Why Groovy?

There are a lot goodies in Groovy language. Two examples:

MetaClass - http://blog.terrencemiao.com/archives/metaclass-in-groovy
MockFor mock object - http://blog.terrencemiao.com/archives/mock-object-return-non-sense-error-in-unit-test-in-grails

Remember in last project because Mr.B hated getter and setter that pollute codes in DTO (Data Transfer Object) all over the world, he used Java Reflection, finding out all properties in POJO class, getProperty and setProperty in POJO without having get and set codes one by one.

In Groovy, it's built in feature, and one line only:

metaClass.setProperties(this, map.findAll { key, value -> this.hasProperty(key) })

Also you don't need Mockito to mock objects in unit test. Groovy already have MockFor which supports (typically unit) testing of classes in isolation by allowing a strictly ordered expectation of the behavior of collaborators to be defined.

Save your time, with less and clean codes. That's you call "Advance Groovy language".