Java / Android 基于Http的多线程下载的实现

转载请标明出处：http://blog.csdn.net/lmj623565791/article/details/26994463

有个朋友需要个多线程现在的例子，就帮忙实现了，在此分享下~

先说下原理，原理明白了，其实很简单：

a、对于网络上的一个资源，首先发送一个请求，从返回的Content-Length中回去需要下载文件的大小，然后根据文件大小创建一个文件。

 this.fileSize = conn.getContentLength();// 根据响应获取文件大小
 File dir = new File(dirStr);
 this.localFile = new File(dir, filename);
 RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");
 raf.setLength(fileSize);
 raf.close();

b、根据线程数和文件大小，为每个线程分配下载的字节区间，然后每个线程向服务器发送请求，获取这段字节区间的文件内容。

 conn.setRequestProperty("Range", "bytes=" + startPos + "-"
                         + endPos);// 设置获取实体数据的范围

c、利用RandomAccessFile的seek方法，多线程同时往一个文件中写入字节。

 raf.seek(startPos);
 while ((len = is.read(buf)) != -1)
 {
     raf.write(buf, 0, len);
 }

分析完了原理就很简单了，我封装了一个类，利用这个类的实例进行下载，所需参数：下载资源的URI， 本地文件路径，线程的数量。

 package com.zhy.mutilthread_download;

 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.RandomAccessFile;
 import java.net.HttpURLConnection;
 import java.net.URL;

 public class MultipartThreadDownloador
 {

     /**
      * 需要下载资源的地址
      */
     private String urlStr;
     /**
      * 下载的文件
      */
     private File localFile;
     /**
      * 需要下载文件的存放的本地文件夹路径
      */
     private String dirStr;
     /**
      * 存储到本地的文件名
      */
     private String filename;

     /**
      * 开启的线程数量
      */
     private int threadCount;
     /**
      * 下载文件的大小
      */
     private long fileSize;

     public MultipartThreadDownloador(String urlStr, String dirStr,
             String filename, int threadCount)
     {
         this.urlStr = urlStr;
         this.dirStr = dirStr;
         this.filename = filename;
         this.threadCount = threadCount;
     }

     public void download() throws IOException
     {
         createFileByUrl();

         /**
          * 计算每个线程需要下载的数据长度
          */
         long block = fileSize % threadCount == 0 ? fileSize / threadCount
                 : fileSize / threadCount + 1;

         for (int i = 0; i < threadCount; i++)
         {
             long start = i * block;
             long end = start + block >= fileSize ? fileSize : start + block - 1;

             new DownloadThread(new URL(urlStr), localFile, start, end).start();
         }

     }

     /**
      * 根据资源的URL获取资源的大小，以及在本地创建文件
      */
     public void createFileByUrl() throws IOException
     {
         URL url = new URL(urlStr);
         HttpURLConnection conn = (HttpURLConnection) url.openConnection();
         conn.setConnectTimeout(15 * 1000);
         conn.setRequestMethod("GET");
         conn.setRequestProperty(
                 "Accept",
                 "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
         conn.setRequestProperty("Accept-Language", "zh-CN");
         conn.setRequestProperty("Referer", urlStr);
         conn.setRequestProperty("Charset", "UTF-8");
         conn.setRequestProperty(
                 "User-Agent",
                 "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
         conn.setRequestProperty("Connection", "Keep-Alive");
         conn.connect();

         if (conn.getResponseCode() == 200)
         {
             this.fileSize = conn.getContentLength();// 根据响应获取文件大小
             if (fileSize <= 0)
                 throw new RuntimeException(
                         "the file that you download has a wrong size ... ");
             File dir = new File(dirStr);
             if (!dir.exists())
                 dir.mkdirs();
             this.localFile = new File(dir, filename);
             RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");
             raf.setLength(fileSize);
             raf.close();

             System.out.println("需要下载的文件大小为 :" + this.fileSize + " , 存储位置为： "
                     + dirStr + "/" + filename);

         } else
         {
             throw new RuntimeException("url that you conneted has error ...");
         }
     }

     private class DownloadThread extends Thread
     {
         /**
          * 下载文件的URI
          */
         private URL url;
         /**
          * 存的本地路径
          */
         private File localFile;
         /**
          * 是否结束
          */
         private boolean isFinish;
         /**
          * 开始的位置
          */
         private Long startPos;
         /**
          * 结束位置
          */
         private Long endPos;

         public DownloadThread(URL url, File savefile, Long startPos, Long endPos)
         {
             this.url = url;
             this.localFile = savefile;
             this.startPos = startPos;
             this.endPos = endPos;
         }

         @Override
         public void run()
         {
             System.out.println(Thread.currentThread().getName() + "开始下载...");
             try
             {
                 HttpURLConnection conn = (HttpURLConnection) url
                         .openConnection();
                 conn.setConnectTimeout(15 * 1000);
                 conn.setRequestMethod("GET");
                 conn.setRequestProperty(
                         "Accept",
                         "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
                 conn.setRequestProperty("Accept-Language", "zh-CN");
                 conn.setRequestProperty("Referer", url.toString());
                 conn.setRequestProperty("Charset", "UTF-8");
                 conn.setRequestProperty("Range", "bytes=" + startPos + "-"
                         + endPos);// 设置获取实体数据的范围

                 conn.setRequestProperty(
                         "User-Agent",
                         "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
                 conn.setRequestProperty("Connection", "Keep-Alive");
                 conn.connect();

                 /**
                  * 代表服务器已经成功处理了部分GET请求
                  */
                 if (conn.getResponseCode() == 206)
                 {
                     InputStream is = conn.getInputStream();
                     int len = 0;
                     byte[] buf = new byte[1024];

                     RandomAccessFile raf = new RandomAccessFile(localFile,
                             "rwd");
                     raf.seek(startPos);
                     while ((len = is.read(buf)) != -1)
                     {
                         raf.write(buf, 0, len);
                     }
                     raf.close();
                     is.close();
                     System.out.println(Thread.currentThread().getName()
                             + "完成下载  ： " + startPos + " -- " + endPos);
                     this.isFinish = true;
                 } else
                 {
                     throw new RuntimeException(
                             "url that you conneted has error ...");
                 }
             } catch (IOException e)
             {
                 e.printStackTrace();
             }
         }

     }



 }

createFileByUrl方法，就是我们上述的原理的步骤1，得到文件大小和创建本地文件。我在程序使用了一个内部类DownloadThread继承Thread，专门负责下载。download()方法，根据线程数量和文件大小计算每个线程需要下载的字节区间，然后开启线程去下载。

服务器端：我就扔了几个文件在Tomcat根目录做实验，下面是测试代码：

 package com.zhy.mutilthread_download;

 import java.io.IOException;

 public class Test
 {

     public static void main(String[] args)
     {
         try
         {
             new MultipartThreadDownloador("http://localhost:8080/nexus.zip",
                     "f:/backup/nexus", "nexus.zip", 2).download();
         } catch (IOException e)
         {
             e.printStackTrace();
         }

     }
 }

输出结果：

 需要下载的文件大小为 :31143237 , 存储位置为： f:/backup/nexus/nexus.zip
 Thread-1开始下载...
 Thread-2开始下载...
 Thread-3开始下载...
 Thread-4开始下载...
 Thread-4完成下载  ： 23357430 -- 31143237
 Thread-2完成下载  ： 7785810 -- 15571619
 Thread-1完成下载  ： 0 -- 7785809
 Thread-3完成下载  ： 15571620 -- 23357429

截图：

ok，多线程下载介绍完毕，如果代码设计不合理，以及方法使用错误，欢迎各位留言，，，

源码点击下载