Adder累加器

Java8 引入的，相比是比较新的一个类
高并发下LongAdder比AtomicLong效率高，不过本质是空间换时间
竞争激烈的情况下，LongAdder把不同线程对应到不同的cell上进行修改，降低了冲突的概率，是多段锁的概念，提高了并发性

多线程情况下AtomicLong的性能，有16个线程对同一个AtomicLong累加
由于竞争很激烈，每一次加法，都要flush和refresh，导致很耗费资源

AtomicLong弊端：每次加法都需要做同步

// AtomicLong
public class AtomicLongDemo {
    public static void main(String[] args) {
        AtomicLong atomicLong = new AtomicLong(0);
        ExecutorService executorService = Executors.newFixedThreadPool(20);
        long start = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            executorService.submit(new Task(atomicLong));
        }
        executorService.shutdown();
        while (!executorService.isTerminated()){
        }
        long end = System.currentTimeMillis();
        System.out.println(end-start);
        System.out.println(atomicLong.get());
    }
}
 class Task implements Runnable{
     public AtomicLong atomicLong;
     public Task(AtomicLong atomicLong) {
         this.atomicLong = atomicLong;
     }
     @Override
     public void run() {
         for (int i = 0; i < 10000; i++) {
             atomicLong.getAndIncrement();
         }
     }
 }
//LongAdder
public class LongAdderDemo {
    public static void main(String[] args) {
        LongAdder longAdder = new LongAdder();
        ExecutorService executorService = Executors.newFixedThreadPool(20);
        long start = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            executorService.submit(new TaskAdder(longAdder));
        }
        executorService.shutdown();
        while (!executorService.isTerminated()){
        }
        long end = System.currentTimeMillis();
        System.out.println(end-start);
        System.out.println(longAdder.sum());
    }
}
class TaskAdder implements Runnable{
    public LongAdder longAdder;
    public TaskAdder(LongAdder atomicLong) {
        this.longAdder = atomicLong;
    }
    @Override
    public void run() {
        for (int i = 0; i < 10000; i++) {
            longAdder.increment();
        }
    }
}