Java ArrayBlockingQueue获取源代码

Question

When I was reading source code of ArrayBlockingQueue.take method, I got a problem.

I think then two threads call take method concurrently, only one thread could successfully get lock, and the other thread would waiting on the lock at the line: lock.lockInterruptibly(); this is source code of take:

public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        while (count == 0)
            notEmpty.await();
        return dequeue();
    } finally {
        lock.unlock();
    }
}

But when I make a thread dump of the two threads, I found that both threads got lock successfully, and waiting on line: notEmpty.await(); (because queue is empty) This is the thread dump:

"test-thread-18" #6357 daemon prio=5 os_prio=0 tid=0x00007f8f54543000 nid=0x58ef waiting on condition [0x00007f901bc70000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x00007f93ae695410> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039) at java.util.concurrent.ArrayBlockingQueue.take(ArrayBlockingQueue.java:403) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745)

"test-thread-17" #6356 daemon prio=5 os_prio=0 tid=0x00007f8f54542000 nid=0x58ee waiting on condition [0x00007f901beb9000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x00007f93ae695410> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039) at java.util.concurrent.ArrayBlockingQueue.take(ArrayBlockingQueue.java:403) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745)

So, why two different thread could get the same lock on same time? What's wrong with my understand?

Answer 1

Only one thread can hold the ReentrantLock on the same time.

notEmpty is an Condition object, current thread will release the lock when calling Condition.await() :

Causes the current thread to wait until it is signalled or interrupted. The lock associated with this Condition is atomically released and the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:

• Some other thread invokes the signal() method for this Condition and the current thread happens to be chosen as the thread to be awakened;
• or Some other thread invokes the signalAll() method for this Condition;
• or Some other thread interrupts the current thread, and interruption of thread suspension is supported; -or A "spurious wakeup" occurs.

In all cases, before this method can return the current thread must re-acquire the lock associated with this condition. When the thread returns it is guaranteed to hold this lock.