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. One feature of the invokeConstructor allows us to create groovy objects using a map argument to set the properties of the object.
This example overides 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".