是否可以使用带有两个AtomicInteger的线程安全方法？

Question

I want to write a Thread-safe method sum() but I'm not sure if I can use two AtomicIntegers to make it safe or do I have to use a synchronized block?

class A {

   private AtomicInteger a = new AtomicInteger();
   private AtomicInteger b = new AtomicInteger();

   public void sum(int c) {
      a.set(a.get() + b.get() + c);
   }
}

Answer 1

The rules for evaluation order in Java mean that you'll always follow a series of non-overlapping locks: A (get) -> B (get) -> A (set). So you won't have any issues with blocking, because the locks won't interfere with one another. Note that there is, however, a form of race condition: it is possible for thread 1 to read A, then read B. Before thread 1 sets A, thread 2 can read A and B. The final value will be one of the threads' resulting sum, but the other will be lost.

Answer 2

There are specific methods of AntomicInteger to take care of this. You don't need to break out of the AtomicInteger word and it's thread safety. Just checking the JavaDocs of AtomicInteger will show you methods like AtomicInteger.getAndAdd that you can use for this.