Java上传文件到服务器

普通上传文件以下代码就够用了：

/**     * 上传文件     * @param urlStr     * @param file     */    public static String upload(String urlStr, File file, String fileName){        String rv = "";        try {            URL url = new URL(urlStr);            HttpURLConnection conn = (HttpURLConnection) url.openConnection();              String boundary = "----aryg4pBUG8gGY9qgAAs";            // 发送POST请求必须设置如下两行              conn.setDoOutput(true);              conn.setDoInput(true);              conn.setUseCaches(false);              conn.setRequestMethod("POST");            conn.setRequestProperty("Connection", "keep-alive");            conn.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.89 Safari/537.1");            conn.setRequestProperty("Charsert", "UTF-8");            conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);            OutputStream out = new DataOutputStream(conn.getOutputStream());              StringBuilder sb = new StringBuilder();            sb.append("--"+boundary);            sb.append("\r\n");            sb.append("Content-Disposition: form-data;name=\"media\";filename=\""+ fileName + "\"\r\n");            sb.append("Content-Type:application/octet-stream\r\n\r\n");            //开始            byte[] data = sb.toString().getBytes();            out.write(data);            //将文件写入            DataInputStream in = new DataInputStream(new FileInputStream(file));            int bytes = 0;            byte[] bufferOut = new byte[1024];            while ((bytes = in.read(bufferOut)) != -1) {                out.write(bufferOut, 0, bytes);            }            out.write("\r\n".getBytes());            in.close();            //结束分割符            byte[] end_data = ("\r\n--" + boundary + "--\r\n").getBytes();              out.write(end_data);            out.flush();            out.close();             // 定义BufferedReader输入流来读取URL的响应            BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));            StringBuffer sbf = new StringBuffer();            String line = "";            while ((line = reader.readLine()) != null) {                sbf.append(line);            }            rv = sbf.toString();            reader.close();        } catch (Exception e) {            e.printStackTrace();        }        return rv;    }

但是如果上面的方式上传不了，返回400，那么就需要使用工具。
典型应用是：SpringMVC中的文件上传就不能用上面的代码，只能使用工具才行。

xUtils是一个github开源项目，封装了发送http请求的细节，也就是干了浏览器发送http请求干的事情。

https://github.com/wyouflf/xUtils