Multithreaded code and concurrency (original) (raw)

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 from the article 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 exceptions = Collections.synchronizedList(new ArrayList()); 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()); }

A JUnit extension that generalizes this pattern into a library is ConcurrentUnit.

Another article giving an overview of alternative stragies at http://tempusfugitlibrary.org/recipes/2012/05/20/testing-concurrent-code/ might also be useful.

Java Concurrency Bookshelf