Wait until boolean value changes it state

Question

I have a thread which wait for a boolean value to change like this:

while(!value) {   Thread.sleep(1000) } // Do some work after change of the value 

This is not my prefered way to do this, cause of massive CPU consumption.

Is there any way to block the Thread, until the boolean value changes it state?

Answer 1

This is not my prefered way to do this, cause of massive CPU consumption.

If that is actually your working code, then just keep it like that. Checking a boolean once a second causes NO measurable CPU load. None whatsoever.

The real problem is that the thread that checks the value may not see a change that has happened for an arbitrarily long time due to caching. To ensure that the value is always synchronized between threads, you need to put the volatile keyword in the variable definition, i.e.

private volatile boolean value; 

Note that putting the access in a synchronized block, such as when using the notification-based solution described in other answers, will have the same effect.

Answer 2

You need a mechanism which avoids busy-waiting. The old wait/notify mechanism is fraught with pitfalls so prefer something from the java.util.concurrent library, for example the CountDownLatch:

public final CountDownLatch latch = new CountDownLatch(1);  public void run () {   latch.await();   ... } 

And at the other side call

yourRunnableObj.latch.countDown(); 

However, starting a thread to do nothing but wait until it is needed is still not the best way to go. You could also employ an ExecutorService to which you submit as a task the work which must be done when the condition is met.