join() 和 sleep() 区别

Posted 2020-10-26 otherCoding

来源于《Java多线程编程核心技术》

一、join() 作用

在很多情况，主线程创建并启动子线程，如果子线程中需要进行大量的耗时计算，主线程往往早于子线程结束。这时，如果主线程想等待子线程执行结束之后再结束，比如子线程处理一个数据，主线程要取得这个数据，就要用待jion() 方法。

方法join()的作用是等待线程对象的销毁

二、区别

　　sleep(long)方法在睡眠时不释放对象锁

　　join(long)方法在等待的过程中释放对象锁

三、实例

　　创建join_sleep_1项目

　　

　　类ThreadA.java代码如下

 1 package extthread;
 2 
 3 public class ThreadA extends Thread {
 4 
 5     private ThreadB b;
 6 
 7     public ThreadA(ThreadB b) {
 8         super();
 9         this.b = b;
10     }
11 
12     @Override
13     public void run() {
14         try {
15             synchronized (b) {
16                 b.start();
17                 Thread.sleep(6000);
18             }
19         } catch (InterruptedException e) {
20             e.printStackTrace();
21         }
22     }
23 }

类ThreadB.java代码如下

 1 package extthread;
 2 
 3 public class ThreadB extends Thread {
 4 
 5     @Override
 6     public void run() {
 7         try {
 8             System.out.println("   b run begin timer="
 9                     + System.currentTimeMillis());
10             Thread.sleep(5000);
11             System.out.println("   b run   end timer="
12                     + System.currentTimeMillis());
13         } catch (InterruptedException e) {
14             e.printStackTrace();
15         }
16     }
17 
18     synchronized public void bService() {
19         System.out.println("��ӡ��bService timer=" + System.currentTimeMillis());
20     }
21 
22 }

类ThreadC.java代码如

 1 package extthread;
 2 
 3 public class ThreadC extends Thread {
 4 
 5     private ThreadB threadB;
 6 
 7     public ThreadC(ThreadB threadB) {
 8         super();
 9         this.threadB = threadB;
10     }
11 
12     @Override
13     public void run() {
14         threadB.bService();
15     }
16 }

Run.java 代码

 1 package test.run;
 2 
 3 import extthread.ThreadA;
 4 import extthread.ThreadB;
 5 import extthread.ThreadC;
 6 
 7 public class Run {
 8     public static void main(String[] args) {
 9 
10         try {
11             ThreadB b = new ThreadB();
12 
13             ThreadA a = new ThreadA(b);
14             a.start();
15 
16             Thread.sleep(1000);
17 
18             ThreadC c = new ThreadC(b);
19             c.start();
20         } catch (InterruptedException e) {
21             e.printStackTrace();
22         }
23     }
24 }

结果

由于线程ThreadA使用Thread.sleep(long)方法一直持有ThreadB对象的锁，时间达到6秒，所以线程C只有在ThreadA时间到达6秒后释放ThreadB的锁时，才可以调用ThreadB中的同步方法Synchronized public void bService()

上面实验证明Thread.sleep(long)不释放锁

---

 

 下面实验修改ThreadA.java

 1 package extthread;
 2 
 3 public class ThreadA extends Thread {
 4 
 5     private ThreadB b;
 6 
 7     public ThreadA(ThreadB b) {
 8         super();
 9         this.b = b;
10     }
11 
12     @Override
13     public void run() {
14         try {
15             synchronized (b) {
16                 b.start();
17                 b.join();
18                 for (int i = 0; i < Integer.MAX_VALUE; i++) {
19                     String newString = new String();
20                     Math.random();
21                 }
22             }
23         } catch (InterruptedException e) {
24             e.printStackTrace();
25         }
26     }
27 }

结果如下

由于线程ThreadA释放了ThreadB的锁，所以线程ThreadC可以调用ThreadB中的同步方法synchronized public void bService()

实验证明join(long)方法具有释放锁的特点