A few weeks ago I wrote a blog on synchronizing multithreaded integration tests, which was republished on DZone Javalobby from where it received a comment from Robert Saulnier who quite rightly pointed out that you can also use join() to synchronize a worker thread and its unit tests. This got me thinking, just how many ways can you synchronise multi-threaded integration tests? So, I started counting... and came up with: Using a random delay. Adding a CountDownLatch Thread.join() Acquiring a Semaphore With a Future and ExecutorService Now, I’m not going to explain all the following in great detail, I’ll let the code speak for itself, except to say that all the code samples do roughly the same thing: the unit test creates a ThreadWrapper instance and then calls its doWork() method (or call() in the case of the Future). The unit test’s main thread then waits for the worker thread to complete before asserting that the test has passed. For the sample code demonstrating points 1 and 2 take a look at my original blog on Synchronizing Multithreaded Integration Tests, though I wouldn’t recommend point 1: using a random delay. Thread.join() public class ThreadWrapper { private Thread thread; /** * Start the thread running so that it does some work. */ public void doWork() { thread = new Thread() { /** * Run method adding data to a fictitious database */ @Override public void run() { System.out.println("Start of the thread"); addDataToDB(); System.out.println("End of the thread method"); } private void addDataToDB() { try { Thread.sleep(4000); } catch (InterruptedException e) { e.printStackTrace(); } } }; thread.start(); System.out.println("Off and running..."); } /** * Synchronization method. */ public void join() { try { thread.join(); } catch (InterruptedException ex) { Thread.currentThread().interrupt(); } } } public class ThreadWrapperTest { @Test public void testDoWork() throws InterruptedException { ThreadWrapper instance = new ThreadWrapper(); instance.doWork(); instance.join(); boolean result = getResultFromDatabase(); assertTrue(result); } /** * Dummy database method - just return true */ private boolean getResultFromDatabase() { return true; } } Acquiring a Semaphore public class ThreadWrapper { /** * Start the thread running so that it does some work. */ public void doWork() { doWork(null); } @VisibleForTesting void doWork(final Semaphore semaphore) { Thread thread = new Thread() { /** * Run method adding data to a fictitious database */ @Override public void run() { System.out.println("Start of the thread"); addDataToDB(); System.out.println("End of the thread method"); semaphore.release(); } private void addDataToDB() { try { Thread.sleep(4000); } catch (InterruptedException e) { e.printStackTrace(); } } }; aquire(semaphore); thread.start(); System.out.println("Off and running..."); } private void aquire(Semaphore semaphore) { try { semaphore.acquire(); } catch (InterruptedException e) { e.printStackTrace(); } } } public class ThreadWrapperTest { @Test public void testDoWork() throws InterruptedException { ThreadWrapper instance = new ThreadWrapper(); Semaphore semaphore = new Semaphore(1); instance.doWork(semaphore); semaphore.acquire(); boolean result = getResultFromDatabase(); assertTrue(result); } /** * Dummy database method - just return true */ private boolean getResultFromDatabase() { return true; } } With a Future public class ThreadWrapper implements Callable { @Override public Boolean call() throws Exception { System.out.println("Start of the thread"); Boolean added = addDataToDB(); System.out.println("End of the thread method"); return added; } /** * Add to the DB and return true if added okay */ private Boolean addDataToDB() { try { Thread.sleep(4000); } catch (InterruptedException e) { e.printStackTrace(); } return Boolean.valueOf(true); } } public class ThreadWrapperTest { @Test public void testCall() throws ExecutionException, InterruptedException { ThreadWrapper instance = new ThreadWrapper(); ExecutorService executorService = Executors.newFixedThreadPool(1); Future future = executorService.submit(instance); Boolean result = future.get(); assertTrue(result); } } Having listed all these methods, the next thing to consider is which one is the best? In asking that question you have to define the word “best” in terms of best for what? Best for simplicity? Maintainability? Speed or code size? After all Programming the Art of Making the Right Decision. You may have guessed that I don’t like the random delay idea and prefer the use of a CountDownLatch. Thread.join() is a bit old school; remember that the likes of Semaphore and CountDownLatch were written to improve on the original Java threading techniques. ExecutorService seems a little heavy weight for what we need here. At the end of the day the choice of technique really comes down to personal preference. Finally, I’ve listed five ways of synchronizing multi-threaded integration tests; however, if you can think of any others please let me know... The source code for this blog is available on Github.

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