How to measure Testability of Java Classes? | Metrics [on hold]

I have been googling, but can't find anything specific. Let's say I have classes already written in an Spring Webproject.

Now I want to implement JUnit-tests, in order to maybe do some refactoring. So the thing is, of course you would first want to check, how good the class, you want to test is implemented for testability. I heard, that there are some metrics, where you can measure, how good the code is for testing.

But unfortunately I don't seem to find anything specific. Is there anything, which is suitable for Java-classes, to maybe do a complete project-analysis?