java - synchronized 关键字给出了预期的输出，但对不同线程调用方法的顺序不满意

Question

下面是我实现 Runnable 接口的 3 个类（线程）：

public class Thread1 implements Runnable {
Shared s;

public Thread1(Shared s){
    this.s = s;
}

@Override   
public void run() {
    System.out.println("Sum of 10 and 20 by " + Thread.currentThread().getName() + " is "+ s.add(10, 20));
}

}

public class Thread2 implements Runnable {
Shared s;

public Thread2(Shared s){
    this.s = s;
}
    
@Override
public void run() {
    System.out.println("Sum of 100 and 200 by " + Thread.currentThread().getName() + " is " + s.add(100, 200));
}

}

public class Thread3 implements Runnable {
Shared s;

public Thread3(Shared s){
    this.s = s;
}
    
@Override
public void run() {
    System.out.println("Sum of 1000 and 2000 by " + Thread.currentThread().getName() + " is " + s.add(1000, 2000));
}

}

下面是这些线程之间共享其对象的类：

public class Shared {
private int x;
private int y;

synchronized public int add(int a, int b) {
    x = a;
    y = b;

    try {
        System.out.println(Thread.currentThread().getName() + "is going into sleep state");
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    return x + y;
}

}

最后这就是我启动线程并传递共享对象的方式：

    public static void main(String[] args) {
    Shared s = new Shared();
    Thread t1 = new Thread(new Thread1(s), "Thread-1");
    Thread t2 = new Thread(new Thread2(s), "Thread-2");
    Thread t3 = new Thread(new Thread3(s), "Thread-3");
    
    t1.start();
    t2.start();
    t3.start();
}

这是我得到的正确输出：

      Thread-2is going into sleep state
      Thread-1is going into sleep state
      Sum of 100 and 200 by Thread-2 is 300
      Thread-3is going into sleep state
      Sum of 10 and 20 by Thread-1 is 30
      Sum of 1000 and 2000 by Thread-3 is 3000

但是如果你在这里看到，Thread-1 在 Thread-2 完成它的工作之前就开始执行 add 方法（它是同步的）。由于 Thread-2 进入了睡眠状态，所以它自己也获得了锁，不应允许其他线程进入 add(...) 方法。Thread-1 或 Thread-3 只能在 Thread-2 完成后开始执行 add(...) 方法。所以，我期待的输出是：

      Thread-2is going into sleep state
      Sum of 100 and 200 by Thread-2 is 300
      Thread-1is going into sleep state
      Sum of 10 and 20 by Thread-1 is 30
      Thread-3is going into sleep state
      Sum of 1000 and 2000 by Thread-3 is 3000

请告诉我做错了什么，或者这就是输出的方式，如果是，请告诉原因。

标签： javamultithreadingsynchronizationsynchronized

Answer 1

原因是System.out.println()速度很慢。

您可以使用它从 run() 方法和add(). 无法保证 sysout 中的消息一致。

我重写Shared如下：

public class Shared {
    private int x;
    private int y;

    synchronized public int add(int a, int b) {
        x = a;
        y = b;

        try {
            long now = System.currentTimeMillis() - start;

            Thread.sleep(1000);

            // notice FIRST sleep and THEN sysout
            System.out.println(Thread.currentThread().getName() + " calculation has taken place at time " + now);

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return x + y;
    }

    public static long start;

    public static void main(String[] args) {

        start = System.currentTimeMillis();

        Shared s = new Shared();
        Thread t1 = new Thread(new Thread1(s), "Thread-1");
        Thread t2 = new Thread(new Thread2(s), "Thread-2");
        Thread t3 = new Thread(new Thread3(s), "Thread-3");

        t1.start();
        t2.start();
        t3.start();
    }
}

每个线程现在看起来像：

public class Thread1 implements Runnable {
    Shared s;

    public Thread1(Shared s) {
        this.s = s;
    }

    @Override
    public void run() {
        int result = s.add(10, 20);
        long now= System.currentTimeMillis()-Shared.start;
        System.out.println("Sum of 10 and 20 by " + Thread.currentThread().getName() + " is " + result+ " found at "+ now);
    }
}

生成以下输出：

Thread-1 calculation has taken place at time 2
Sum of 10 and 20 by Thread-1 is 30 found at 1005
Thread-2 calculation has taken place at time 1005
Sum of 100 and 200 by Thread-2 is 300 found at 2006
Thread-3 calculation has taken place at time 2006
Sum of 1000 and 2000 by Thread-3 is 3000 found at 3007

add()每个线程都按应有的方式阻塞。

只有稍后显示结果时，下一个线程已经开始计算。

请注意，add()现在首先是 sysout（很慢），它发生在睡眠期间，这给了它足够的时间。