CODEWITHSUNDEEP
javamultithreadingsynchronizedinterrupted-exceptionspurious-wakeup
user21820
user21820

Reputation: 567

Java: Does this pause the thread correctly?

I am curious whether it is possible to pause a thread t in Java and allow another thread to resume it later, by having t run the following pause code:

while(true) {
  try {
    synchronized(t) { 
      t.wait();
    }
  } catch(InterruptedException e) {
    break;
  }
}

And then resuming the thread t by calling .interrupt() on it. However, I have read about spurious wake-ups, and so I wondered whether my code can fail, in the sense of exiting the while-loop despite no other thread calling .interrupt() on it. While this answer and this answer state that there are no spurious interrupts, and hence my code will never fail, the Java docs does not seem to address this. My question probably boils down to whether InterruptedException is ever thrown without the thread being interrupted by .interrupt(). Is there any official source or documentation that confirms this?

Upvotes: 0

Views: 113

Answers (1)

Impurity
Impurity

Reputation: 1132

Summary

So, although technically this works, there are numerous reasons why this should not be done. Oracle's documentation states that interruption should only be utilized for cancellations. But if you were to do this, it will clear the interrupt status and the previously waiting thread will receive an InterruptedException.

Alternative

Lets step through a brief, simplified example.

Object obj = new Object;
synchronized (obj) {
    while(condition) {
        obj.wait();
    }
}

  1. A thread here will obtain the monitor.

  2. The thread will begin to wait via wait(), and release the monitor. Always utilize wait() inside a conditional because threads are subject to get spurious wake-ups from wait(). At this point, you have achieved forcing a thread to wait.

Lets investigate how we go about returning the thread to work.

synchronized(obj) {
    obj.notify();
}

The notify() will wake up the first waiting thread on the monitor. Now, if you want all waiting threads to wake up, utilize notifyAll() instead. This is the intended purpose and functionality of wait()/notify() and thus should be utilized over wait()/interrupt(). For an additional example, see this article.

Upvotes: 1

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