Multithreaded code and concurrency

Java code can be difficult to test for thread safety when multithreading.

The article at http://www.planetgeek.ch/2009/08/25/how-to-find-a-concurrency-bug-with-java/ describes a method of exposing concurrency bugs that adds a new assertion method assertConcurrent.

To use this you pass in a Collection of Runnables that are your arrange\act\assert test on the SUT, they all run at the same time in the assertConcurrent method; the chances of triggering a multithreading code error, and thereby failing some assertion are greatly increased:

The assertConcurrent method is:

	public static void assertConcurrent(final String message, final List<? extends Runnable> runnables, final int maxTimeoutSeconds) throws InterruptedException {
	  final int numThreads = runnables.size();
	  final List<Throwable> exceptions = Collections.synchronizedList(new ArrayList<Throwable>());
	  final ExecutorService threadPool = Executors.newFixedThreadPool(numThreads);
	  try {
		final CountDownLatch allExecutorThreadsReady = new CountDownLatch(numThreads);
		final CountDownLatch afterInitBlocker = new CountDownLatch(1);
		final CountDownLatch allDone = new CountDownLatch(numThreads);
		for (final Runnable submittedTestRunnable : runnables) {
		  threadPool.submit(new Runnable() {
			public void run() {
			  allExecutorThreadsReady.countDown();
			  try {
				afterInitBlocker.await();
				submittedTestRunnable.run();
			  } catch (final Throwable e) {
				exceptions.add(e);
			  } finally {
				allDone.countDown();
			  }
			}
		  });
		}
		// wait until all threads are ready
		assertTrue("Timeout initializing threads! Perform long lasting initializations before passing runnables to assertConcurrent", allExecutorThreadsReady.await(runnables.size() * 10, TimeUnit.MILLISECONDS));
		// start all test runners
		afterInitBlocker.countDown();
		assertTrue(message +" timeout! More than" + maxTimeoutSeconds + "seconds", allDone.await(maxTimeoutSeconds, TimeUnit.SECONDS));
	  } finally {
		threadPool.shutdownNow();
	  }
	  assertTrue(message + "failed with exception(s)" + exceptions, exceptions.isEmpty());
	}