为什么我在运行时会出现这种方法不一致的情况

Question

有时，当运行时变量总数将等于其他值而不是 50005000 它总是像 50005001 一样短，有时运行时为什么会发生这种情况不应该同步（这个）创建一个锁，只有在锁被释放后才能更新线程？

import java.util.concurrent.atomic.AtomicLong;

public class CurrentThread {
    public static AtomicLong c = new AtomicLong(0L);
    public static AtomicLong total = new AtomicLong(0L);
    public static void main(String[] args) {

        Thread t = Thread.currentThread();
        System.out.println(t);
        t.setName("My Thread");
        System.out.println(t);

        for (int x = 0; x < 10; x++) {
            System.out.println("Instance " + x);
            new Thread(new Updater(x, "Thread: " + String.valueOf(x))).start();
        }
        try {
            Thread.sleep(1000);
        }
        catch (InterruptedException e) {
        }    
    }
}
class Updater implements Runnable {
    public int na;   
    private String threadName;  
    public Updater(int n, String threadName) {
        this.na = n;
        this.threadName = threadName;  
    }  
    @Override
    public void run() {
        this.updateCount();
        if(CurrentThread.total.get() == 50005000) {
            System.out.println("Passed");
        }
        else {
            System.out.println("Failed");
        }
    }    
    public  void  updateCount() {

            while (CurrentThread.c.get() < 10000) {
                synchronized(this) {
                    CurrentThread.c.getAndIncrement();
                    CurrentThread.total.addAndGet(CurrentThread.c.get());
                    System.out.println(this.threadName + " " + String.valueOf(CurrentThread.c.get()) + " " + CurrentThread.total.get() + " " + System.nanoTime());
                }
            }
      }  
}

Answer 1

您在this 上进行同步，实际上根本没有同步，因为每个线程都有不同的Runnable 实例。

改为同步所有Updater 实例之间共享的内容，例如Updater.class.

---

但是，请注意，在 AtomicLong 上进行同步有点代码味道 - 它意味着已经以原子方式执行操作。

您可以改用compareAndSet，并避免完全同步，例如：

while (CurrentThread.c.get() < 10000) {
  while (true) {
    long currValue = CurrentThread.c.get();
    if (currValue >= 10000) break;

    long newValue = currValue + 1;

    // Only sets c to newValue if its value is still currValue.
    if (CurrentThread.c.compareAndSet(currValue, newValue)) {
      long total = CurrentThread.total.addAndGet(newValue);
      System.out.println(
          this.threadName + " " + newValue + " " + total + " " + System.nanoTime());
      break;
    }
  }
}

请注意，这会使用“已知”值，例如 newValue 和 total，而不是从 AtomicLong 再次获取它们。