During an execution of a test suite TestNG keeps references to all executed test class instances. Some of our test classes keep references to instantiated caches, cache managers, and other heavy weight objects thus preventing those references from ever being garbage collected.

Our test classes should never keep references to heavyweight objects other than in a test method scope. If we must keep reference to a cache or a cache manager in a class scope these references should be cleared and nulled in a method annotated with @AfterClass.