GroovyDuckPunching.groovy

 
class MyClass {
 
    void tweeter() {
        println "Tweet, tweet"
    }
}
 
def my_object = new MyClass()

def my_replacement_method = { println "Honk, honk" }

class ReplacingInterceptor implements Interceptor {

        ReplacingInterceptor(String aMethodName, Closure aReplacementMethod) {
            replacementMethod = aReplacementMethod
            methodName = aMethodName
            }

        final String methodName
        final Closure replacementMethod
    
        Object beforeInvoke(Object object, String methodName, Object[] arguments) {
            if(this.methodName.equals(methodName)) {
                replacementMethod.call(arguments)
                }
            return object
            }
        boolean doInvoke() { false }
        Object afterInvoke(Object object, String methodName, Object[] arguments, Object result) { result}
    }

proxy = ProxyMetaClass.getInstance(my_object.class)
   
proxy.interceptor = new ReplacingInterceptor('tweeter', my_replacement_method)
 
proxy.use {
    new MyClass().tweeter()
    my_object.tweeter()
    }
    
new MyClass().tweeter()