delegating Meta Class

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 MetaClassClass delegateaclass)
    {
        super(delegateaclass);
        initialize()
    }

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

package groovy.runtime.metaclass.java.lang

class StringMetaClass extends groovy.lang.DelegatingMetaClass
{
    StringMetaClass(MetaClassRegistry a_registry, final Class a_classMetaClass delegate)
    {
        super(a_classdelegate);
    }

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