Java同步帐户示例没有按预期工作

Question

class Account {

public int dollars;

public Account(int d) {
    dollars = d;
}

// might fail due to concurrently accessing this method
public synchronized void deduct2(int amount) {
      dollars -= amount;
}

@Override
public String toString() {
    return "" + dollars;
}

 class MyThread extends Thread {
static Account account = new Account(10000);

private int id;

public MyThread(int i) {
    id = i;
}

@Override
public void run() {
    System.out.println("performing my job ....");
    for (int i = 0; i < 100; i++) {
        account.deduct2(1);
        System.out.println("account " + account + "  " + getName() +"performing....");
    }
}

ThreadTest类

public class ThreadTest {

public static void main(String args[]) {
    new ThreadTest().exec();
    System.out.println("main finished");
}

private void exec() {
    test1();
}

private void test1() {
    Thread thread1 = new MyThread(1);
    Thread thread2 = new MyThread(2);
    Thread thread3 = new MyThread(3);
    thread1.start();
    thread2.start();
    thread3.start();

}}

结果

主要完成
执行我的作业....
执行我的作业....
执行我的作业....
帐户9997线程-0执行....
帐户9997线程-2执行....
帐户9995线程-1执行....
帐户9993线程-2执行....
帐户9994线程-1执行....
帐户9992线程-1执行....
帐户9992线程-1执行....
帐户9992线程-2执行....
帐户9992线程-2执行....
帐户9989线程-2执行....2执行...
帐户9988线程-0执行...
帐户9985线程-2执行...
帐户9986线程-1执行...
...
帐户9713线程-1执行...
帐户9708线程-1执行...
帐户9709线程-0执行...
帐户9706线程-0执行...
帐户9707线程-1执行...
帐户9707线程-1执行...
帐户9704线程-1执行...
帐户9704线程-1执行...
帐户9705线程-0执行....
帐户9702线程-0执行....
帐户9703线程-1执行....
帐户9701线程-0执行....
帐户9700线程-1执行....

我运行三个不同的新线程，每个新线程100次扣1。所以帐户美元的结果是正确的（9700）。
但我很困惑为什么账户美元被扣除的过程没有像我预期的那样工作。我想它会像9999 999 8 999 7......

Answer 1

您的system.out.println不同步，例如：

1. 开始时dollers是10000，线程1和线程2执行帐户扣减2(1)；现在dollers是9998。
2. 线程3执行帐户扣减2(1)；现在dollers是9997。
3. 线程1和线程2开始打印帐户#美元，您将看到两个9997.

如果要按顺序打印，请将synchronized放入for循环。(帐户#duct2不再需要添加synchronized。）：

       for (int i = 0; i < 100; i++) {
            synchronized (account) {
                account.deduct2(1);
                System.out.println("account " + account + "  " + getName() + "performing....");
            }
        }