java - 使用wait()和notify()进行同步

Question

我需要使用 3 个不同的线程打印以下图案:

线程 1 打印“I”

线程 2 打印“LOVE”

线程 3 打印“EARTH”

I LOVE EARTH
I LOVE EARTH
I LOVE EARTH

使用wait()和notify()方法。我从以下代码开始，但似乎只打印一次，因为它们都在每个循环的第一次迭代结束时等待。

public class MultiThreading_2 {
static volatile boolean flag=false;
static volatile String  word = "I";


public static void main(String[] args) throws InterruptedException {

    MultiThreading_2 m = new MultiThreading_2();


    Runnable a = new Runnable() {

        public void run() {
            if(word.equals("I"))
            {
                synchronized(m)
                {
                for(int i=1;i<=2;i++) {

                    if(word.equals("I"))    {   
                        System.out.println("I ");
                        word="LOVE";

                        try {
                            m.wait();
                        } catch (InterruptedException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                        }
                    }
                    m.notify();

                }

                }
                }
        }


    };

    Runnable b = new Runnable() {

        public void run() {

            if(word.equals("LOVE"))
            {
                synchronized(m)
                {

                for(int j=1;j<=2;j++) {

                    if(word.equals("LOVE")) {
                        System.out.println("LOVE ");
                        word="WORLD";
                        try {
                            m.wait();
                        } catch (InterruptedException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                        }
                        m.notify();


                    }

                }


                }
                }
        }



    };

    Runnable c = new Runnable() {

        public void run() {

            if(word.equals("WORLD"))
            {
                synchronized(m)
                {

                for(int k=1;k<=2;k++) {
                        System.out.println("WORLD ");
                        word="I";
                        try {
                            m.wait();
                        } catch (InterruptedException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                        }

                            m.notify();



                    }


                }

                }
                }



    };


    new Thread(a).start();
    Thread.sleep(100);
    new Thread(b).start();
    Thread.sleep(100);
    new Thread(c).start();

}


}

有人可以解释一下如何解决这个问题吗？

Answer 1

我已经修复了你的代码。您已将“m.notify()”放在“m.wait()”之后，以便它们都互相等待。我轻轻地将它移到“m.wait()”之前，并将 for 循环转换为无限的 while 循环，以使线程永远运行。

更新1

我已经更新了代码，以便线程将文本写入三次。

public class MultiThreading_2 {
    static volatile boolean flag = false;
    static volatile String word = "I";

    public static void main(String[] args) throws InterruptedException {

        MultiThreading_2 m = new MultiThreading_2();

        Runnable a = new Runnable() {

            public void run() {

                for (int i = 0; i < 3; i++) {
                    synchronized (m) {
                        if (word.equals("I")) {
                            System.out.print("I ");
                            word = "LOVE";

                            m.notify();
                            try {
                                m.wait();
                            } catch (InterruptedException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            }
                        } else {
                            i--;
                        }

                    }

                }
            }

        };

        Runnable b = new Runnable() {

            public void run() {

                for (int i = 0; i < 3; i++) {
                    synchronized (m) {
                        if (word.equals("LOVE")) {
                            System.out.print("LOVE ");
                            word = "WORLD";
                            m.notify();
                            try {
                                m.wait();
                            } catch (InterruptedException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            }

                        } else {
                            i--;
                        }

                    }

                }
            }

        };

        Runnable c = new Runnable() {

            public void run() {

                for (int i = 0; i < 3; i++) {
                    synchronized (m) {
                        if (word.equals("WORLD")) {
                            System.out.println("WORLD ");
                            word = "I";
                            m.notify();
                            try {
                                m.wait();
                            } catch (InterruptedException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            }

                        } else {
                            i--;
                        }
                    }

                }

            }

        };

        new Thread(a).start();
        Thread.sleep(100);
        new Thread(b).start();
        Thread.sleep(100);
        new Thread(c).start();

    }

}