Question

How do I ensure in a JUnit test case, that all the the threads spawned directly/indirectly by the method under test are done with there job, so that I can assert the final result?

@Test
public void testMethod() {
 Result result=method();// may spawn multiple threads to set result.value

 Assert.assertTrue(result.getValue()==4); //should execute only after result.value is set to its final value.
}

Answer 1

The real question is, how do you deal with this case in your non-test code? What does the API of method() guarantee to its callers about when result.value will be set?

Bear that in mind strongly when writing tests – the purpose is to assert that the class and its methods behave as they advertise. Sometimes, working out what the advertised interface is can be half of the challenge.

In a situation like this, I would strongly recommend that your Result object behave like a Future, in that its get() method blocks until the result is available. (Alternatively, give it a waitFor() method or similar).

If your method doesn’t provide any specific guarantees or blocking calls, all you can really do in the test is to keep checking the value every x seconds in a loop, putting a @Timeout on the test to ensure that the value is set in a “reasonable” time. But this is all a client would be able to do too, so

1. it’s a very valid test;
2. it highlights that the interface isn’t very usable for clients and modifying it would be a nice idea.