问题描述:

Suppose I have the following situation:

synchronized void someMethod() {

...

try {

wait();

}catch(InterruptedException e) {

System.out.println("InterruptedException caught");

}

...

}

and

synchronized void someOtherMethod() {

...

notify();

}

And the Thread accesses first someMethod, goes into wait and then someOtherMethod notifies it and returns to Runnable state. Does the position of the notify() call in the method matter? I noticed no change in behavior even when I positioned the notify() call at different positions inside the method.

Shouldn't the Thread be notified immediately when the call to notify() is made?

网友答案:

The position of the notify() call within the synchronized block does not matter because by definition, if you are still in the synchronized block, then you still hold the lock.

Shouldn't the Thread be notified immediately when the call to notify() is made?

Yes. Calling notify() puts one of the threads (if any) from the wait queue into the blocked queue. This does happen immediately, but then the awoken thread needs to get the lock before it can start running. So it is immediately moved out of the wait queue but is still waiting to get the lock.

Btw, I would recommend writing this as this.wait() and this.notify() just to be explicit about which object is being affected.

网友答案:

No, the position of the notify() call within the synchronized block does not matter.

I recommend the style:

class SomeClass {
  synchronized void someMethod() throws InterruptedException{
    ...
    while (! someCondition) {
      wait();
    } 
    ...
  }

  synchronized void someOtherMethod() {
    ...
    makeConditionValid();
    notifyAll();  
  }
}

Notice the use of a while loop around the wait call. Some JVMs can issue spurious notifies, so there is no guarantee that when a thread is notified, the original condition which caused it to wait is valid. Also, the woken up thread does not get to run until the notifying thread relinquishes the lock; so it is possible that by the time the waiting thread executes the condition is again invalid.

网友答案:

These calls (i.e. Object#wait and Object#notify) need to be made within a synchronized block. Since your method is synchronized, the scope of the synchronized block includes everything within the method. Therefore, positioning is irrelevant to it.

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