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());
        }

    }