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FencedLock

FencedLock is a member of CP Subsystem API. For detailed information, see the CP Subsystem chapter.

FencedLock is a linearizable and distributed implementation of java.util.concurrent.locks.Lock, meaning that if you lock using a FencedLock, the critical section that it guards is guaranteed to be executed by only one thread in the entire cluster. Even though locks are great for synchronization, they can lead to problems if not used properly. Also note that Hazelcast Lock does not support fairness.

For detailed information and configuration, see the FencedLock section under the CP Subsystem chapter.

Using Try-Catch Blocks with Locks

Always use locks with try-catch blocks. This ensures that locks are released if an exception is thrown from the code in a critical section. Also note that the lock method is outside the try-catch block because we do not want to unlock if the lock operation itself fails.

        HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();

        Lock lock = hazelcastInstance.getCPSubsystem().getLock("myLock");
        lock.lock();
        try {
            // do something here
        } finally {
            lock.unlock();
        }

Releasing Locks with tryLock Timeout

If a lock is not released in the cluster, another thread that is trying to get the lock can wait forever. To avoid this, use tryLock with a timeout value. You can set a high value (normally it should not take that long) for tryLock. You can check the return value of tryLock as follows:

if ( lock.tryLock ( 10, TimeUnit.SECONDS ) ) {
  try {
    // do some stuff here..
  } finally {
    lock.unlock();
  }
} else {
  // warning
}

Understanding Lock Behavior

  • Locks are fail-safe. If a member holds a lock and some other members go down, the cluster will keep your locks safe and available. Moreover, when a member leaves the cluster, all the locks acquired by that dead member will be removed so that those locks are immediately available for live members.

  • Locks are not automatically removed. If a lock is not used anymore, Hazelcast does not automatically perform garbage collection in the lock. This can lead to an OutOfMemoryError. If you create locks on the fly, make sure they are destroyed.

  • Locks are re-entrant. The same thread can lock multiple times on the same lock. Note that for other threads to be able to require this lock, the owner of the lock must call unlock as many times as the owner called lock.