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Normally when you add a MetaMethod, it is added for all instances of that class. However, for GroovyObjects, you can dynamically add methods to individual instances by giving that instance its own MetaClass:.

titleFor Groovy version 1.6 and higher

See "What's New in Groovy 1.6"

Code Block
def str = "hello"
str.metaClass.test = { "test" }
assert "test" == str.test()
titleFor Groovy prior to version 1.6

Code Block

def test = "test"
def gstr = "hello $test"     // this is a GString, which implements GroovyObject

def emc = new ExpandoMetaClass( gstr.class, false )
emc.test = { println "test" }

gstr.metaClass = emc
gstr.test()                  // prints "test"