java - Java中同步10个线程

Question

为什么我会得到

22291 3091 0 351 0 1423 0 0 0 0

而不是

0 0 0 0 0 0 0 0 0 0

当我在 Java 中运行此代码时:

import java.lang.Thread;

class MainThread {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) new MyThread().start();
    }
}

class MyThread extends Thread {
    static int counter = 0;
    static Object mutex = new Object();
    public void run() {
        synchronized (mutex) {
            for (int i = 0; i < 1000000; i++) counter = counter + 1;
            for (int i = 0; i < 1000000; i++) counter = counter - 1;
        }
        System.out.print(counter + " ");
    }
}

Answer 1

您得到该输出是因为 print 语句未同步。 线程完成两个 for 循环并存在于 syncronized block 后，计数器 = 0。
但在打印出 counter 之前，其他一些线程会进入 syncronized block 并开始递增 counter。
这就是为什么前面的线程打印递增的计数器。

public void run() {
    synchronized (mutex) {
        for (int i = 0; i < 1000000; i++) counter = counter + 1;
        for (int i = 0; i < 1000000; i++) counter = counter - 1;
    }
    // Here the current thread exited the sync block and some other thread get into this block  
    // and started incrementing the counter

    System.out.print(counter + " "); // your thread prints incremented counter
}