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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