Thread Weaver is a framework for writing multithreaded unit tests for Java classes. It lets you create breakpoints inside your code and to stop individual threads when they reach a breakpoint. This allows you to produce controlled repeatable tests that can help to verify thread safety and detect race conditions.
PIT is a fast bytecode based mutation testing system for Java that makes it possible to test the effectiveness of your unit tests. You can think of mutation testing as either as an automated test of your tests or as a much more in-depth form of code coverage. Unlike traditional line and branch coverage tools, PIT does not just confirm that your tests execute your code, it confirms that your tests are actually able to detect faults in it.