JUnit is an open source regression-testing framework for Java: see http://www.junit.org. JUnit is more of a testing tool than a troubleshooting tool, but if you are developing an application of any complexity, it could be useful to create a suite of JUnit tests to quickly verify your Java code. For example, you could write a JUnit test for each ADF BC view object, to instantiate the view object and execute its query.
JDeveloper provides two JUnit extensions: JUnit integration and JUnit integration for business components. You can install these extensions from Tools Preferences Extensions; click Check for Updates.