How To Add A Groovy Method To A Java Class

2017-11-03

This aims to be a quick tutorial for those who want to add a Groovy method to an exisiting Java class, and call the method from Groovy. Calling from Java might be similiar. There are exisiting tutorials for similiar purposes but at the time I was unable to find anything tailored to my need. To reduce time needed for documentation searching, and experimenting (failing), I am reproducing my example here in a general way.

Say you have a Java class defined as follows:

import java.util.Date public class myJavaClass { private static String s = new String("foo"); private static String getDate() { final Date data = new Date(); return date.format("yyyyMMdd-HHmmss"); } }

Now if you want to add a Groovy method, in your Groovy script, do

import myJavaClass myJavaClass.metaClass.myGroovyMethod << { // for getAttribute(), the target attribute has to be static final String javaString = myJavaClass.metaClass.getAttribute(myJavaClass, "s") // getMetaMethod() is used instead for non-static methods final MetaMethod getDate = myJavaClass.metaClass.getStaticMetaMethod("getDate", null) final String time = getDate.invoke(delegate, null) println "The time is " + time // the attribute to set cannot be final myJavaClass.metaClass.setAttribute(myJavaClass.metaClass.getTheClass(), "s", time) }

Documentations for this are located here. Search for "Groovy metaprogramming" for related tutorials.