Search code examples
javasynchronizationvisibilitycyclicbarrier

Visibility synchronization at CyclicBarrier?

When using a CyclicBarrier for synchronizing threads in Java, do they synchronize non-volatile variables?

int a = 0;
int b = 0;
CyclicBarrier barrier = new CyclicBarrier(2);

/*** Thread 1 ***/
public void run() {
    a = 2;
    barrier.await();

    doSomeStuff(b); // no side-effects
}

/*** Thread 2 ***/
public void run() {
    b = 3;
    barrier.await();

    doSomeStuff(a); // no side-effects
}

Can we be sure that on Thread 1's doSomeStuff call b has been set to 3? When trying it's always 3...

Solution

  • Yes, visibility happens as you would expect, as you can see from the javadoc for the CyclicBarrier class:

    Memory consistency effects: Actions in a thread prior to calling await() happen-before actions that are part of the barrier action, which in turn happen-before actions following a successful return from the corresponding await() in other threads.