Java多线程之线程间协作 notify与wait的使用

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（转载请注明出处：http://blog.csdn.net/buptgshengod）

1.背景

Java多线程操作运用很广，特别是在android程序方面。线程异步协作是多线程操作的难点也是关键，也是找工作面试经常考到的地方。下面分享一下我的使用心得。

介绍几个关键字：

synchronized：线程锁，使得系统只执行当前线程。

notifyAll():唤醒其它被锁住的线程

wait():挂起线程

ExecutorService exec=Executors.newCachedThreadPool();:创建线程池

exec.execute( new Runnable() ):将线程放到线程池中管理

2.代码部分

（1）

public class Thread2 {     public void m4t1() {          synchronized(this) {               int i = 5;               while(i>0) {                    System.out.println("1");                    i--;                    try {                         Thread.sleep(500);                    } catch (InterruptedException ie) {                    }               }          }     }     public void m4t2() {        // synchronized(this){         int i = 5;          while( i > 0) {               System.out.println("2");               i--;               try {                    Thread.sleep(500);               } catch (InterruptedException ie) {               }          }     }     public static void main(String[] args) {          final Thread2 myt2 = new Thread2();          Thread t1 = new Thread(  new Runnable() {  public void run() {  myt2.m4t1();  }  }, "t1"  );          Thread t2 = new Thread(  new Runnable() {  public void run() { myt2.m4t2();   }  }, "t2"  );          t1.start();          t2.start();     }}

我们发现，两个线程其中一个上了锁，另一个没有上锁。运行结果如下。说明了虽然一个线程上了锁，但是还是能被让其它未上锁线程访问。

（2）

当我们给两个方法都加上锁

public class Thread2 {     public void m4t1() {          synchronized(this) {               int i = 5;               while(i>0) {                    System.out.println("1");                    i--;                    try {                         Thread.sleep(500);                    } catch (InterruptedException ie) {                    }               }          }     }     public void m4t2() {        synchronized(this){         int i = 5;          while( i > 0) {               System.out.println("2");               i--;               try {                    Thread.sleep(500);               } catch (InterruptedException ie) {               }          }        }     }     public static void main(String[] args) {          final Thread2 myt2 = new Thread2();          Thread t1 = new Thread(  new Runnable() {  public void run() {  myt2.m4t1();  }  }, "t1"  );          Thread t2 = new Thread(  new Runnable() {  public void run() { myt2.m4t2();   }  }, "t2"  );          t1.start();          t2.start();     }}

运行结果变成了

先执行完第一个锁中的内容后执行第二个锁中的内容。

(3)

这时候我们使用notifyAll()和wait()

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class Thread2 {     public void m4t1() {          synchronized(this) {               int i = 5;               while(i>0) {                    System.out.println("1");                    i--;                    try {                         Thread.sleep(500);                         notifyAll();                         wait();                    } catch (InterruptedException ie) {                    }               }          }     }     public void m4t2() {        synchronized(this){         int i = 5;          while( i > 0) {               System.out.println("2");               i--;               try {                    Thread.sleep(500);                    notifyAll();                    wait();               } catch (InterruptedException ie) {               }          }        }     }     public static void main(String[] args) {          final Thread2 myt2 = new Thread2();          Thread t1 = new Thread(  new Runnable() {  public void run() {  myt2.m4t1();  }  }, "t1"  );          Thread t2 = new Thread(  new Runnable() {  public void run() { myt2.m4t2();   }  }, "t2"  );          t1.start();          t2.start();     }}

通过不断的唤醒再挂起，两个线程又交替运行。

（4）

如果想让代码更完善，可以将两个线程放到线程池

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class Thread2 {     public void m4t1() {          synchronized(this) {               int i = 5;               while(i>0) {                    System.out.println("1");                    i--;                    try {                         Thread.sleep(500);                         notifyAll();                         wait();                    } catch (InterruptedException ie) {                    }               }          }     }     public void m4t2() {        synchronized(this){         int i = 5;          while( i > 0) {               System.out.println("2");               i--;               try {                    Thread.sleep(500);                    notifyAll();                    wait();               } catch (InterruptedException ie) {               }          }        }     }     public static void main(String[] args) {          final Thread2 myt2 = new Thread2();         ExecutorService exec=Executors.newCachedThreadPool();         exec.execute( new Runnable() {  public void run() {  myt2.m4t1();  }  });         exec.execute( new Runnable() {  public void run() {  myt2.m4t2();  }  });     }}

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