Weird problem using JUnit in multi-thread environment

Question

I meet a weired problem when using JUnit in multi-thread environment. The following code should fail, but it actually pass in eclipse.

public class ExampleTest extends TestCase {

    private ExecutorService executor = Executors.newFixedThreadPool(10);

    private volatile boolean isDone = false;

    public void test() throws InterruptedException, ExecutionException {
        executor.submit(new Runnable() {

            @Override
            public void run() {
                try {
                    fail();
                } finally {
                    isDone = true;
                }
            }
        });

        while (!isDone) {
            Thread.sleep(1000);
        }
    }
}

And here'a another piece of code, here I use Future.get() to wait for thread stop, in this case it will fail.

public class ExampleTest extends TestCase {

    private ExecutorService executor = Executors.newFixedThreadPool(10);

    private volatile boolean isDone = false;

    public void test() throws InterruptedException, ExecutionException {
        Future future=executor.submit(new Runnable() {

            @Override
            public void run() {
                try {
                    fail();
                } finally {
                    isDone = true;
                }
            }
        });

        future.get();
    }
}

I googled it and found that JUnit can not handle Multiple-thread unit testing,but what's the differences between these two pieces of code ? Thanks

Answer 1

JUnit cannot see the exceptions that occur in threads other than the thread in which the tests are running. In the first case, through an exception occurs by calling fail, it occurs in a separate thread run by the executor. Hence it is not visible to JUnit and the test passes.

In the second case, the same exception happens in the separate thread run by the executor but the exception is effectively "reported back" to the test thread when you call future.get. This is because future.get throws an ExecutionException if the computation of the future failed due to any exception. JUnit is able to see this exception and hence the test fails.