JUnit - should I assign null to resources in tearDown that were instantiated in setUp?

Question

I am reading one book about JUnit now and writer advises nulling resources in tearDown method. Why? Isn't this GC's job? Can it seriously make any harm?

Lets think of example like this:

public class SomeTest extends TestCase {
  Vector vector;
  List<Object> list;  

  protected void setUp() {
    vector = new Vector();
    list = new ArrayList<Object>();
  }

  // messing with resources
  // adding, deleting, testing whatever

  protected void tearDown() {
    vector = null;
    list = null;
  }
}

What do you think? Is that code in tearDown necessary?

Answer 1

Yes, this can indeed be necessary.

You see, JUnit will actually create a separate instance of the Test class for each test method, and the Junit3 test runner (not so with JUnit4) will keep these instances around until the entire test suite has finished.

Therefore, if your (JUnit3) test class has fields that take up a lot of memory, you can easily run out of heap space when you have a large number of test methods. Of course, if those collections in your example code only ever contain a handful of short strings, it doesn't matter.

Answer 2

It depends what you consider a resource. Whilst heap space is a resource, you can probably get away with the GC cleaning up after you (YMMV).

Things that might cause issues are Closables like database connections / open files and streams etc. which should always be closed after use to prevent nasties in long running code.

I once had a situation that an integration test for some hibernate code didn't cleanup properly and resulted in some really strange errors. It took many hours to find and angered me so badly that I'll never make the same mistake again.