For Griffon managed classes like controllers, models and so forth you can add methods, constructors etc. using the ExpandoMetaClass mechanism by accessing each controller's MetaClass:

    class ExampleAddon {
        def addonPostInit(app) {
            app.artifactManager.controllerClasses.each { controllerClass ->
                 controllerClass.metaClass.myNewMethod = {-> println "hello world" }
            }
        }
    }

In this case we use the `app.artifactManager` object to get a reference to all of the controller classes' MetaClass instances and then add a new method called `myNewMethod` to each controller. Alternatively, if you know before hand the class you wish add a method to you can simple reference that classes `metaClass` property:

    class ExampleAddon {
        def addonPostInit(app) {
            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()
            }        assert "UpAndDown" == "uPaNDdOWN".swapCase()
        }
    }

In this example we add a new method `swapCase` to `java.lang.String` directly by accessing its `metaClass`.