Java list of waiting threads

Question

Is there a way to get a list of waiting threads/number of waiting threads on an object?

Answer 1

If you are using the synchronized keyword - no. But if you are using the java.util.concurrent locks, you can.

ReentrantLock has a protected method getWaitingThreads(). If you extend it, you can make it public.

Update: You are using .wait() and .notify(), so you can manually fill and empty a List<Thread> - before wach .wait() call list.add(Thread.currentThread(), and remove it before each notify. It's not perfect, but actually you shouldn't need such a list.

Answer 2

You can use the JMX classes to inspect the threads:

ThreadInfo[] infos = ManagementFactory.getThreadMXBean().dumpAllThreads(true, true);

Each blocked thread has a non null LockInfo associated that will let you identify on what object it's waiting:

for (ThreadInfo info : infos) {
    LockInfo lockInfo = info.getLockInfo();
    if (lockInfo != null 
            && lockInfo.getClassName().equals(lock.getClass().getName()) 
            && lockInfo.getIdentityHashCode() == System.identityHashCode(lock)) {

        System.out.println("Thread waiting on " + lock + " : " + info.getThreadName());
    }
}