Java使用Thread.sleep（）或继续询问

Question

I have scenario: I want to wait, until something is false. Usually takes likes 20 seconds or so.

while(foo) {
    try {
        Thread.sleep(5000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

Or just go like this?

while(foo) {

}

Answer 1

Busy waiting for 20 seconds is not a good idea - and sleeping for 5 seconds means you might miss the signal by 5 seconds. Can't you use a different way, such as a CountdownLatch :

CountDownLatch latch = new CountDownLatch(1);

//code that triggers the signal
latch.countDown();

//code that waits for the signal
try {
    latch.await();
} catch (InterruptedException e) {
    //handle interruption
}

Answer 2

It this is just a unit test, I would use the first example. It doesn't have to be pretty.

If this is production code, you are better off using a different structure such a Condition or a Future.

Answer 3

The second form is going to burn a lot of CPU cycles. It will check as fast as it can, maybe a hundred thousand times per second. That's not a good idea.

Thread.sleep() is better, a little. You still have another problem here if foo is a non-volatile field and/or this is not in a synchronized block. It is not actually guaranteed that it will ever see an update from another thread.

Use primitives in java.util.concurrent for this like CountDownLatch .

Answer 4

For sure, the second form can not be used. That can hog the CPU and cause problems. The first form is better.

What is going to change the value?

If it is just another thread then you can consider using wait / notify or any of the concurrency constructs.

Answer 5

Does nobody like Wait/Notify any more? It's effcient, simpler than the Sleep() loop bodges and less typing than 'CountDownLatch'.,