转载请标明出处：
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.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，，，