java - 没有等待线程的信号条件

Question

我使用 ReentrantLock 和 Condition 来同步两个线程。每当消费者线程在生产者线程执行 signal() 之前对条件执行 await() 时，一切似乎都工作正常。但是，当生产者线程在消费者线程执行 await() 之前执行 signal() 时，消费者线程就会永远等待。这是应该发生的还是我做错了什么？当发出条件信号并且没有等待线程时，预期的行为是什么？

这是消费者的代码:

this.lock.lock();
Log.d("websocket", "Sender acquired lock");
try
{
    Log.d("websocket", "Sender waiting for message from server");
    this.resultReady.await();
    Log.d("websocket", "Sender waking up");
    return this.result;

} catch (InterruptedException e)
{
    e.printStackTrace();
}
finally {
    this.lock.unlock();
}

这是生产者的代码:

Log.d("websocket", "Receiver acquiring lock");
this.lock.lock();
Log.d("websocket", "Receiver acquired lock");
try
{
    Log.d("websocket", "Receiver setting result");
    result = serviceResult;
    Log.d("websocket", "Receiver waking up waiting threads");
    this.resultReady.signal();
} finally
{
    this.lock.unlock();
}

Answer 1

Is this supposed to happen or am I doing something wrong? What is the expected behaviour when a condition is signalled and there are no waiting threads?

这应该会发生。如果没有线程等待，则没有任何效果。 Java 文档说:

Wakes up one waiting thread.

If any threads are waiting on this condition then one is selected for waking up. That thread must then re-acquire the lock before returning from await.

如果没有线程唤醒，那么就没有效果吗？

您有一个缓冲区，其中包含一项，即您的结果。您必须测试此结果是否独立于 await() 方法而有效。或者切换到诸如Semaphore之类的东西，它确实有一个可以设置的内部状态。

// Consumer
this.lock.lock();
Log.d("websocket", "Sender acquired lock");
try
{
    Log.d("websocket", "Sender waiting for message from server");
    while( result == null )   // CHANGE HERE
        resultReady.await();
    Log.d("websocket", "Sender waking up");
    return this.result;

} catch (InterruptedException e)
{
    e.printStackTrace();
}
finally {
    this.lock.unlock();
}