Gist.java

@Test
public void shouldRemoveThreadsAfterCoreThreadTimeoutPeriod() throws Throwable {
    CountDownLatch untilAllThreadsDone = startaBunchOfThreads();

    untilAllThreadsDone.await();

    waitUntilTimeoutOccurs();

    assertEquals(countExecutorThreads(), 0);
}

private CountDownLatch startaBunchOfThreads() {
    final CountDownLatch allThreadsDoneWaiter = new CountDownLatch(threadPoolSize);
    for (int i = 0; i < threadPoolSize; i++) {
        threadPoolExecutor.submit(new Runnable() {
            @Override
            public void run() {
                allThreadsDoneWaiter.countDown();
            }
        });
    }
    return allThreadsDoneWaiter;
}

private void waitUntilTimeoutOccurs() throws InterruptedException {
    Thread.sleep(timeoutMillis * 2);
}