Atomics and compare-and-swap question 60 of 180 senior

How would you write a thread-safe non-blocking counter without atomics?

It is impossible without synchronized
Correct: With a VarHandle (or, historically, Unsafe) doing the compare-and-set on a volatile field yourself
With a ThreadLocal
With Collections.synchronizedList

The atomic classes are thin wrappers. Since Java 9 the supported way to do it yourself is VarHandle:

private volatile long value;
private static final VarHandle VALUE;
static {
    try {
        VALUE = MethodHandles.lookup().findVarHandle(Counter.class, "value", long.class);
    } catch (ReflectiveOperationException e) {
        throw new ExceptionInInitializerError(e);
    }
}

void increment() {
    long old;
    do {
        old = value;
    } while (!VALUE.compareAndSet(this, old, old + 1));
}

VarHandle also exposes the weaker modes that make this interesting at the senior level: getPlain, getOpaque, getAcquire/setRelease and the full volatile mode. Release/acquire is enough for most publication and is cheaper than a full fence on weakly ordered hardware. In real code, use AtomicLong unless you have measured a reason not to.

cas varhandle

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