Java下载文件获取真实文件名

String imgurl = "文件地址";
URL url = new URL(imgurl);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.connect();

//获取文件名和扩展名
            //先连接一次，解决跳转下载
            conn.getResponseCode();
            imgurl = conn.getURL().toString();
            //第一种方式，针对 img.png
            fileName = imgurl.substring(imgurl.lastIndexOf("/")+1);
            if(fileName.lastIndexOf(".")>0){
                extName = fileName.substring(fileName.lastIndexOf(".")+1);
            }
            if(extName==null || extName.length()>4 || extName.indexOf("?")>-1){
                //第二种方式，获取header 确定文件名和扩展名
                fileName = conn.getHeaderField("Content-Disposition");
                if(fileName==null || fileName.indexOf("filename")<0){
                    fileName = enclosure.getId().toString();
                    extName = "jpg";
                }else{
                    fileName = URLDecoder.decode(fileName.substring(fileName.indexOf("filename")+10,fileName.length()-1),"UTF-8");
                    extName = fileName.substring(fileName.lastIndexOf(".")+1);
                }
            }

//还找不到的话，我也没辙了。