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:

String.metaClass.swapCase = {->
      def sb = new StringBuffer()
      delegate.each {
           sb << (Character.isUpperCase(it as char) ? Character.toLowerCase(it as char) : 
                   Character.toUpperCase(it as char))
      }
      sb.toString()
}

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:

• ExpandoMetaClass - Borrowing Methods — Borrowing methods from other classes
• ExpandoMetaClass - Constructors — Adding or overriding constructors
• ExpandoMetaClass - Dynamic Method Names — Dynamically creating method names
• ExpandoMetaClass - GroovyObject Methods — Overriding invokeMethod, getProperty and setProperty
• ExpandoMetaClass - Interfaces — Adding methods on interfaces
• ExpandoMetaClass - Methods — Adding or overriding instance methods
• ExpandoMetaClass - Overriding static invokeMethod — Overriding invokeMethod for static methods
• ExpandoMetaClass - Properties — Adding or overriding properties
• ExpandoMetaClass - Runtime Discovery — Overriding invokeMethod for static methods
• ExpandoMetaClass - Static Methods — Adding or overriding static methods