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 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<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());
}

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

• The Java Memory Model and Thread Specification JSR-133
• Threads and Locks, Java Virtual Machine Specification
• The book Java Concurrency in Practice by Brian Goetz (Amazon UK).
• Double Checked Locking article by Doug Lea
• Doug Lea's homepage