LongAdder和AtomicLong比较

public class LongAdderMain {


    public static void main(String[] args) throws InterruptedException {
        atomicAlongVsLongAdderTest(1, 10000000);
        System.out.println("-----------------------------------");
        atomicAlongVsLongAdderTest(10, 10000000);
        System.out.println("-----------------------------------");
        atomicAlongVsLongAdderTest(50, 10000000);

    }

    private static void atomicAlongVsLongAdderTest(int threadNum, int times)
    											 throws InterruptedException {

        long start = System.currentTimeMillis();
        long result = longAdderRun(threadNum, times);
        System.out.println("LongAdder  线程数: " + threadNum + " 结果: " 
        		+ result + " 耗时: " + (System.currentTimeMillis() - start));

        long start1 = System.currentTimeMillis();
        long result1 = atomicLongRun(threadNum, times);
        System.out.println("AtomicLong 线程数: " + threadNum + " 结果: " 
        + result1 + " 耗时: " + (System.currentTimeMillis() - start1));

    }

    private static Long atomicLongRun(int threadNum, int times) 
    									throws InterruptedException {

        AtomicLong counter = new AtomicLong();
        List<Thread> list = new ArrayList<>();
        for (int i = 0; i < threadNum; i++) {
            Thread t = new Thread(() -> {
                int j = 0;
                while (j < times) {
                    counter.incrementAndGet();
                    j++;
                }

            });
            list.add(t);
        }


        for (Thread thread : list) {
            thread.start();
        }

        for (Thread thread : list) {
            thread.join();
        }
        return counter.get();
    }

    private static long longAdderRun(int threadNum, int times)
    								 throws InterruptedException {
        LongAdder counter = new LongAdder();
        List<Thread> list = new ArrayList<>();
        for (int i = 0; i < threadNum; i++) {
            Thread t = new Thread(() -> {
                int j = 0;
                while (j < times) {
                    counter.increment();
                    //如果每次都取值，longAdder效率不如atomicLong高
//                    counter.longValue();
                    j++;
                }

            });
            list.add(t);
        }


        for (Thread thread : list) {
            thread.start();
        }

        for (Thread thread : list) {
            thread.join();
        }
        return counter.longValue();
    }


}

结果:

LongAdder  线程数: 1 结果: 10000000 耗时: 125 ms
AtomicLong 线程数: 1 结果: 10000000 耗时: 108 ms
-----------------------------------
LongAdder  线程数: 10 结果: 100000000 耗时: 137 ms
AtomicLong 线程数: 10 结果: 100000000 耗时: 2153 ms
-----------------------------------
LongAdder  线程数: 50 结果: 500000000 耗时: 338 ms
AtomicLong 线程数: 50 结果: 500000000 耗时: 10134 ms

可以发现， LongAdder的效率还是很高的

LongAdder

实现方式， 相比较AtomicLong, LongAdder 实现不仅仅使用了CAS, 而且也使用了 类似ConcurrentHashMap 分段锁一样的机制：

• Base + Cell[] 数组
• 如果没有线程竞争，result = Base
• 如果存在线程竞争， result = Base+ Cells.value