为什么同步无法正常工作? [英] Why is synchronized not working properly?

问题描述

这是我的代码:

private int count = 0;

  public synchronized void increment() {
      count++;
  }

 public void doWork() throws InterruptedException {

    Thread t1 = new Thread(new Runnable() {
        public void run() {
            for (int i = 0; i < 5; i++) {
                increment();
                System.out.println(count+"  "+Thread.currentThread().getName());
            }}});

    Thread t2 = new Thread(new Runnable() {
        public void run() {
            for (int i = 0; i < 5; i++) {
                increment();
                System.out.println(count+"  "+Thread.currentThread().getName());
            }}});

    t1.start();
    t2.start();
}

这是我的输出:

2  Thread-1
2  Thread-0
3  Thread-1
5  Thread-1
6  Thread-1
4  Thread-0
8  Thread-0
9  Thread-0
7  Thread-1
10  Thread-0

我的理解是increment是synchronized.因此，它应该首先increment一个数字，然后释放lock，然后将lock赋予线程t1或t2.因此，应该一次increment一个数字，对吧?

My understanding is that increment is synchronized. So, it should first increment one number and then release the lock and then give the lock to the thread t1 or t2. So, it should increment one number at a time, right?

但是为什么我的代码incrementing一次是两个或三个数字?我做错什么了吗(我是新手)?

But why is my code incrementing two or three numbers at a time? Am I doing something wrong (I'm a newbie)?

推荐答案

虽然count++;实际上是同步的，但System.out.println(count+" "+Thread.currentThread().getName());不是同步的，但是它可以访问count变量.

While count++; indeed is synchronized System.out.println(count+" "+Thread.currentThread().getName()); is not, but it access to count variable.

即使您同步了访问权限，它也无济于事，因为下一种情况仍然可能:

Even if you synchronize access, it won't help you because next scenario will be still possible:

• 线程1增量
• 线程2增量
• 线程1的打印值2
• 线程2的打印值2

• Thread 1 increment
• Thread 2 increment
• Thread 1 print value 2
• Thread 2 print value 2

要解决此问题，您需要在相同的同步部分中进行递增和打印.例如，您可以将System.out.println(count+" "+Thread.currentThread().getName());放入increment方法中.

To fix this issue you need to increment and print in same synchronized section. For example you can put System.out.println(count+" "+Thread.currentThread().getName()); into increment method.

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