Question

I have a function that needs to be called once a boolean variable is true. I tried using a while loop in a thread but it doesn’t work. Here is what I’ve tried:

public class MyRunnable implements Runnable {

public void run() {
    while (true) {
         if (conditions == true) { 
             System.out.println("second");
             break;
         }
    }
}

public static void main(String args[]) {
    boolean condition = false;
    (new Thread(new MyRunnable())).start();
    System.out.println("first\n");
    // set conndition to true
    condition = true;

    }

}

The result shoud be:

first
second

Answer 1

Do not busy-wait for such conditions. Use a blocking idiom. For your simple case you would get away with a new CountDownLatch(1). First, here’s your code, but fixed to compile and run the way you expect:

public class MyRunnable implements Runnable {
  volatile boolean condition = false;

  public void run() {
    while (true) {
      if (condition) {
        System.out.println("second");
        break;
      }
    }
  }
  public static void main(String args[]) {
    final MyRunnable r = new MyRunnable();
    new Thread(r).start();
    System.out.println("first\n");
    r.condition = true;
  }
}

For comparison, a program with a CountDownLatch:

public class MyRunnable implements Runnable {
  final CountDownLatch latch = new CountDownLatch(1);

  public void run() {
    try { latch.await(); } catch (InterruptedException e) {}
    System.out.println("second");
  }

  public static void main(String args[]) {
    final MyRunnable r = new MyRunnable();
    new Thread(r).start();
    System.out.println("first\n");
    r.latch.countDown();
  }
}

To truly notice the difference, add a Thread.sleep(20000) after println("first") and hear the difference in the sound of your computer’s fan working hard to dissipate the energy the first program is wasting.