Java實現多執行緒輪流列印1-100的數字操作
首先列印1-100數字如果用一個單執行緒實現那麼只要一個for迴圈即可,那麼如果要用兩個執行緒打印出來呢?(一個執行緒列印奇數,一個執行緒列印偶數)於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?這裡我用一個變數來控制兩個執行緒的輸出
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; } } } } }
那麼如果要實現三個執行緒輪流列印1-100的數字呢?是不是也可以用上面的方法實現呢?程式碼如下
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; } } } } }
執行結果
發現三個執行緒只打印了一次就停止不輸出了,是什麼原因呢?
可以用jdk自帶的jstack來看看執行緒的狀態,在windows系統中可以開啟cmd然後進入jdk所在目錄,然後執行Jsp,能檢視到各執行緒id,然後執行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,什麼原因呢?
尷尬發現flag變數和i變數前面忘記加volatile,導致flag和i被執行緒讀取修改時,其他執行緒不可見,所以才導致上面的問題出現。
在JVM中每個執行緒讀取變數到cache中時相互都是不可見的,也就是java五大記憶體區中的程式計數器區域對於每個執行緒都是獨立的不共享的,只有堆記憶體區和方法區是對所有執行緒都是共享的。
當執行緒1讀取了flag和i的值,並對其進行修改的時候,執行緒2併發執行,並不知道flag和i值已經改變,導致多執行緒資料不一致的情況,所以加了volatile後,當執行緒讀取變數進行修改後會“通知”其它執行緒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,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; } } } } }
執行結果
-----未完-----
補充知識:Java n個執行緒輪流列印數字的問題
一、兩個執行緒輪流列印數字。
加鎖實現:
package lianxi; /* * 用鎖實現兩個執行緒輪流列印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,用一個變數來控制列印的順序。
package lianxi; /* * 用兩個執行緒輪流列印1——10;用所實現效率太低,用變數來控制 */ 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、那麼如果要實現三個執行緒輪流列印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實現多執行緒輪流列印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,也希望大家多多支援我們。