HttpURLConnection getContentLength值为-1

一个android项目有一个点击更新可以下载更新apk，在更新时需要显示一个进度，这里用：

		URL url = new URL(downLoadPath);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
       conn.setRequestMethod("GET");
       conn.setConnectTimeout(8 * 1000);
        conn.setReadTimeout(8 * 1000);
       conn.connect();
        long length = conn.getContentLength();

这样取得文件的总长度，结果发现conn.getConentLength()取到的值 为-1，查了一下有说设置 conn.setRequestProperty(“Accept-Encoding”, “identity”); 就可 了正常获取长度了，添加后值还是-1。
于是把downLoadPath,下载地址放在浏览器上下载可以成功下展，看他NetWork:

这图是我修改后的，修改前发现response里并没有content_lenght这个头信息，所以获取一直就是-1，然后去修改服务端，加上这个值：
resp.setHeader(“Content-Length”, String.valueOf(new FileInputStream(file).available()));
加上后便如上图所示可以发现头信息里有了contentLength这个值，然后客户端就可以成功获取了，下面展示一下详细代码
服务端

private void fileLoad(HttpServletRequest req, HttpServletResponse resp) throws IOException {
		String param = req.getParameter("param") == null ? "" : req.getParameter("param");
		String callback = req.getParameter("callback") == null ? "" : req.getParameter("callback");
		String filename = req.getParameter("filename") == null ? "" : req.getParameter("filename");
		String tv = req.getParameter("tv") == null ? "" : req.getParameter("tv");

		JSONObject obj = new JSONObject();
		int recode = 0;
		String text = "";
		
		ServletOutputStream out = null;
		if (!filename.equals("")) {
			try {
				resp.setContentType("application/force-download");
				resp.setCharacterEncoding("utf-8");
				
				
				File file = null;
				if ("008".equals(tv)) {
					file = new File(MyIP.tv8_apk_download_url + filename);
				} else if ("009".equals(tv)) {
					file = new File(MyIP.tv9_apk_download_url + filename);
				}
				resp.setHeader("Content-Disposition", "attachment;filename=" + new String(filename.getBytes()));
				
				resp.setHeader("Content-Length", String.valueOf(new FileInputStream(file).available()));
				
				out = resp.getOutputStream();
				InputStream bis = null;
				BufferedOutputStream bos = null;
				try {
					bis = new FileInputStream(file);
					bos = new BufferedOutputStream(out);
					byte[] buff = new byte['?'];

					int bytesRead;
					int k = 0;
					while (-1 != (bytesRead = bis.read(buff, 0, buff.length))) {
						k+=1;
						bos.write(buff, 0, bytesRead);
					}
				} catch (Exception e) {
					e.printStackTrace();
				} finally {
					if (bis != null)
						bis.close();
					if (bos != null) {
						bos.close();
					}
				}
			} catch (NumberFormatException e) {
				recode = 1;
				text = "参数格式错误";
				e.printStackTrace();
			} catch (Exception e) {
				recode = 99;
				text = "系统异常";
				e.printStackTrace();

				OutputMethod.ExceptionControl(e, parameter);
			}
		} else {
			recode = 1;
			text = "缺少参数";
		}
		obj.put("recode", Integer.valueOf(recode));
		obj.put("text", text);
		System.out.println("####fileLoad："+obj.toString());
		out.print(callback + "(" + obj.toString() + ")");
		out.close();
	}

客户端

  private class downloadApkThread extends Thread {
        @Override
        public void run() {
            try {
                if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                    System.out.println("App的下载路径为：" + downLoadPath);

                    URL url = new URL(downLoadPath);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    conn.setRequestMethod("GET");
                    conn.setRequestProperty("Accept-Encoding", "identity");
                    conn.setRequestProperty("User-Agent", " Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.120 Safari/537.36");
                    conn.setConnectTimeout(8 * 1000);
                    conn.setReadTimeout(8 * 1000);
                    conn.connect();
                    long length = conn.getContentLength();

                    if(length < 0){
                        String values = conn.getHeaderField("Content-Length");
                        if (values != null && !values.isEmpty()) {
                            length = Long.parseLong(values);
                        }
                    }
                    InputStream is = conn.getInputStream();

                    System.out.println("#######conn.getContentLength:"+length);
                    //获取App文件名
                    apkName = downLoadPath.substring(downLoadPath.lastIndexOf("=") + 1, downLoadPath.length());
                    System.out.println("####apkName:"+apkName);
                    //下载后保存的路径
                    saveFileDir = Environment.getExternalStorageDirectory() + "/APP";
                    File apkFileDir = new File(saveFileDir);
                    if (!apkFileDir.exists()) {
                        //如果/App文件夹不存在就创建一个新的App子文件夹
                        apkFileDir.mkdir();
                    }
                    File apkFile = new File(apkFileDir, apkName);
                    FileOutputStream fos = new FileOutputStream(apkFile);

                    int count = 0;
                    byte buf[] = new byte[10 * 1024];
                    do {
                        int numread = is.read(buf);
                        count += numread;
                        progress = (int) (((float) count / length) * 100);
                        //System.out.println("#####load progress:" + progress);
                        updateProgress.setProgress(progress);
                        if (numread <= 0) {

                            installAPk(apkFile);
                            break;
                        }
                        fos.write(buf, 0, numread);
                    } while (true);
                    fos.close();
                    is.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    ;