IBM tutorials

Tech library

Multicore and threads

http://www.ibm.com/developerworks/library/j-nothreads/

Tutorials

Java concurrency

http://www.ibm.com/developerworks/training/kp/j-kp-concurrency/

• 5 things didn't know about j.u.c part 1
  • TimeUnit
  • CopyOnWriteArrayList
  • BlockingQueue
  • ConcurrentMap
  • SynchronousQueues
• 5 things... part 2
  • Semaphore for throttling
  • CountDownLatch
  • Executor instead of creating Threads directly
    • Executor exec = getAnExecutorFromSomeplace(); exec.execute(new Runnable() { ... });
    • ExecutorService
      • ExecutorService executor = Executors.newFixedThreadPool(..)
    • Callable and Future
  • ScheduledExecutorService
  • Timeout methods
  • also look at .locks and .atomic package, CyclicBarrier
• Java concurrency bug patterns for multicore systems
  1. volatile keyword only guarantees that a variable is visible; it does not guarantee atomicity.
  2. simultaneous locking on updated fields (to fix, use private final to ensure the lock object remains unchanged and mutex is guaranteed)
  • read through the example but the gist is the synchronized field is updated with a 'different' instance
  3. java.util.concurrent lock leak
  • i.e. Lock.lock() invocation without unlock() on the same instance causes a lock leak (see below).

Examples

1. Lock leak

private final Lock lock = new ReentrantLock();

public void lockLeak() {
  lock.lock();
  try {
    accessSharedResource(...);
    lock.unlock();
  } catch (Exception e) {}
}

public void accessSharedResource() throws InterruptedException {...}

Instead put the unlock invocation in the finally block.

public void lockLeak() {
  lock.lock();
  try {
    accessSharedResource(...);
  } catch (Exception e) {}
  finally {
    lock.unlock();
  }
}

2. another

Actor concurrency

http://www.ibm.com/developerworks/training/kp/j-kp-actorconcurrency/