Mockito versus Spock
In this recipe, we will write a simple test using Spock that verifies the behavior of the system under test when an exception is thrown. Before going into details, it's worth mentioning that Spock is a Groovy-based (http://groovy.codehaus.org/) tool. Therefore, in order to use it, you need to know at least the basics of the Groovy language. Spock is based on JUnit and is much more than a mocking framework. It gives you a beautiful BDD (Behavior Driven Development) syntax that will convert your tests to Specifications (capital S since Specification is a class that you need to extend to work with Spock).
If you want to try out Spock without installing it on your machine, check out the Spock Web Console at http://meetspock.appspot.com/, where you can write your tests online!
Spock is a perfect tool for you if you want to introduce Groovy into your project. You can start off with writing tests and then gradually progress towards production code (if that is what you want...
