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Using ExpandoMetaClass to add behaviour

Groovy 1.1 includes a special MetaClass called an ExpandoMetaClass that allows you to dynamically add methods, constructors, properties and static methods using a neat closure syntax.

How does it work? Well every java.lang.Class is supplied with a special "metaClass" property that when used will give you a reference to an ExpandoMetaClass instance.

For example given the Java class java.lang.String to obtain its ExpandoMetaClass you can use:

This adds a method called swapCase to the String class.


By default ExpandoMetaClass doesn't do inheritance. To enable this you must call ExpandoMetaClass.enableGlobally() before your app starts such as in the main method or servlet bootstrap

Further Reading:

  1. Adding or overriding instance methods
  2. Adding or overriding static methods
  3. Adding or overriding properties
  4. Adding or overriding constructors
  5. Adding methods on interfaces
  6. Dynamically creating method names
  7. Overriding invokeMethod, getProperty and setProperty
  8. Evaluating MetaClass instances at runtime
  9. Borrowing methods from other classes
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