Question

Is this a safe publication?

Given the class:

@NotThreadSafe
public class Pub {
   int drinkers;
   public Pub(int drinkers) {
      this.drinkers = drinkers;
   }
}

T1 invokes publish() and T2 invokes inspect:

private final Object lock = new Object();

@GuardedBy("lock")
private Pub shared;

void publish() {
  Pub p = new Pub(12);
  synchronized(lock) {
      this.shared = p;  // publish  
  }
}

Pub inspect() {
  synchronized(lock) {
     return shared;
  }
}    

Is T2 guaranteed to see that shared.drinkers == 12? Or was the fact that we did not construct the Pub while holding the lock constitute an unsafe publication?

Answer 1

That’s a perfectly valid operation. Pub will either be null or initialised properly. This is because initialisation and assignment happen after each other on the same thread.