Using the Proxy Meta Class

import org.codehaus.groovy.runtime.InvokerHelper

class ProxyMetaClassTest extends GroovyTestCase
{
    void testProxyMetaClass()
    {
        def proxy = ProxyMetaClass.getInstance(String.class);
        proxy.interceptor = new MyInterceptor()

                def text = "hello world"

        assertEquals "hello world", text.toString()

        proxy.use {
            assertEquals "changed hello world", text.toString()
        }

        assertEquals "hello world", text.toString()
    }
}

class MyInterceptor implements groovy.lang.Interceptor
{
    Object beforeInvoke(Object a_object, String a_methodName, Object[] a_arguments)
    {
    }

    boolean doInvoke()
    {
        return true
    }

    Object afterInvoke(Object a_object, String a_methodName, Object[] a_arguments, Object a_result)
    {
         return "changed ${a_result}"
    }
}