Java并发 - 虚假唤醒和等待（超时）

Question

I have learned that when waiting on an object, the condition should be placed in a loop instead of within a single if condition . One reason for that is to avoid various spurious wakeup calls that might occur.

For example:

synchronized (obj) {
    while (condition is false) {
        obj.wait(5 * 1000);
    }
    ... do something ...
}

However, I only want to wait 5 seconds for the thread to be notified. In the code above, the thread waits for 5 seconds. But after that period, if the condition is still false, we wait again. This would not be the case if I had the following:

synchronized (obj) {
    if (condition is false) {
        obj.wait(5 * 1000);
    }
    ... do something ...
}

But this contradicts the guideline we should use a loop instead of a single if condition!

How is this solved?

Answer 1

here's how i typically handle this:

long timeout = ....;
synchronized(obj) {
  long start = System.currentTimeMillis();
  while(!condition && (timeout > 0)) {
    obj.wait(timeout);
    long now = System.currentTimeMillis();
    timeout -= (now - start);
    start = now;
  }
}

Answer 2

Answering my own question, this might work:

synchronized (obj) {
    for (long time = System.currentTimeMillis() + timeout; !condition && System.currentTimeMillis() < time; ) {
        obj.wait(time - System.currentTimeMillis());
    }
    if (condition) {
        ... do something ...
    }
}