Question

I often use the following pattern to create a cancellable thread:

public class CounterLoop implements Runnable {

    private volatile AtomicBoolean cancelPending = new AtomicBoolean(false);

    @Override
    public void run() {
        while (!cancelPending.get()) {
            //count
        }
    }

    public void cancel() {
        cancelPending.set(true);
    }
}

But I’m not sure that cancelPending MUST be a AtomicBoolean. Can we just use a normal boolean in this case?

Answer 1

Using both volatile and AtomicBoolean is unnecessary. If you declare the cancelPending variable as final as follows:

private final AtomicBoolean cancelPending = new AtomicBoolean(false);

the JLS semantics for final fields mean that synchronization (or volatile) will not be needed. All threads will see the correct value for the cancelPending reference. JLS 17.5 states:

“An object is considered to be completely initialized when its constructor finishes. A thread that can only see a reference to an object after that object has been completely initialized is guaranteed to see the correctly initialized values for that object’s final fields.”

… but there are no such guarantees for normal fields; i.e. not final and not volatile.

You could also just declare cancelPending as a volatile boolean … since you don’t appear to be using the test-and-set capability of AtomicBoolean.

However, if you used a non-volatile boolean you would need to use synchronized to ensure that all threads see an up-to-date copy of the cancelPending flag.