在Java中同步10個線程

Question

我為什么得到

22291 3091 0 351 0 1423 0 0 0 0

並不是

0 0 0 0 0 0 0 0 0 0

當我在Java中運行此代碼時：

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

您得到該輸出，因為print語句未同步。 thread完成兩個for循環並從syncronized塊存在之后， counter = 0 。
但是，在輸出counter之前，其他線程進入了syncronized塊並開始遞增counter 。
這就是為什么前一個thread顯示增量counter 。

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
}