Why not JTest? I recommend listing the constraints you have in place. I'm guessing JTest is too expensive? (Nobody does it for free that I know of if that's your real question.)
It also depends on what you are trying to accomplish. If you want something that verifies the behavior of a legacy class doesn't change for regression purposes, you are better off with writing an integration or end to end test. Generated tests tend to throw basic data such as nulls at a method. Or they give you the option of suggesting the data yourself - in which case you are writing the test. Have you noticed I'm not a big fan of automatically generated unit test.
If you are just looking for something to generate the template of a unit test for you to fill in, Eclipse does that with the built in plugin.