I have a mock:
static MyGroovyBasedHttpClient createClient(){
MyGroovyBasedHttpClient client = mock(MyGroovyBasedHttpClient.class);
Answer
There is a problem with mockito and groovy, you need additional dependency or you can mock only interfaces.
For more info visit : https://github.com/cyrusinnovation/mockito-groovy-support
Issue 303 about mockito problem with groovy classes : https://code.google.com/p/mockito/issues/detail?id=303