java后台调用url无协议

url格式不正确，可能有"www.baidu.com"    "这个不能有

// 下载pdf

    public void downpdf(String URL, String fileName) {

        try {

            // byte[] data = jsonStr.getBytes();

            java.net.URL url = new java.net.URL(URL);

            java.net.HttpURLConnection conn = (java.net.HttpURLConnection) url

                    .openConnection();

            conn.setRequestMethod("GET");

            conn.setConnectTimeout(5 * 1000);// 设置连接超时时间为5秒

            conn.setReadTimeout(20 * 1000);// 设置读取超时时间为20秒

            // 使用 URL 连接进行输出，则将 DoOutput标志设置为 true

            conn.setDoOutput(true);

            conn.setRequestProperty("Content-Type",

                    "application/x-www-form-urlencoded");

            // 如果请求响应码是200，则表示成功

            if (conn.getResponseCode() == 200) {

                InputStream stream = conn.getInputStream();

                FileOutputStream fos = null;

                BufferedInputStream bis = null;

                // HttpURLConnection httpUrl = null;

                // URL url = null;

                int BUFFER_SIZE = 1024;

                byte[] buf = new byte[BUFFER_SIZE];

                int size = 0;

                bis = new BufferedInputStream(stream);

                // //建立文件

                fos = new FileOutputStream(fileName);

                // //保存文件

                while ((size = bis.read(buf)) != -1)

                    fos.write(buf, 0, size);

                //

                fos.close();

                bis.close();

                // httpUrl.disconnect();

            }

            conn.disconnect();// 断开连接

        } catch (Exception e) {

            System.out.println(e.getMessage());

        }

    }