java - 使用 ReentrantLock 时出现死锁

Question

我使用ReentrantLock实现生产者消费者问题

public class Processor {
   Lock lock =  new ReentrantLock(true);
   Condition condn = lock.newCondition();
    public void produce() throws InterruptedException{
       lock.lock();
            System.out.println("inside producer method");
            condn.await();
            System.out.println("thread again wakeup");
        lock.unlock();
    }

    public void consume() throws InterruptedException{
          lock.lock();
            Thread.sleep(1000);
            condn.signal();
            System.out.println("will i ever be ok ");
            lock.unlock();
    }


}

ReentrantLock 同步了两个方法，但有时进程会死锁

o/p我会好吗内部生产者方法

运行堆转储时

引用处理程序”#2 守护进程 prio=10 os_prio=31 cpu=0.26ms elapsed=540.35s tid=0x00007fd5f186f800 nid=0x4603 等待条件 [0x0000700002357000]

 java.lang.Thread.State: RUNNABLE
    at java.lang.ref.Reference.waitForReferencePendingList(java.base@11.0.1/Native Method)
    at java.lang.ref.Reference.processPendingReferences(java.base@11.0.1/Reference.java:241)
    at java.lang.ref.Reference$ReferenceHandler.run(java.base@11.0.1/Reference.java:213)

"Finalizer" #3 daemon prio=8 os_prio=31 cpu=0.67ms elapsed=540.35s tid=0x00007fd5f1883000 nid=0x4303 in Object.wait()  [0x000070000245a000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(java.base@11.0.1/Native Method)
    - waiting on <0x0000000787f08f80> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(java.base@11.0.1/ReferenceQueue.java:155)
    - waiting to re-lock in wait() <0x0000000787f08f80> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(java.base@11.0.1/ReferenceQueue.java:176)
    at java.lang.ref.Finalizer$FinalizerThread.run(java.base@11.0.1/Finalizer.java:170)

"Signal Dispatcher" #4 daemon prio=9 os_prio=31 cpu=0.34ms elapsed=540.27s tid=0x00007fd5f1884000 nid=0x3903 waiting on condition  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"Thread-0" #12 prio=5 os_prio=31 cpu=1.72ms elapsed=540.07s tid=0x00007fd5f00c7000 nid=0xa303 waiting on condition  [0x0000700002d75000]
   java.lang.Thread.State: WAITING (parking)
    at jdk.internal.misc.Unsafe.park(java.base@11.0.1/Native Method)
    - parking to wait for  <0x0000000787ed4030> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(java.base@11.0.1/LockSupport.java:194)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@11.0.1/AbstractQueuedSynchronizer.java:2081)
    at Processor.produce(Processor.java:11)
    at RunnableExample$1.run(RunnableExample.java:13)
    at java.lang.Thread.run(java.base@11.0.1/Thread.java:834)

为什么即使我使用释放锁后仍然会出现死锁

    condn.signal();

Answer 1

如果消费者先调用 consume，然后生产者调用 product，那么生产者将错过信号并陷入困境。