Java线程使用同步锁交替执行打印奇数偶数的方法

对同一个对象进行多线程操作时，如何保证线程执行结果的一致性？我们需要对线程操作对象加同步锁。（这是一道面试题）

需求描述

1-20个数字

A线程打印奇数：1，3，5，7，9，11，13，15，17，19

B线程打印偶数：2，4，6，8，10，12，14，16，18，20

C线程在AB两个线程执行完了之后打印结果：“success”。

线程代码实现

Num.java

package com.boonya.thread.test;

/**
 * @ClassName: Num
 * @Description: TODO(加锁计算对象)
 * @author: pengjunlin
 * @motto: 学习需要毅力，那就秀毅力
 * @date 2019-01-14 22:47
 */
public class Num {
  int value=1;
}

AThread.java

package com.boonya.thread.test;

/**
* @ClassName: AThread
* @Description: TODO(奇数线程)
* @author: pengjunlin
* @motto: 学习需要毅力，那就秀毅力
* @date 2019-01-14 22:26
*/
public class AThread implements Runnable{

  Num num;

  public AThread(Num num){
    this.num=num;
  }

  public void run() {
    while (num.value<20){
      synchronized (num){
        if(num.value%2!=0){
          System.out.println("AThread:"+ num.value);
          num.value++;
        }
      }
    }

  }
}

BThread.java

package com.boonya.thread.test;

/**
 * @ClassName: BThread
 * @Description: TODO(偶数线程)
 * @author: pengjunlin
 * @motto: 学习需要毅力，那就秀毅力
 * @date 2019-01-14 22:26
 */
public class BThread implements Runnable {

    Num num;

    public BThread(Num num){
      this.num=num;
    }

    public void run() {
      while (num.value<=20){
        synchronized (num){
          if(num.value%2==0){
            System.out.println("BThread:"+ num.value);
            num.value++;
          }
        }
      }
    }
 }

CThread.java

package com.boonya.thread.test;

/**
 * @ClassName: CThread
 * @Description: TODO(等待结果线程)
 * @author: pengjunlin
 * @motto: 学习需要毅力，那就秀毅力
 * @date 2019-01-14 22:26
 */
public class CThread implements Runnable {

    Num num;

    public CThread(Num num){
      this.num=num;
    }

    public void run() {
      while (num.value<20){
        try {
          Thread.sleep(1);
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
      System.out.println("CThread:success!");
    }
  }

MainTest.java

package com.boonya.thread.test;

import java.util.ArrayList;
import java.util.List;

/**
 * @ClassName: MainTest
 * @Description: TODO(线程测试)
 * @author: pengjunlin
 * @motto: 学习需要毅力，那就秀毅力
 * @date 2019-01-14 21:54
 */
public class MainTest {

  public static void main(String[] args) {
    Num num=new Num();
    Thread a=new Thread(new AThread(num));
    Thread b=new Thread(new BThread(num));
    Thread c=new Thread(new CThread(num));
    a.start();
    b.start();
    c.start();
  }
}

测试结果

"C:\Program Files\Java\jdk1.8.0_121\bin\java" "-javaagent:C:\Users\boonya\AppData\Roaming\JetBrains\IntelliJ IDEA 2017.2.5\lib\idea_rt.jar=51911:C:\Users\boonya\AppData\Roaming\JetBrains\IntelliJ IDEA 2017.2.5\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_121\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\rt.jar;C:\Users\boonya\Desktop\JavaAlgorithm\target\test-classes;C:\Users\boonya\Desktop\JavaAlgorithm\target\classes" com.boonya.thread.test.MainTest
AThread:1
BThread:2
AThread:3
BThread:4
AThread:5
BThread:6
AThread:7
BThread:8
AThread:9
BThread:10
AThread:11
BThread:12
AThread:13
BThread:14
AThread:15
BThread:16
AThread:17
BThread:18
AThread:19
BThread:20
CThread:success!

Process finished with exit code 0

注意：synchronized 作为方法块使用时需要只能对对象加锁，不能是常用数据类型。

AB线程改进:使用线程等待和通知

AThread.java

package com.boonya.thread.test;

/**
* @ClassName: AThread
* @Description: TODO(奇数线程)
* @author: pengjunlin
* @motto: 学习需要毅力，那就秀毅力
* @date 2019-01-14 22:26
*/
public class AThread implements Runnable{

  Num num;

  public AThread(Num num){
    this.num=num;
  }

  public void run() {
    while (num.value<20){
      synchronized (num){
        if(num.value%2!=0){
          System.out.println("AThread:"+ num.value);
          num.value++;
          //num.notify();
          num.notifyAll();
        }else{
          try {
            num.wait();
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }
      }
    }

  }
}

BThread.java

package com.boonya.thread.test;

/**
 * @ClassName: BThread
 * @Description: TODO(偶数线程)
 * @author: pengjunlin
 * @motto: 学习需要毅力，那就秀毅力
 * @date 2019-01-14 22:26
 */
public class BThread implements Runnable {

    Num num;

    public BThread(Num num){
      this.num=num;
    }

    public void run() {
      while (num.value<=20){
        synchronized (num){
          if(num.value%2==0){
            System.out.println("BThread:"+ num.value);
            num.value++;
            //num.notify();
            num.notifyAll();
          }else{
            try {
              num.wait();
            } catch (InterruptedException e) {
              e.printStackTrace();
            }
          }
        }
      }
    }
 }

注：线程等待和通知的操作是节省CPU运算的一种方式，避免CPU空转（线程方法一直不停地跑类似于死循环是很可怕的），如果是要让线程停下来需要调用线程的wait()。

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