This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
That should work. All Groovy classes can be treated exactly as Java classes.
Can you look in your output directory to ensure there is a Rule_rule1.class file in the tester package? If there is a class file there with the correct name, it doesn't matter whether Java or Groovy created it.
Of course, the Groovy jar must be present in the project classpath to load a Groovy class, but if you've got Groovy already in your project, it should be there.
Also, can you give more context to this issue and maybe a stacktrace? Tell us why you are loading a Groovy class from Java and maybe we can help you find a better way.