使用wait（）和notify（）进行同步

Question

我需要使用3个不同的线程来打印以下模式：

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。我从下面的代码开始，但是似乎它们只打印一次，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)

                {

   

有人可以解释如何解决这个问题吗？

Answer 1

我已经修复了您的代码。您已将“ m.notify（）”放在“ m.wait（）”之后，因此它们都互相等待。我在“ m.wait（）”之前轻轻移动了它，并将for循环转换为无尽的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3次。public class MultiThreading_2 {&nbsp; &nbsp; static volatile boolean flag = false;&nbsp; &nbsp; static volatile String word = "I";&nbsp; &nbsp; public static void main(String[] args) throws InterruptedException {&nbsp; &nbsp; &nbsp; &nbsp; MultiThreading_2 m = new MultiThreading_2();&nbsp; &nbsp; &nbsp; &nbsp; Runnable a = new Runnable() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public void run() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for (int i = 0; i < 3; i++) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; synchronized (m) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (word.equals("I")) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; System.out.print("I ");&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; word = "LOVE";&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.notify();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.wait();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } catch (InterruptedException e) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // TODO Auto-generated catch block&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; e.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } else {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; i--;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; };&nbsp; &nbsp; &nbsp; &nbsp; Runnable b = new Runnable() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public void run() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for (int i = 0; i < 3; i++) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; synchronized (m) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (word.equals("LOVE")) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; System.out.print("LOVE ");&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; word = "WORLD";&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.notify();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.wait();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } catch (InterruptedException e) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // TODO Auto-generated catch block&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; e.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } else {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; i--;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; };&nbsp; &nbsp; &nbsp; &nbsp; Runnable c = new Runnable() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public void run() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for (int i = 0; i < 3; i++) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; synchronized (m) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (word.equals("WORLD")) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; System.out.println("WORLD ");&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; word = "I";&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.notify();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.wait();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } catch (InterruptedException e) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // TODO Auto-generated catch block&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; e.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } else {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; i--;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; };&nbsp; &nbsp; &nbsp; &nbsp; new Thread(a).start();&nbsp; &nbsp; &nbsp; &nbsp; Thread.sleep(100);&nbsp; &nbsp; &nbsp; &nbsp; new Thread(b).start();&nbsp; &nbsp; &nbsp; &nbsp; Thread.sleep(100);&nbsp; &nbsp; &nbsp; &nbsp; new Thread(c).start();&nbsp; &nbsp; }}

Answer 2

Object.notify()选择一个随机线程唤醒。由于两次启动之间的睡眠，您的代码只能工作一次，因此您可以人为地确保预期的执行顺序。您应该使用notifyAll()，唤醒所有线程，因此，线程应该wait()循环循环，直到轮到为止：Runnable a = new Runnable() {&nbsp; public void run() {&nbsp; &nbsp; synchronized (m) {&nbsp; &nbsp; &nbsp; for (int i = 1; i <= 2; i++) {&nbsp; &nbsp; &nbsp; &nbsp; while(!word.equals("I"))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.wait();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; catch(InterruptedException ie){&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ie.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; System.out.print("I ");&nbsp; &nbsp; &nbsp; &nbsp; word = "LOVE";&nbsp; &nbsp; &nbsp; &nbsp; m.notifyAll();&nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; }};Runnable b = new Runnable() {&nbsp; public void run() {&nbsp; &nbsp; synchronized (m) {&nbsp; &nbsp; &nbsp; for (int i = 1; i <= 2; i++) {&nbsp; &nbsp; &nbsp; &nbsp; while(!word.equals("LOVE"))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.wait();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; catch(InterruptedException ie){&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ie.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; System.out.print("LOVE ");&nbsp; &nbsp; &nbsp; &nbsp; word = "WORLD";&nbsp; &nbsp; &nbsp; &nbsp; m.notifyAll();&nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; }};Runnable c = new Runnable() {&nbsp; public void run() {&nbsp; &nbsp; synchronized (m) {&nbsp; &nbsp; &nbsp; for (int i = 1; i <= 2; i++) {&nbsp; &nbsp; &nbsp; &nbsp; while(!word.equals("WORLD"))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.wait();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; catch(InterruptedException ie){&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ie.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; System.out.println("WORLD ");&nbsp; &nbsp; &nbsp; &nbsp; word = "I";&nbsp; &nbsp; &nbsp; &nbsp; m.notifyAll();&nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; }};new Thread(a).start();new Thread(b).start();new Thread(c).start();在if从一开始就已经被删除，在真正的并发执行难保有在预期值word。然后，其中一个线程抓住该锁，检查是否word是自己的那个线程，或者开始等待，或者打印其文本和步骤word以进入下一阶段，从而唤醒其他线程（两个线程都带有notifyAll()）。然后它要么退出，要么继续wait()再次。这是重要的部分：我想尽可能少地修改您的代码，以便所有事情都在同步块内发生，这意味着线程仅在其他两个正在等待或完成时才可以运行。对于此锁步，它可以工作，但通常而言，同步块应该位于for循环内，可能是两个单独的块，一个围绕equals-wait循环，另一个围绕set + notifyAll语句。