浅析 Java Thread.join()

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thread Java 活动 JDK Blog

一、在研究join的用法之前，先明确两件事情。

1.join方法定义在Thread类中，则调用者必须是一个线程，

例如：

Thread t = new CustomThread();//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

customThread.start();

customThread.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：

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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)

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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：

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

然后进入join(0)方法：

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   /**
    * 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;
     }
}
    }