java - 它是线程安全机制吗？

这个类是线程安全的吗？

class Counter {
  private ConcurrentMap<String, AtomicLong> map =
    new ConcurrentHashMap<String, AtomicLong>();
  public long add(String name) {
    if (this.map.get(name) == null) {
      this.map.putIfAbsent(name, new AtomicLong());
    }
    return this.map.get(name).incrementAndGet();
  }
}

你怎么看？

最佳答案

是的，前提是您将 map 制作成最终版。 if 不是必需的，但如果需要，您可以出于性能原因保留它，尽管它很可能不会产生显着的差异:

public long add(String name) {
  this.map.putIfAbsent(name, new AtomicLong());
  return this.map.get(name).incrementAndGet();
}

编辑

为此，我快速测试了两种实现(有和没有检查)。同一字符串上的 1000 万次调用需要:

250 毫秒，检查

480 ms 无检查

这证实了我所说的:除非你调用这个方法数百万次或者它是你代码的性能关键部分，否则它没有区别。

编辑 2

完整的测试结果 - 请参阅 BetterCounter ，它会产生更好的结果 。现在该测试非常具体(无争用 + get 始终有效)并且不一定与您的用法相对应。

public class Test {

    public static void main(String args[]) throws IOException {
        Counter count = new Counter();
        LazyCounter lazyCount = new LazyCounter();
        MPCounter mpCount = new MPCounter();
        BetterCounter betterCount = new BetterCounter();

        //WARM UP
        for (int i = 0; i < 10_000_000; i++) {
            count.add("abc");
            lazyCount.add("abc");
            mpCount.add("abc");
            betterCount.add("abc");
        }

        //TEST
        long start = System.nanoTime();
        for (int i = 0; i < 10_000_000; i++) {
            count.add("abc");
        }
        long end = System.nanoTime();
        System.out.println((end - start) / 1000000);

        start = System.nanoTime();
        for (int i = 0; i < 10_000_000; i++) {
            lazyCount.add("abc");
        }
        end = System.nanoTime();
        System.out.println((end - start) / 1000000);

        start = System.nanoTime();
        for (int i = 0; i < 10_000_000; i++) {
            mpCount.add("abc");
        }
        end = System.nanoTime();
        System.out.println((end - start) / 1000000);

        start = System.nanoTime();
        for (int i = 0; i < 10_000_000; i++) {
            betterCount.add("abc");
        }
        end = System.nanoTime();
        System.out.println((end - start) / 1000000);
    }

    static class Counter {

        private final ConcurrentMap<String, AtomicLong> map =
                new ConcurrentHashMap<String, AtomicLong>();

        public long add(String name) {
            this.map.putIfAbsent(name, new AtomicLong());
            return this.map.get(name).incrementAndGet();
        }
    }

    static class LazyCounter {

        private final ConcurrentMap<String, AtomicLong> map =
                new ConcurrentHashMap<String, AtomicLong>();

        public long add(String name) {
            if (this.map.get(name) == null) {
                this.map.putIfAbsent(name, new AtomicLong());
            }
            return this.map.get(name).incrementAndGet();
        }
    }

    static class BetterCounter {

        private final ConcurrentMap<String, AtomicLong> map =
                new ConcurrentHashMap<String, AtomicLong>();

            public long add(String name) {
                AtomicLong counter = this.map.get(name);
                if (counter != null)
                    return counter.incrementAndGet();

                AtomicLong newCounter = new AtomicLong();
                counter = this.map.putIfAbsent(name, newCounter);

                return (counter == null ? newCounter.incrementAndGet() : counter.incrementAndGet());
            }
    }

    static class MPCounter {

        private final ConcurrentMap<String, AtomicLong> map =
                new ConcurrentHashMap<String, AtomicLong>();

        public long add(String name) {
            final AtomicLong newVal = new AtomicLong(),
                    prevVal = map.putIfAbsent(name, newVal);
            return (prevVal != null ? prevVal : newVal).incrementAndGet();
        }
    }
}

关于java - 它是线程安全机制吗？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/10498303/