Groovy is very dynamic. We can add methods to classes at runtime that don't exist at compile time. We can add our own custom MetaClass at startup time of our application if we follow the magic package naming convention. The naming convention is groovy.runtime.metaclass.[package].[class]MetaClass. For example if we want to change the behavior of the java.lang.String class, then we must write a custom MetaClass with the package name groovy.runtime.metaclass.java.lang and class name StringMetaClass. We can do the same for classes we create ourselves. For example if we have a class myapp.RunApp than the custom metaclass implementation RunAppMetaClass would be in package groovy.runtime.metaclass.myapp.

// File: StringMetaClass.groovy
package groovy.runtime.metaclass.java.lang

class StringMetaClass extends DelegatingMetaClass {

    StringMetaClass(MetaClass meta) {
        super(meta)
    }

    Object invokeMethod(Object object, String method, Object[] arguments) {
        if (method == 'hasGroovy') {
            object ==~ /.*[Gg]roovy.*/
        } else {
            super.invokeMethod object, method, arguments
        }
    }
}

The code that will use the delegating metaclass implementation:

// File: StringDelegateSample.groovy

// Original methods are still invoked.
assert 'mrhaki'.toUpperCase() == 'MRHAKI'

// Invoke 'hasGroovy' method we added via the DelegatingMetaClass.
assert !'Java'.hasGroovy()
assert 'mrhaki loves Groovy'.hasGroovy()
assert 'Groovy'.toLowerCase().hasGroovy()

First we compile our StringMetaClass:

$ groovyc StringMetaClass.groovy

Next we can run the StringDelegateSample.groovy file if we put the generated class file in our classpath:

$ groovy -cp . StringDelegateSample

From http://mrhaki.blogspot.com/2011/11/groovy-goodness-magic-package-to-add.html