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;}@Overridepublic 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.zipThread-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 -- 31143237Thread-2完成下载  ： 7785810 -- 15571619Thread-1完成下载  ： 0 -- 7785809Thread-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