首先打印1-100數字如果用一個單線程實現那么只要一個for循環即可，那么如果要用兩個線程打印出來呢？（一個線程打印奇數，一個線程打印偶數）于是大家會想到可以通過加鎖實現，但是這樣的效率是不是不高？這里我用一個變量來控制兩個線程的輸出

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; } } } }}

那么如果要實現三個線程輪流打印1-100的數字呢？是不是也可以用上面的方法實現呢？代碼如下

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; } } } }}

運行結果

發現三個線程只打印了一次就停止不輸出了，是什么原因呢？

可以用jdk自帶的jstack來看看線程的狀態，在windows系統中可以打開cmd然后進入jdk所在目錄，然后執行Jsp，能查看到各線程id，然后執行jstack -F pid就可以看的狀態了

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

尷尬發現flag變量和i變量前面忘記加volatile，導致flag和i被線程讀取修改時，其他線程不可見，所以才導致上面的問題出現。

在JVM中每個線程讀取變量到cache中時相互都是不可見的，也就是java五大內存區中的程序計數器區域對于每個線程都是獨立的不共享的，只有堆內存區和方法區是對所有線程都是共享的。

當線程1讀取了flag和i的值，并對其進行修改的時候，線程2并發運行，并不知道flag和i值已經改變，導致多線程數據不一致的情況，所以加了volatile后，當線程讀取變量進行修改后會“通知”其它線程這個值已經進行了修改。

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; } } } }}

運行結果

-----未完-----

補充知識：Java n個線程輪流打印數字的問題

一、兩個線程輪流打印數字。

加鎖實現：

package lianxi; /* * 用鎖實現兩個線程輪流打印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(); } }

用鎖效率太低，用一個變量來控制打印的順序。

package lianxi;/* * 用兩個線程輪流打印1——10；用所實現效率太低，用變量來控制 */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、那么如果要實現三個線程輪流打印1-100的數字呢？

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(); }}

以上這篇Java實現多線程輪流打印1-100的數字操作就是小編分享給大家的全部內容了，希望能給大家一個參考，也希望大家多多支持好吧啦網。