文件下载是开发中比较常见的功能，当后端无法告诉前端下载文件大小时，就需要前端自己获取文件大小

简单的方法为：

final URLConnection connection = url.openConnection();
final int length = connection.getContentLength();

当文件比较多且文件较大时，使用getContentLength()获取大小你会发现，神马情况？太NM慢了

记录一下解决文件下载前获取多个网络文件总大小时太慢的问题

    private long getAllFileLength(String[] fileList){
        try {
            URL u = null;
            long fileLength = 0;
            for (int i=0;i<fileList.length;i++){
                u = new URL(fileList[i]);
                HttpURLConnection urlcon = (HttpURLConnection) u.openConnection();
                 fileLength =  fileLength + handleFileLen(urlcon.getHeaderFields());
            }
            return fileLength;
        } catch (MalformedURLException e) {
            e.printStackTrace();
            return 0;
        } catch (IOException e) {
            e.printStackTrace();
            return 0;
        }
    }

public long handleFileLen(Map<String, List<String>> headers) {
        if (headers == null || headers.isEmpty()) {
            Log.e(TAG, "header为空，获取文件长度失败");
            return -1;
        }
        List<String> sLength = headers.get("Content-Length");
        if (sLength == null || sLength.isEmpty()) {
            return -1;
        }
        String temp = sLength.get(0);
        long len = TextUtils.isEmpty(temp) ? -1 : Long.parseLong(temp);
        // 某些服务，如果设置了conn.setRequestProperty("Range", "bytes=" + 0 + "-");
        // 会返回 Content-Range: bytes 0-225427911/225427913
        if (len < 0) {
            List<String> sRange = headers.get("Content-Range");
            if (sRange == null || sRange.isEmpty()) {
                len = -1;
            } else {
                int start = temp.indexOf("/");
                len = Long.parseLong(temp.substring(start + 1));
            }
        }

        return len;
    }