使用Apache HttpClient实现多线程下载的小例子
网上类似的文章很多，参考了很多人的，大部分人都是用URLConnection写的。
原理一：HTTP多线程下载原理1、发送一个含有Rang头的Head请求，如果返回状态码为206，则允许多线程下载
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原理二：多线程下载原理1、使用HttpClient的Head请求获取请求文件的信息
2、发送一个Rang的Head请求判断是否允许多线程下载
3、通过主任务创建多个分段下载线程，分段下载文件，然后用Java的随机读写文件类保存下载的内容
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（等有时间了再添加内容吧，先简单写这么多）
调度功能代码片段
/** * 开始下载 * @throws Exception */public void startDown() throws Exception{HttpClient httpClient = new DefaultHttpClient();try {//获取下载文件信息getDownloadFileInfo(httpClient);//启动多个下载线程startDownloadThread();//开始监视下载数据monitor();} catch (Exception e) {throw e;} finally {httpClient.getConnectionManager().shutdown();}}/** * 获取下载文件信息 */private void getDownloadFileInfo(HttpClient httpClient) throws IOException,ClientProtocolException, Exception {HttpHead httpHead = new HttpHead(url);HttpResponse response = httpClient.execute(httpHead);//获取HTTP状态码int statusCode = response.getStatusLine().getStatusCode();if(statusCode != 200) throw new Exception("资源不存在!");if(getDebug()){for(Header header : response.getAllHeaders()){System.out.println(header.getName()+":"+header.getValue());}}//Content-LengthHeader[] headers = response.getHeaders("Content-Length");if(headers.length > 0)contentLength = Long.valueOf(headers[0].getValue());httpHead.abort();httpHead = new HttpHead(url);httpHead.addHeader("Range", "bytes=0-"+(contentLength-1));response = httpClient.execute(httpHead);if(response.getStatusLine().getStatusCode() == 206){acceptRanges = true;}httpHead.abort();}/** * 启动多个下载线程 * @throws IOException * @throws FileNotFoundException */private void startDownloadThread() throws IOException,FileNotFoundException {//创建下载文件File file = new File(localPath);file.createNewFile();RandomAccessFile raf = new RandomAccessFile(file, "rw");raf.setLength(contentLength);raf.close();//定义下载线程事件实现类DownloadThreadListener listener = new DownloadThreadListener() {public void afterPerDown(DownloadThreadEvent event) {//下载完一个片段后追加已下载字节数synchronized (object) {DownloadTask.this.receivedCount += event.getCount();}}public void downCompleted(DownloadThreadEvent event) {//下载线程执行完毕后从主任务中移除threads.remove(event.getTarget());if(getDebug()){System.out.println("剩余线程数："+threads.size());}}};//不支持多线程下载时if (!acceptRanges) {if(getDebug()){System.out.println("该地址不支持多线程下载");}//定义普通下载DownloadThread thread = new DownloadThread(url, 0, contentLength, file, false);thread.addDownloadListener(listener);thread.start();threads.add(thread);return;}//每个请求的大小long perThreadLength = contentLength / threadCount + 1;long startPosition = 0;long endPosition = perThreadLength;//循环创建多个下载线程do{if(endPosition >= contentLength)endPosition = contentLength - 1;DownloadThread thread = new DownloadThread(url, startPosition, endPosition, file);thread.addDownloadListener(listener);thread.start();threads.add(thread);startPosition = endPosition + 1;//此处加 1,从结束位置的下一个地方开始请求endPosition += perThreadLength;} while (startPosition < contentLength);}?分段下载线程代码片段：/** * 现在过程代码 */public void run() {if(DownloadTask.getDebug()){System.out.println("Start:" + startPosition + "-" +endPosition);}HttpClient httpClient = new DefaultHttpClient();try {HttpGet httpGet = new HttpGet(url);if(isRange){//多线程下载httpGet.addHeader("Range", "bytes="+startPosition+"-"+endPosition);}HttpResponse response = httpClient.execute(httpGet);int statusCode = response.getStatusLine().getStatusCode();if(DownloadTask.getDebug()){for(Header header : response.getAllHeaders()){System.out.println(header.getName()+":"+header.getValue());}System.out.println("statusCode:" + statusCode);}if(statusCode == 206 || (statusCode == 200 && !isRange)){InputStream inputStream = response.getEntity().getContent();//创建随机读写类RandomAccessFile outputStream = new RandomAccessFile(file, "rw");//跳到指定位置outputStream.seek(startPosition);int count = 0;byte[] buffer=new byte[1024];while((count = inputStream.read(buffer, 0, buffer.length))>0){outputStream.write(buffer, 0, count);//触发下载事件fireAfterPerDown(new DownloadThreadEvent(this,count));}outputStream.close();}httpGet.abort();} catch (Exception e) {e.printStackTrace();} finally {//触发下载完成事件fireDownCompleted(new DownloadThreadEvent(this, endPosition));if(DownloadTask.getDebug()){System.out.println("End:" + startPosition + "-" +endPosition);}httpClient.getConnectionManager().shutdown();}}?附件说明：1、Download.jar为编译好的可运行程序
2、Download.zip为Eclipse项目文件
3、运行截图
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