java - 在 Java 中制作偶数和奇数线程以自然顺序打印数字

Question

我知道之前有人问过这个问题，但我无法弄清楚为什么我的解决方案对我不起作用。我有两个线程偶数和奇数，一个打印偶数，另一个打印奇数。当我启动线程时，我希望输出按数字的自然顺序排列，例如 0 1 2 3.. 等。这是我的代码:- [更新]

public class ThreadCommunication {


public static void main(String... args) throws InterruptedException
{
    final ThreadCommunication obj = new ThreadCommunication();
    Thread even = new Thread(){
        @Override
        public void run()
        {
            for(int i=0;i<10;i=i+2){
                synchronized(obj){
                    System.out.println(i);
                    try {
                        obj.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
        }
    };
    Thread odd = new Thread(){
        @Override
        public void run()
        {
            for(int i=1;i<10;i=i+2){
                synchronized(obj){
                    System.out.println(i);
                    obj.notify();
                }
            }

        }
    };
    even.start();
    odd.start();
}

当我运行上面的代码时，有时它会按预期的自然顺序打印数字，但有时会以其他顺序打印，例如:

0
1
3
5
7
9
2

我在这里做错了什么？

Answer 1

编辑:

volatile static boolean isAlreadyWaiting = false;



Thread even = new Thread() {
            @Override
            public void run() {
                synchronized (obj) {
                    for (int i = 0; i < 10; i = i + 2) {
                        System.out.println(i);
                        try {
                            if (!isAlreadyWaiting) {
                                isAlreadyWaiting = true;
                                obj.wait();
                            }
                            obj.notify();
                            isAlreadyWaiting=false;

                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }

                    }
                }
            }
        };
        Thread odd = new Thread() {
            @Override
            public void run() {
                synchronized (obj) {
                    for (int i = 1; i < 10; i = i + 2) {
                        System.out.println(i);
                        try {
                            if(isAlreadyWaiting){
                            obj.notify();
                            isAlreadyWaiting = false;
                            }
                            if (!isAlreadyWaiting) {
                                isAlreadyWaiting = true;
                                obj.wait();
                            }

                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }

                    }
                }
            }
        };

检查文档

public class IllegalMonitorStateException extends RuntimeException

Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.

Monitor 属于obj

所以你应该打电话

obj.wait();

和

obj.notify();

有关所有权的更多信息

This methods (wait or notify) should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:

1. By executing a synchronized instance method of that object.

2. By executing the body of a synchronized statement that synchronizes on the object.

   3. For objects of type Class, by executing a synchronized static method of that class.

一次只有一个线程可以拥有一个对象的监视器。