应用 java AtomicIntegeraccumulateAndGet

Question

我有一个 Counter 类，它将值存储为 AtomicInteger。该类应该是线程安全的。我有方法boolean consume(int number)应该减少计数器并返回 true如果counter >= number ，并且不应更改计数器并返回 false如果counter < number

class Counter {
   AtomicInteger counter = new AtomicInteger(initialValue);

   boolean consume(int number) {
     counter.accumulateAndGet(number, (prev, next) -> {
            if (number <= prev) {
                return prev - number;
            } else {
                // not modify the previous number;
                return prev;
            }
        });
       return ???
   }
}

而且我不知道该功能是否适用。我找到了以下解决方案

boolean consume(int number) {
    AtomicBoolean result = new AtomicBoolean(false);
    counter.accumulateAndGet(number, (prev, next) -> {
            if (number <= prev) {
                result.set(true);
                return prev - number;
                // function applied
            } else {
                result.set(false);
                // not modify the previous number;
                return prev;
            }
    });
    return result.get();
}

但是 accumulateAndGet 的 javadoc赛伊:

The function should be side-effect-free, since it may be re-applied when attempted updates fail due to contention among threads.

所以，我的解决方案有副作用。使用安全吗？如果不是，我怎样才能得到相同的结果？

Answer 1

从描述来看，听起来好像您想要类似的东西:

class Counter {
    private final int initialValue = 42; // make compile

    /** Non-negative count. */
    private final AtomicInteger counter = new AtomicInteger(initialValue);

    public boolean consume(int number) {
        for (;;) {
           int old = counter.get();
           int next = old-number;

           if (next >= 0) {
               if (counter.compareAndSet(old, next)) {
                   return true;
               };
           } else {
               return false;
           }
        }
    }
}

since it may be re-applied when attempted updates fail due to contention among threads.

问题中的代码可以执行一次或多次，但如果方便方法不方便，则使用这些方法无济于事。