Page History

...

public final class CountBoxes implements Runnable {
  private static volatile int counter;
  // ...
  private final Object lock = new Object();

  @Override public void run() {
    synchronized (lock) {
      counter++;
      // ...
    }
  }

  public static void main(String[] args) {
     for (int i = 0; i < 2; i++) {
      new Thread(new CountBoxes()).start();
    }
  }
}

This example fails to prevent either thread from observing an inconsistent value of counter because the increment operation on volatile fields fails to be atomic in the absence of proper synchronization. (See rule VNA02-J. Ensure that compound operations on shared variables are atomic for more information.)

Noncompliant Code Example (Method Synchronization for Static Data)

...

In this case, the method synchronization uses the intrinsic lock associated with each instance of the class rather than the intrinsic lock associated with the class itself. Consequently, threads constructed using different Runnable instances may observe inconsistent values of the counter.

Compliant Solution (Static Lock Object)

...

...