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

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());
    }
}