Java 线程无法正确运行

Question

我想验证 Thread机制，所以我做了一个Company类如下:

public class Company {
    static void p(String s){ System.out.println(s); }

    interface IWork{ void work(); }
    interface OnReportListener{ int onEnd(Worker w); }
    static class Job{ int effertCount, budget=100; }

    static class Worker implements IWork{
        String name; Job job=new Job(); OnReportListener listener; boolean isOver;
        public Worker(String n, OnReportListener l) {
            name = n; listener = l;
        }

        public void work() {
            new Thread(){
                public void run() {
                    while (!isOver) {
                        int spent = (int) Math.round(Math.random()*7-2) ;
                        if (spent<0) p(name+": I earned $"+(-spent));
                        isOver = (job.budget-=spent) <=0;
                        job.effertCount++;
                    }
                    p(name+": OMG, I got the salary $"+ listener.onEnd(Worker.this));
                }
            }.start();
        }
    }

    static class Boss implements IWork, OnReportListener{
        Set<Worker> members; int endCount;
        public Boss(Set<Worker> s){ members = s;}
        public int onEnd(Worker w) {
            p("Boss: "+w.name+", thanks for your effort, you deserve it!");
            endCount++;
            return w.job.effertCount*10;
        }

        public void work() {
            new Thread(){
                public void run() {
                    while (endCount<members.size()) { /*fool around*/ }
                    p("Boss: It's time to go home!");
                }
            }.start();
        }
    }

    public static void main(String[] args) {
        Set<Worker> workers = new HashSet<Worker>();
        Boss boss = new Boss(workers);
        Worker tom = new Worker("Tom", boss); 
        workers.add(tom); // hire Tom
        Worker mary = new Worker("Mary", boss); 
        workers.add(mary); // hire Mary

        p("Company.main: Start to work!");
        boss.work();
        tom.work();
        mary.work();
        p("Company.main: End of the assigning");
    }
}

当我运行该应用程序时，我得到了意想不到的结果:

Company.main: Start to work!
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $2
Tom: I earned $2
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $2
Tom: I earned $2
Tom: I earned $1
Tom: I earned $1
Tom: I earned $1
Tom: I earned $2
Tom: I earned $1
Boss: Tom, thanks for your effort, you deserve it!
Tom: OMG, I got the salary $770
Mary: I earned $1
Mary: I earned $2
Mary: I earned $2
Mary: I earned $1
Mary: I earned $1
Mary: I earned $1
Mary: I earned $2
Mary: I earned $1
Mary: I earned $2
Mary: I earned $1
Mary: I earned $1
Mary: I earned $2
Boss: Mary, thanks for your effort, you deserve it!
Mary: OMG, I got the salary $510
Company.main: End of the assigning

但在另一种实践中，ThreadTest类:

public class ThreadTest extends Thread{
    static void p(String s){ System.out.println(s); }
    public ThreadTest(String s){ super(s); }
    public void run() {
        for (int i = 0; i < 25; i++) p(getName()+": "+i);
    }

    public static void main(String[] args) {
        p("Main: Start!");
        new ThreadTest("t1").start();
        new ThreadTest("t2").start();
        p("Main: Finish!");
    }
}

我运行它并得到:

Main: Start!
t1: 0
t1: 1
t1: 2
Main: Finish!
t2: 0
t2: 1
t2: 2
t2: 3
t2: 4
t2: 5
t1: 3
t1: 4
t1: 5
t1: 6
t2: 6
t2: 7
t2: 8
t2: 9
t2: 10
t2: 11
t2: 12
t2: 13
t2: 14
t2: 15
t2: 16
t2: 17
t2: 18
t2: 19
t2: 20
t2: 21
t2: 22
t2: 23
t2: 24
t1: 7
t1: 8
t1: 9
t1: 10
t1: 11
t1: 12
t1: 13
t1: 14
t1: 15
t1: 16
t1: 17
t1: 18
t1: 19
t1: 20
t1: 21
t1: 22
t1: 23
t1: 24

这些让我困惑:

1. 我期望Company的主线程类应该在每个 IWork 对象开始工作后结束，但似乎没有。
2. 我希望汤姆和玛丽应该一起工作，但结果是玛丽在汤姆工作结束后工作。
3. 看来老板从来没有停止过工作......
4. [更新/追加这个问题:]我不必为 ThreadTest 添加 Thread.yield() 或 Thread.sleep() t1/t2/主线程可以单独运行。

如何修改我的代码 Company就让它如我所愿(问题1~3)，为什么？

非常感谢。

Answer 1

唯一的问题是 endCount 上没有同步，所以当 Tom和Mary调用onEnd方法Boss和增量endCount ，工作Boss可能没有注意到。

您可以使用AtomicInteger ,

AtomicInteger endCount

和

• 使用endCount.incrementAndGet()相反 endCount++
• 使用endCount.get() < members.size()反而 endCount<members.size()

因此 JMM 可以保证 Boss在循环中获取新值。

<小时/>

并且，正如评论中所建议的，您可以将其添加到 Worker 的循环中，会更容易模拟multi-thread环境:

try {
    Thread.sleep(10);
} catch (Exception e) {

}

<小时/>

更新

当启动多个线程时，如果不进行同步，则无法控制各行的执行顺序。它们的执行顺序是由CPU调度的。您可以查看this .

即使在你的第二次测试中，除了 Main: Start!保证显示在第一行，其他行的顺序仍然不确定。

并且，Thread.sleep或Thread.yield只会让模拟并发执行变得更容易，但仍然不能保证 Tom和Mary将在控制台上逐行输出一些内容。

<小时/>

这是我电脑上的测试结果:

Company.main: Start to work!
Company.main: End of the assigning
Tom: I earned $2
Mary: I earned $1
Mary: I earned $1
Mary: I earned $1
Tom: I earned $1
Tom: I earned $2
Mary: I earned $1
Tom: I earned $1
Tom: I earned $2
Mary: I earned $1
Mary: I earned $2
Tom: I earned $1
Mary: I earned $2
Mary: I earned $1
Mary: I earned $1
Mary: I earned $2
Mary: I earned $2
Mary: I earned $2
Tom: I earned $1
Tom: I earned $1
Mary: I earned $1
Tom: I earned $1
Mary: I earned $1
Tom: I earned $1
Mary: I earned $2
Tom: I earned $1
Mary: I earned $1
Tom: I earned $1
Mary: I earned $2
Mary: I earned $1
Tom: I earned $1
Boss: Tom, thanks for your effort, you deserve it!
Tom: OMG, I got the salary $590
Mary: I earned $1
Boss: Mary, thanks for your effort, you deserve it!
Mary: OMG, I got the salary $770
Boss: It's time to go home!