用Java同步10个线程

Question

我为什么要

22291 3091 0 351 0 1423 0 0 0

而不是

0 0 0

当我在Java中运行这段代码时：

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

得到这个输出是因为print语句不是同步的。在thread完成两个for循环并从syncronized块中存在之后，counter = 0。

但是在输出counter之前，其他一些线程进入syncronized块并开始递增counter。

这就是前一个thread输出增量counter的原因。

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
}