Grails Unit Testing Issues

I am having issues while testing my grails controllers, as it depends on one service which seems not to be injected. I tried several ways (for ex. Extending classess like grailsunitestcase, specification) but I keep getting errors. The thing is that that service variable is null and I cant test my controller index method (which calls a render view) due to the exception... I really need to know how to do this but I don't have a clue where to start...