List of Java Pending Threads

Question

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

Answer 1

If you use the synchronized - no. But if you use java.util.concurrent locks, you can.

ReentrantLock has a protected getWaitingThreads() method. If you expand it, you can make it publicly available.

Update. You use .wait() and .notify() so you can manually populate and empty the List<Thread> - before calling .wait() list.add(Thread.currentThread() and delete it before each notification. This is not ideal, but you don’t really need such a list.

Answer 2

You can use JMX classes to check threads:

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

Each blocked thread has an associated non-zero LockInfo , which will allow you to determine on which object it is 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()); } } 

Answer 3

If you are on JDk 1.6, then ManagementFactory.getThreadMXBean() is the best way to find out about all threads waiting on an object. For JDK up to 1.6, you can use a group of threads to find out all the threads, and then check the stack stack to find out about the object. on which they are waiting.