Java实现多线程轮流打印1-100的数字操作

首先打印1-100数字如果用一个单线程实现那么只要一个for循环即可，那么如果要用两个线程打印出来呢？（一个线程打印奇数，一个线程打印偶数）于是大家会想到可以通过加锁实现，但是这样的效率是不是不高？这里我用一个变量来控制两个线程的输出

public class ThreadTest {
 volatile int flag=0;
 public void runThread() throws InterruptedException{
   Thread t1=new Thread(new Thread1());
   Thread t2=new Thread(new Thread2());
   t1.start();
   t2.start();
 }
 public class Thread1 implements Runnable{
 
 public void run() {
  int i=0;
  while(i<=99){
  if(flag==0)
  {
   System.out.println("t1="+i+"flag="+flag);
   i+=2;
   flag=1;
  }
  }
 } 
 }
 
 public class Thread2 implements Runnable{
 
 public void run() {
  int i=1;
  while(i<=99){
  if(flag==1)
  {
   System.out.println("t2="+i+"flag="+flag);
   i+=2;
   flag=0;
  }
  }
 }
 
 }
}

那么如果要实现三个线程轮流打印1-100的数字呢？是不是也可以用上面的方法实现呢？代码如下

public class ThreadTest {
 private int i=0;
 private Thread thread1,thread2,thread3;
 private int flag=0;
 public void runThread() throws InterruptedException{
   thread1=new Thread(new Thread1());
   thread2=new Thread(new Thread2());
   thread3=new Thread(new Thread3());
   thread1.start();
   thread2.start();
   thread3.start();
 }
 public class Thread1 implements Runnable{
 
 public void run() {
  
  while(i<=100){
  if(flag==0) {
   System.out.println("t1="+i);
   i++;
   flag=1;
  }
  }
 } 
 }
 
 public class Thread2 implements Runnable{
 
 public void run() {
  
  while(i<=100){
  if(flag==1) {
   System.out.println("t2="+i);
   i++;
   flag=2;
  }
  }
 } 
 }
 
 public class Thread3 implements Runnable{
 
 public void run() {
  
  while(i<=100){
  if(flag==2) {
   System.out.println("t3="+i);
   i++;
   flag=0;
  }
  }
 }
 
 }
}

运行结果

发现三个线程只打印了一次就停止不输出了，是什么原因呢？

可以用jdk自带的jstack来看看线程的状态，在windows系统中可以打开cmd然后进入jdk所在目录，然后执行Jsp，能查看到各线程id，然后执行jstack -F pid就可以看的状态了

可以看到几个Thread state是BLOCKED，就是阻塞了，什么原因呢？

尴尬发现flag变量和i变量前面忘记加volatile，导致flag和i被线程读取修改时，其他线程不可见，所以才导致上面的问题出现。

在JVM中每个线程读取变量到cache中时相互都是不可见的，也就是java五大内存区中的程序计数器区域对于每个线程都是独立的不共享的，只有堆内存区和方法区是对所有线程都是共享的。

当线程1读取了flag和i的值，并对其进行修改的时候，线程2并发运行，并不知道flag和i值已经改变，导致多线程数据不一致的情况，所以加了volatile后，当线程读取变量进行修改后会“通知”其它线程这个值已经进行了修改。

import java.util.concurrent.atomic.AtomicInteger; 
public class ThreadTest {
 private volatile int i=0;
 private Thread thread1,thread2,thread3;
 private volatile int flag=0;
 public void runThread() throws InterruptedException{
   thread1=new Thread(new Thread1());
   thread2=new Thread(new Thread2());
   thread3=new Thread(new Thread3());
   thread1.start();
   thread2.start();
   thread3.start();
 }
 public class Thread1 implements Runnable{
 
 public void run() {
  while(i<100){
  if(flag==0) {
   System.out.println("t1="+i);
   i++;
   flag=1;
  }
  }
 }
 
 }
 
 public class Thread2 implements Runnable{
 
 public void run() {
  
  while(i<100){
  if(flag==1){
   System.out.println("t2="+i);
   i++;
   flag=2;
  }
  }
 }
 
 }
 
 public class Thread3 implements Runnable{
 
 public void run() {
  
  while(i<100){
  if(flag==2){
   System.out.println("t3="+i);
   i++;
   flag=0;
  }
  }
 }
 
 }
}

运行结果

-----未完-----

补充知识：Java n个线程轮流打印数字的问题

一、两个线程轮流打印数字。

加锁实现：

package lianxi;
 
/*
 * 用锁实现两个线程轮流打印1——100
 */
public class Print1TO100TwoThread {
 private Object lock = new Object();
 private int i = 0;
 
 Thread threadA = new Thread(new Runnable() {
 @Override
 public void run() {
  while (i <= 100) {
  synchronized (lock) {
 
   try {
   if (i > 100)
    break;
   System.out.println("threadA :" + (i++));
   lock.notify();
   lock.wait();
   } catch (InterruptedException e) {
   e.printStackTrace();
   }
  }
  }
 }
 
 });
 
 Thread threadB = new Thread(new Runnable() {
 @Override
 public void run() {
  while (i <= 100) {
  synchronized (lock) {
 
   try {
   if (i > 100)
    break;
   System.out.println("threadB :" + (i++));
   lock.notify();
   lock.wait();
   } catch (InterruptedException e) {
   e.printStackTrace();
   }
  }
  }
 }
 });
 
 public void startTwoThread() throws InterruptedException {
 threadA.start();
 Thread.sleep(20);
 threadB.start();
 }
 public static void main(String[] args) throws InterruptedException {
 new Print1TO100TwoThread().startTwoThread();
 } 
}

用锁效率太低，用一个变量来控制打印的顺序。

package lianxi;
/*
 * 用两个线程轮流打印1——10；用所实现效率太低，用变量来控制
 */
public class PrinntNumTwoThread {
 
 private volatile int num = 0;
 private volatile boolean flag = false;
 
 Thread threadA = new Thread(new Runnable() {
 
 @Override
 public void run() {
  while (true) {
  if (num > 10)
   return;
  if (!flag) {
   System.out.println("threadA-->" + ":" + (num++));
   flag = !flag;
  }
  }
 }
 
 });
 
 Thread threadB = new Thread(new Runnable() {
 @Override
 public void run() {
  while (true) {
  if (num > 10)
   return;
  if (flag) {
   System.out.println("threadB-->" + ":" + (num++));
   flag = !flag;
  }
  }
 }
 
 });
 
 public void startTwoThread() {
 threadA.start();
 threadB.start();
 }
 
 public static void main(String[] args) {
 new PrinntNumTwoThread().startTwoThread();
 }
}

二、那么如果要实现三个线程轮流打印1-100的数字呢？

package lianxi; 
public class PrintNumThreeThread {
 private volatile int i = 0;
 private volatile int flag = 0;
 Thread threadA = new Thread(new Runnable() {
 @Override
 public void run() {
  while (true) {
  if (i > 100)
   return;
  if (flag == 0) {
   System.out.println("threadA->" + ":" + (i++));
   flag = 1;
  }
  }
 }
 
 });
 
 Thread threadB = new Thread(new Runnable() {
 @Override
 public void run() {
  while (true) {
  if (i > 100)
   return;
  if (flag == 1) {
   System.out.println("threadB->" + ":" + (i++));
   flag = 2;
  }
  }
 }
 
 });
 
 Thread threadC = new Thread(new Runnable() {
 @Override
 public void run() {
  while (true) {
  if (i > 100)
   return;
  if (flag == 2) {
   System.out.println("threadC->" + ":" + (i++));
   flag = 0;
  }
  }
 }
 });
 
 public void startThreeThread() {
 threadA.start();
 threadB.start();
 threadC.start();
 }
 
 public static void main(String[] args) {
 new PrintNumThreeThread().startThreeThread();
 }
}

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