什么时候AtomicInteger比synchronized优先？

问题描述

由于 AtomicInteger 的速度至少比 int c> synchronized ，为什么我想使用AtomicInteger？

Since AtomicInteger can be at at least an order of magnitude slower than an int protected by synchronized, why would I ever want to use AtomicInteger?

例如，如果我想要的是增加一个 int 值以线程安全的方式，为什么不使用：

For example, if all I want is to increment an int value in a thread-safe manner, why not always use:

synchronized(threadsafeint) {
  threadsafeint++;
}

而不是使用慢得多的？

推荐答案

AtomicInteger快得多。

AtomicInteger is much faster.

static final Object LOCK1 = new Object();
static final Object LOCK2 = new Object();
static int i1 = 0;
static int i2 = 0;
static final AtomicInteger ai1 = new AtomicInteger();
static final AtomicInteger ai2 = new AtomicInteger();

public static void main(String... args) throws IOException {
    for(int i=0;i<5;i++) {
        testSyncInt();
        testAtomicInt();
    }
}

private static void testSyncInt() {
    long start = System.nanoTime();
    int runs = 10000000;
    for(int i=0;i< runs;i+=2) {
        synchronized (LOCK1) {
            i1++;
        }
        synchronized (LOCK2) {
            i2++;
        }
    }
    long time = System.nanoTime() - start;
    System.out.printf("sync + incr: Each increment took an average of %.1f ns%n", (double) time/runs);
}

private static void testAtomicInt() {
    long start = System.nanoTime();
    int runs = 10000000;
    for(int i=0;i< runs;i+=2) {
        ai1.incrementAndGet();
        ai2.incrementAndGet();
    }
    long time = System.nanoTime() - start;
    System.out.printf("incrementAndGet: Each increment took an average of %.1f ns%n", (double) time/runs);
}

列印

sync + incr: Each increment took an average of 32.4 ns
incrementAndGet: Each increment took an average of 20.6 ns
sync + incr: Each increment took an average of 31.4 ns
incrementAndGet: Each increment took an average of 12.9 ns
sync + incr: Each increment took an average of 29.6 ns
incrementAndGet: Each increment took an average of 12.9 ns
sync + incr: Each increment took an average of 35.1 ns
incrementAndGet: Each increment took an average of 16.6 ns
sync + incr: Each increment took an average of 29.9 ns
incrementAndGet: Each increment took an average of 13.0 ns

---

assylias提示。它表明当你只真正使用一个线程时，CPU可以优化访问。

---

Adding some contention as @assylias suggests. It shows that when you are only really using one thread the CPU can optimise the access.

static final Object LOCK1 = new Object();
static final Object LOCK2 = new Object();
static int i1 = 0;
static int i2 = 0;
static final AtomicInteger ai1 = new AtomicInteger();
static final AtomicInteger ai2 = new AtomicInteger();

public static void main(String... args) throws  ExecutionException, InterruptedException {
    for(int i=0;i<5;i++) {
        testSyncInt();
        testAtomicInt();
    }
}

private static void testSyncInt() throws ExecutionException, InterruptedException {
    long start = System.nanoTime();
    final int runs = 1000000;
    ExecutorService es = Executors.newFixedThreadPool(2);
    List<Future<Void>> futures = new ArrayList<>();
    for(int t=0;t<8;t++) {
        futures.add(es.submit(new Callable<Void>() {
            public Void call() throws Exception {
                for (int i = 0; i < runs; i += 2) {
                    synchronized (LOCK1) {
                        i1++;
                    }
                    synchronized (LOCK2) {
                        i2++;
                    }
                }
                return null;
            }
        }));
    }
    for (Future<Void> future : futures) {
        future.get();
    }
    es.shutdown();
    long time = System.nanoTime() - start;
    System.out.printf("sync + incr: Each increment took an average of %.1f ns%n", (double) time/runs/2);
}

private static void testAtomicInt() throws ExecutionException, InterruptedException {
    long start = System.nanoTime();
    final int runs = 1000000;
    ExecutorService es = Executors.newFixedThreadPool(2);
    List<Future<Void>> futures = new ArrayList<>();
    for(int t=0;t<8;t++) {
        futures.add(es.submit(new Callable<Void>() {
            public Void call() throws Exception {
                for (int i = 0; i < runs; i += 2) {
                    ai1.incrementAndGet();
                    ai2.incrementAndGet();
                }
                return null;
            }
        }));
    }
    for (Future<Void> future : futures) {
        future.get();
    }
    es.shutdown();
    long time = System.nanoTime() - start;
    System.out.printf("incrementAndGet: Each increment took an average of %.1f ns%n", (double) time/runs/2);
}

列印

sync + incr: Each increment took an average of 478.6 ns
incrementAndGet: Each increment took an average of 191.5 ns
sync + incr: Each increment took an average of 437.5 ns
incrementAndGet: Each increment took an average of 169.8 ns
sync + incr: Each increment took an average of 408.1 ns
incrementAndGet: Each increment took an average of 180.8 ns
sync + incr: Each increment took an average of 511.5 ns
incrementAndGet: Each increment took an average of 313.4 ns
sync + incr: Each increment took an average of 441.6 ns
incrementAndGet: Each increment took an average of 219.7 ns

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