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

?

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

?

（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-Length
		Header[] 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 = ht