API：

join

public final void join()

                throws InterruptedException

等待该线程终止。

抛出：

InterruptedException - 假设不论什么线程中断了当前线程。当抛出该异常时，当前线程的中断状态 被清除。

join

public final void join(long millis)

                throws InterruptedException

等待该线程终止的时间最长为 millis 毫秒。超时为 0 意味着要一直等下去。

參数：

millis - 以毫秒为单位的等待时间。

抛出：

InterruptedException - 假设不论什么线程中断了当前线程。

当抛出该异常时，当前线程的中断状态 被清除。

join

public final void join(long millis,

                       int nanos)

                throws InterruptedException

等待该线程终止的时间最长为 millis 毫秒 + nanos 纳秒。

參数：

millis - 以毫秒为单位的等待时间。

nanos - 要等待的 0-999999 附加纳秒。

抛出：

IllegalArgumentException - 假设 millis 值为负，则 nanos 的值不在 0-999999 范围内。

InterruptedException - 假设不论什么线程中断了当前线程。

当抛出该异常时。当前线程的中断状态 被清除。

解析：

Thread.join()。是用来指定当前主线程等待其它线程运行完成后。再来继续运行Thread.join()后面的代码。

例1：

package com.example;

import java.util.Date;

import java.util.concurrent.TimeUnit;

public class DataSourcesLoader implements Runnable{

	@Override

	public void run() {

		System.out.printf("Beginning data sources loading: %s\n",new Date());

	    try {

	      TimeUnit.SECONDS.sleep(4);

	    } catch (InterruptedException e) {

	      e.printStackTrace();

	    }

	    System.out.printf("Data sources loading has finished: %s\n",new Date());

	}

	public static void main(String[] args){

		DataSourcesLoader dsLoader = new DataSourcesLoader();

	    Thread thread1 = new Thread(dsLoader,"DataSourceThread");

	    thread1.start();

	    try {

	        thread1.join();

	      } catch (InterruptedException e) {

	        e.printStackTrace();

	      }

	      System.out.printf("Main: Configuration has been loaded: %s\n",new Date());

	}

}

运行结果：

Beginning data sources loading: Fri Nov 14 14:27:31 CST 2014

Data sources loading has finished: Fri Nov 14 14:27:35 CST 2014

Main: Configuration has been loaded: Fri Nov 14 14:27:35 CST 2014

假设去掉thread1.join()，运行的结果例如以下：

Main: Configuration has been loaded: Fri Nov 14 14:28:33 CST 2014

Beginning data sources loading: Fri Nov 14 14:28:33 CST 2014

Data sources loading has finished: Fri Nov 14 14:28:37 CST 2014

通过结果。就能够非常明显的说明上面红字的部分：“再来继续运行Thread.join()后面的代码”

例2：

package com.example;

import java.util.Date;

import java.util.concurrent.TimeUnit;

public class DataSourcesLoader implements Runnable{

	@Override

	public void run() {

		System.out.printf("Beginning data sources loading: %s\n",new Date());

	    try {

	      TimeUnit.SECONDS.sleep(4);

	    } catch (InterruptedException e) {

	      e.printStackTrace();

	    }

	    System.out.printf("Data sources loading has finished: %s\n",new Date());

	}

	public static void main(String[] args){

		DataSourcesLoader dsLoader = new DataSourcesLoader();

	    Thread thread1 = new Thread(dsLoader,"DataSourceThread");

	    thread1.start();

	    try {

	        thread1.join(3000);

	      } catch (InterruptedException e) {

	        e.printStackTrace();

	      }

	      System.out.printf("Main: Configuration has been loaded: %s\n",new Date());

	}

}

这里使用的是：

thread1.join(3000);

这句话的意思是，仅仅要满足以下2个条件中的一个时，主线程就会继续运行thread.join()后面的代码：

条件1：thread1 运行完成。

条件2：已经等待 thread1 运行了3000ms.

样例中。thread1 自身的运行时间是4s。而设置的等待时间是3s，所以得到的运行结果例如以下。thread1还没有运行完，主线程就開始运行后面的代码。由于 thread1 等待的时间已经超时了：

Beginning data sources loading: Fri Nov 14 14:35:45 CST 2014

Main: Configuration has been loaded: Fri Nov 14 14:35:48 CST 2014

Data sources loading has finished: Fri Nov 14 14:35:49 CST 2014

那么结合上面的2个样例。我们能够判断出以下代码的运行结果了：

例3：

package com.example;

import java.util.Date;

import java.util.concurrent.TimeUnit;

public class DataSourcesLoader implements Runnable{

	@Override

	public void run() {

		System.out.printf("Beginning data sources loading: %s\n",new Date());

	    try {

	      TimeUnit.SECONDS.sleep(4);

	    } catch (InterruptedException e) {

	      e.printStackTrace();

	    }

	    System.out.printf("Data sources loading has finished: %s\n",new Date());

	}

}

package com.example;

import java.util.Date;

import java.util.concurrent.TimeUnit;

public class NetworkConnectionsLoader implements Runnable{

	@Override

	public void run() {

		System.out.printf("Beginning network connect loading: %s\n",new Date());

	    try {

	      TimeUnit.SECONDS.sleep(6);

	    } catch (InterruptedException e) {

	      e.printStackTrace();

	    }

	    System.out.printf("Network connect loading has finished: %s\n",new Date());

	}

	public static void main(String[] args){

		DataSourcesLoader dsLoader = new DataSourcesLoader();

	    Thread thread1 = new Thread(dsLoader,"DataSourceThread");

	    NetworkConnectionsLoader ncLoader = new NetworkConnectionsLoader();

	    Thread thread2 = new Thread(ncLoader,"NetworkConnectionLoader");

	    thread1.start();

	    thread2.start(); 

	    try {

	        thread1.join();

	        thread2.join(1900);

	      } catch (InterruptedException e) {

	        e.printStackTrace();

	      }

	      System.out.printf("Main: Configuration has been loaded: %s\n",new Date());

	}

}

运行结果：

Beginning data sources loading: Fri Nov 14 14:39:20 CST 2014

Beginning network connect loading: Fri Nov 14 14:39:20 CST 2014

Data sources loading has finished: Fri Nov 14 14:39:24 CST 2014

Main: Configuration has been loaded: Fri Nov 14 14:39:26 CST 2014

Network connect loading has finished: Fri Nov 14 14:39:26 CST 2014

注意：假设把例3的 thread2.join(1900) 部分改动为：

thread2.join(3000);

结果会和上面的一样吗？

依据我最開始指出的“Thread.join()。是用来指定当前主线程等待其它线程运行完成后，再来继续运行Thread.join()后面的代码。”

我们能够看到，运行结果会有区别：

Beginning data sources loading: Fri Nov 14 14:41:21 CST 2014

Beginning network connect loading: Fri Nov 14 14:41:21 CST 2014

Data sources loading has finished: Fri Nov 14 14:41:25 CST 2014

Network connect loading has finished: Fri Nov 14 14:41:27 CST 2014

Main: Configuration has been loaded: Fri Nov 14 14:41:27 CST 2014</span>

至于为什么会有这个区别，我上面也已经说明了，我想这个应该不难理解。

PS：代码部分截取来自《Java 7 Concurrency Cookbook》