Java 实现的断点下载

该断点下载可应用于浏览器或者迅雷等下载工具的下载，实现方式有多种多样的，本文仅仅研究了单线程的下载。迅雷等下载工具会自己主动将下载资源分块并记录每块的起始位置，然后依据系统性能。起多线程下载。

1. 基本原理

从Request Header的Range信息里面获取已经下载的文件大小，然后创建response的outputstream 向client（浏览器或者迅雷等下载工具）写，写的时候又利用header里面的“Content-Range”, 让client知道从哪个位置開始写；

读取网络资源方面，利用HttpClient模拟request请求，发起post或者get请求，仅仅是这个请求跟一般请求有点不一样：须要带上Range信息。告诉程序该从哪个位置開始读数据。

2. 须要使用的Java 组件

• HttpServletRequest / Response
• HttpClient
• ServletOutputStream
• BufferedInputStream

3. 代码实现

/**
	 * @desc 断点下载工具方法
	 * @param request
	 * @param response
	 * @param fileLength
	 * @param contentType
	 * @param fileName
	 * @param fileId
	 */
	public static void resumeDownload(HttpServletRequest request,
			HttpServletResponse response, Long fileLength, String contentType,
			String fileName, String fileId) {
		ServletOutputStream out = null;
		response.reset();

		// 记录断点续传的開始点
		long pos = 0;
		if (null != request.getHeader("Range")) {
			response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT);
			try {
				pos = Long.parseLong(request.getHeader("Range")
						.replaceAll("bytes=", "").replaceAll("-.*", ""));
			} catch (NumberFormatException e) {
				LOGGER.error(e.getMessage(), e);
				pos = 0;
			}
			String contentRange = new StringBuffer("bytes ").append(pos + "")
					.append("-").append((fileLength.intValue() - 1) + "")
					.append("/").append(fileLength.intValue() + "").toString();
			response.setHeader("Content-Range", contentRange);
		}

		response.setHeader("Accept-Ranges", "bytes");
		response.setHeader("Content-Length",
				String.valueOf(fileLength.intValue() - pos));
		response.setCharacterEncoding("UTF-8");
		response.setContentType(contentType);
		response.setHeader("Content-disposition", "attachment;filename=\""
				+ fileName + "\"");
		try {
			out = response.getOutputStream();
		} catch (IOException e) {
			LOGGER.error(e.getMessage(), e);
		}

		// 断点下载
		CloseableHttpClient httpClient = HttpClients.createDefault();

		HttpPost httpPost = new HttpPost(SysConf.getString("fezo.download.url"));

		List<NameValuePair> nvps = new ArrayList<NameValuePair>();
		nvps.add(new BasicNameValuePair(SysConf.getString("fezo.download.param"), fileId));
		
		HttpResponse httpResponse = null;
		BufferedInputStream input = null;
		try {
			httpPost.setEntity(new UrlEncodedFormEntity(nvps));
			
			httpPost.setHeader("Range", "bytes=" + pos + "-");
			httpResponse = httpClient.execute(httpPost);

			input = new BufferedInputStream(httpResponse.getEntity().getContent());

			byte[] buffer = new byte[CommonConstants.BUFFER_SIZE];
			int len = -1;
			while ((len = input.read(buffer)) != -1) {
				out.write(buffer, 0, len);
			}
			out.flush();
			out.close();
			input.close();
		} catch (UnsupportedEncodingException e) {
			LOGGER.error(e.getMessage(), e);
		} catch (ClientProtocolException e) {
			LOGGER.error(e.getMessage(), e);
		} catch (IOException e) {
			// 能够忽略这个异常。有可能是用户暂停下载，或者迅雷等下载工具分块下载
		} finally {
			try {
				if (httpClient != null) httpClient.close();
			} catch(IOException e) {
				LOGGER.error(e.getMessage(), e);
			}
		}
	}

    - 获取response的输出流程来向client提供下载功能，而不是简单的把数据写入到某个详细的文件，核心代码：out = response.getOutputStream();

    - 头信息里面"Range" 和 "Conent-Range" 等信息的处理；