浅析 Java Thread.join()

一、 在研究 join 的用法之前，先明确 两件 事情 。

1.j oin 方法定 义 在 Thread 类 中， 则调 用者必 须 是一个 线 程，

例如：

Thread t = new Custom Thread(); // 这里一般是自定义的线程类

t .start();// 线程起动

t.join();// 此处 会 抛出 InterruptedException 异常

 

2. 上面的两行代码也是在一个线程里面执行的。

 

以上出现了两个线程，一个是我们自定义的线程类，我们实现了 run 方法，做一些我们需要的工作；另外一个线程，生成我们自定义线程类的对象，然后执行

custom Thread .start();

custom Thread .join();

在这种情况下，两个线程的关系是一个线程由另外一个线程生成并起动，所以我们暂且认为第一个线程叫做“子线程 ”，另外一个线程叫做“主线程 ”。

 

二、为什么要用 join() 方法

主线程生成并起动了子线程，而子线程里要进行大量的耗时的运算 ( 这里可以借鉴下线程的作用 ) ，当主线程处理完其他的事务后，需要用到子线程的处理结果，这个时候就要用到 join(); 方法了。

 

 

三、 join 方法的作用

在网上看到有人说“将两个线程合并”。这样解释我觉得理解起来还更麻烦。不如就借鉴下 API 里的说法：

“等待该线程终止。”

解释一下，是主线程 ( 我在“一”里已经命名过了 ) 等待子线程的终止。也就是在子线程调用了 join() 方法后面的代码，只有等到子线程结束了才能执行。(Waits for this thread to die.)

 

 

四、用实例来理解

写一个简单的例子来看一下 join() 的用法，一共三个类：

1.CustomThread 类

2. CustomThread1 类

3. JoinTestDemo 类， main 方法所在的类。

 

代码 1 ：

 package  wxhx.csdn2;
 /**
  *
  * @author bzwm
  *
  */
 class  CustomThread1  extends  Thread {
     public  CustomThread1() {
         super ( "[CustomThread1] Thread" );
     };
     public   void  run() {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         try  {
             for  ( int  i =  0 ; i <  5 ; i++) {
                 System.out.println(threadName + " loop at "  + i);
                 Thread.sleep(1000 );
             }
             System.out.println(threadName + " end." );
         } catch  (Exception e) {
             System.out.println("Exception from "  + threadName +  ".run" );
         }
     }
 }
 class  CustomThread  extends  Thread {
     CustomThread1 t1;
     public  CustomThread(CustomThread1 t1) {
         super ( "[CustomThread] Thread" );
         this .t1 = t1;
     }
     public   void  run() {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         try  {
             t1.join();
             System.out.println(threadName + " end." );
         } catch  (Exception e) {
             System.out.println("Exception from "  + threadName +  ".run" );
         }
     }
 }
 public   class  JoinTestDemo {
     public   static   void  main(String[] args) {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         CustomThread1 t1 = new  CustomThread1();
         CustomThread t = new  CustomThread(t1);
         try  {
             t1.start();
             Thread.sleep(2000 );
             t.start();
             t.join();//在代碼2里，將此處注釋掉
         } catch  (Exception e) {
             System.out.println("Exception from main" );
         }
         System.out.println(threadName + " end!" );
     }
 }

 package  wxhx.csdn2;
 /**
  *
  * @author bzwm
  *
  */
 class  CustomThread1  extends  Thread {
     public  CustomThread1() {
         super ( "[CustomThread1] Thread" );
     };
     public   void  run() {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         try  {
             for  ( int  i =  0 ; i <  5 ; i++) {
                 System.out.println(threadName + " loop at "  + i);
                 Thread.sleep(1000 );
             }
             System.out.println(threadName + " end." );
         } catch  (Exception e) {
             System.out.println("Exception from "  + threadName +  ".run" );
         }
     }
 }
 class  CustomThread  extends  Thread {
     CustomThread1 t1;
     public  CustomThread(CustomThread1 t1) {
         super ( "[CustomThread] Thread" );
         this .t1 = t1;
     }
     public   void  run() {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         try  {
             t1.join();
             System.out.println(threadName + " end." );
         } catch  (Exception e) {
             System.out.println("Exception from "  + threadName +  ".run" );
         }
     }
 }
 public   class  JoinTestDemo {
     public   static   void  main(String[] args) {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         CustomThread1 t1 = new  CustomThread1();
         CustomThread t = new  CustomThread(t1);
         try  {
             t1.start();
             Thread.sleep(2000 );
             t.start();
             t.join();//在代碼2里，將此處注釋掉
         } catch  (Exception e) {
             System.out.println("Exception from main" );
         }
         System.out.println(threadName + " end!" );
     }
 }

package wxhx.csdn2; /** * * @author bzwm * */ class CustomThread1 extends Thread { public CustomThread1() { super("[CustomThread1] Thread"); }; public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); try { for (int i = 0; i < 5; i++) { System.out.println(threadName + " loop at " + i); Thread.sleep(1000); } System.out.println(threadName + " end."); } catch (Exception e) { System.out.println("Exception from " + threadName + ".run"); } } } class CustomThread extends Thread { CustomThread1 t1; public CustomThread(CustomThread1 t1) { super("[CustomThread] Thread"); this.t1 = t1; } public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); try { t1.join(); System.out.println(threadName + " end."); } catch (Exception e) { System.out.println("Exception from " + threadName + ".run"); } } } public class JoinTestDemo { public static void main(String[] args) { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); CustomThread1 t1 = new CustomThread1(); CustomThread t = new CustomThread(t1); try { t1.start(); Thread.sleep(2000); t.start(); t.join();//在代碼2里，將此處注釋掉 } catch (Exception e) { System.out.println("Exception from main"); } System.out.println(threadName + " end!"); } }

 

打印结果：

 

main start. //main 方法所在的线程起动，但没有马上结束，因为调用 t.join(); ，所以要等到 t 结束了，此线程才能向下执行。

[CustomThread1] Thread start. // 线程 CustomThread1 起动

[CustomThread1] Thread loop at 0 // 线程 CustomThread1 执行

[CustomThread1] Thread loop at 1 // 线程 CustomThread1 执行

[CustomThread] Thread start. // 线程 CustomThread 起动，但没有马上结束，因为调用 t1.join(); ，所以要等到 t1 结束了，此线程才能向下执行。

[CustomThread1] Thread loop at 2 // 线程 CustomThread1 继续执行

[CustomThread1] Thread loop at 3 // 线程 CustomThread1 继续执行

[CustomThread1] Thread loop at 4 // 线程 CustomThread1 继续执行

[CustomThread1] Thread end. // 线程 CustomThread1 结束了

[CustomThread] Thread end. // 线程 CustomThread 在 t1.join(); 阻塞处起动，向下继续执行的结果

main end! // 线程 CustomThread 结束，此线程在 t.join(); 阻塞处起动，向下继续执行的结果。

 

修改一下代码，得到代码 2 ：(这里只写出修改的部分)

 public   class  JoinTestDemo {
     public   static   void  main(String[] args) {
         String threadName = Thread.currentThread().getName();
         System.out.println(threadName + " start." );
         CustomThread1 t1 = new  CustomThread1();
         CustomThread t = new  CustomThread(t1);
         try  {
             t1.start();
             Thread.sleep(2000 );
             t.start();
 //          t.join();//在代碼2里，將此處注釋掉
         } catch  (Exception e) {
             System.out.println("Exception from main" );
         }
         System.out.println(threadName + " end!" );
     }
 }

public class JoinTestDemo { public static void main(String[] args) { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); CustomThread1 t1 = new CustomThread1(); CustomThread t = new CustomThread(t1); try { t1.start(); Thread.sleep(2000); t.start(); // t.join();//在代碼2里，將此處注釋掉 } catch (Exception e) { System.out.println("Exception from main"); } System.out.println(threadName + " end!"); } }

 

打印结果：

 

main start. // main 方法所在的线程起动，但没有马上结束，这里并不是因为 join 方法，而是因为 Thread.sleep(2000);

[CustomThread1] Thread start. // 线程 CustomThread1 起动

[CustomThread1] Thread loop at 0 // 线程 CustomThread1 执行

[CustomThread1] Thread loop at 1 // 线程 CustomThread1 执行

main end! // Thread.sleep(2000); 结束，虽然在线程 CustomThread 执行了 t1.join(); ，但这并不会影响到其他线程 ( 这里 main 方法所在的线程 ) 。

[CustomThread] Thread start. // 线程 CustomThread 起动，但没有马上结束，因为调用 t1.join(); ，所以要等到 t1 结束了，此线程才能向下执行。

[CustomThread1] Thread loop at 2 // 线程 CustomThread1 继续执行

[CustomThread1] Thread loop at 3 // 线程 CustomThread1 继续执行

[CustomThread1] Thread loop at 4 // 线程 CustomThread1 继续执行

[CustomThread1] Thread end. // 线程 CustomThread1 结束了

[CustomThread] Thread end. // 线程 CustomThread 在 t1.join(); 阻塞处起动，向下继续执行的结果

 

 

五、从源码看 join() 方法

 

在 CustomThread 的 run 方法里，执行了 t1.join(); ，进入看一下它的 JDK 源码：

 public   final   void  join()  throws  InterruptedException {
 n(0 );
 }

 public   final   void  join()  throws  InterruptedException {
 n(0 );
 }

public final void join() throws InterruptedException { join(0); }

然后进入 join(0) 方法：

 

     /**
     * Waits at most <code>millis</code> milliseconds for this thread to
     * die. A timeout of <code>0</code> means to wait forever. //注意这句
     *
     * @param      millis   the time to wait in milliseconds.
     * @exception  InterruptedException if another thread has interrupted
     *             the current thread.  The <i>interrupted status</i> of the
     *             current thread is cleared when this exception is thrown.
     */
    public   final   synchronized   void  join( long  millis)  //参数millis为0.
    throws  InterruptedException {
 long  base = System.currentTimeMillis();
 long  now =  0 ;
 if  (millis <  0 ) {
            throw   new  IllegalArgumentException( "timeout value is negative" );
 }
 if  (millis ==  0 ) { //进入这个分支
     while  (isAlive()) { //判断本线程是否为活动的。这里的本线程就是t1.
     wait(0 ); //阻塞
     }
 } else  {
     while  (isAlive()) {
     long  delay = millis - now;
     if  (delay <=  0 ) {
         break ;
     }
     wait(delay);
     now = System.currentTimeMillis() - base;
     }
 }
    }

     /**
     * Waits at most <code>millis</code> milliseconds for this thread to
     * die. A timeout of <code>0</code> means to wait forever. //注意这句
     *
     * @param      millis   the time to wait in milliseconds.
     * @exception  InterruptedException if another thread has interrupted
     *             the current thread.  The <i>interrupted status</i> of the
     *             current thread is cleared when this exception is thrown.
     */
    public   final   synchronized   void  join( long  millis)  //参数millis为0.
    throws  InterruptedException {
 long  base = System.currentTimeMillis();
 long  now =  0 ;
 if  (millis <  0 ) {
            throw   new  IllegalArgumentException( "timeout value is negative" );
 }
 if  (millis ==  0 ) { //进入这个分支
     while  (isAlive()) { //判断本线程是否为活动的。这里的本线程就是t1.
     wait(0 ); //阻塞
     }
 } else  {
     while  (isAlive()) {
     long  delay = millis - now;
     if  (delay <=  0 ) {
         break ;
     }
     wait(delay);
     now = System.currentTimeMillis() - base;
     }
 }
    }

/** * Waits at most <code>millis</code> milliseconds for this thread to * die. A timeout of <code>0</code> means to wait forever. //注意这句 * * @param millis the time to wait in milliseconds. * @exception InterruptedException if another thread has interrupted * the current thread. The <i>interrupted status</i> of the * current thread is cleared when this exception is thrown. */ public final synchronized void join(long millis) //参数millis为0. throws InterruptedException { long base = System.currentTimeMillis(); long now = 0; if (millis < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (millis == 0) {//进入这个分支 while (isAlive()) {//判断本线程是否为活动的。这里的本线程就是t1. wait(0);//阻塞 } } else { while (isAlive()) { long delay = millis - now; if (delay <= 0) { break; } wait(delay); now = System.currentTimeMillis() - base; } } }

 

 

 

单纯从代码上看，如果线程被生成了，但还未被起动，调用它的 join() 方法是没有作用的。将直接继续向下执行，这里就不写代码验证了。

 

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