分别给出了post发送文件和json数据的函数，其中使用到了Jackson库来转化Json数据，使用log4j2来打印日记，可自行剔除。

public class HttpUtils {
    static private ObjectMapper objectMapper=new ObjectMapper();
    static private Logger logger= LogManager.getLogger();

    /**
     * 以post请求方式发送文件
     * @param url 
     * @param name 文件的请求参数名
     * @param fileName 文件名
     * @param fileInput 文件的输入流
     * @return 默认返回参数为Json格式的数据
     * @throws IOException
     * @throws InterruptedException
     */
    static public Map<String,Object> postFile2(String url,String name,String fileName,InputStream fileInput) throws IOException, InterruptedException {
        String charset = "UTF-8";
        String boundary = Long.toHexString(System.currentTimeMillis()); // Just generate some unique random value.
        String CRLF = "\r\n"; // Line separator required by multipart/form-data.

        URLConnection connection = new URL(url).openConnection();
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);

        try (
                OutputStream output = connection.getOutputStream();
                PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, charset), true);
        ) {
            // Send binary file.
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\""+URLEncoder.encode(name,"UTF-8")+"\"; filename=\"" + URLEncoder.encode(fileName,"UTF-8") + "\"").append(CRLF);
            writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(fileName)).append(CRLF);
            writer.append("Content-Transfer-Encoding: binary").append(CRLF);
            writer.append(CRLF).flush();
            fileInput.transferTo(output);
            output.flush(); // Important before continuing with writer!
            writer.append(CRLF).flush(); // CRLF is important! It indicates end of boundary.

            // End of multipart/form-data.
            writer.append("--" + boundary + "--").append(CRLF).flush();
        }

        // Request is lazily fired whenever you need to obtain information about response.
        int responseCode = ((HttpURLConnection) connection).getResponseCode();
        System.out.println(responseCode); // Should be 200
        //读取响应消息
        BufferedReader reader=new BufferedReader(new InputStreamReader(connection.getInputStream()));
        StringBuilder stringBuilder=new StringBuilder();
        String tempStr=null;
        while((tempStr=reader.readLine())!=null){
            stringBuilder.append(tempStr);
        }
        //转化为JSON
        Map<String,Object> response= objectMapper.readValue(stringBuilder.toString(),Map.class);
        return response;
    }

	/**
	* 以post请求发送Json文件
	*/
    public static Map<String,Object> sendJsonPost(String url,String jsonStr) throws IOException, Utils.ReceiveBadCodeException {
        HttpURLConnection httpURLConnection= (HttpURLConnection) new URL(url).openConnection();
        //配置连接
        httpURLConnection.setDoOutput(true);
        httpURLConnection.setRequestMethod("POST");
        httpURLConnection.setRequestProperty("Content-Type","application/json;charset=UTF-8");
        OutputStream out=httpURLConnection.getOutputStream();
        //发送
        out.write(jsonStr.getBytes("UTF-8"));
        //检查响应
        InputStream in=httpURLConnection.getInputStream();
        StringBuilder stringBuilder=new StringBuilder();
        byte[] bytes=new byte[1024];
        int num;
        while((num=in.read(bytes))!=-1){
            stringBuilder.append(new String(bytes,"utf-8"));
        }
        Map<String,Object> response=objectMapper.readValue(stringBuilder.toString(),Map.class);
        return response;
    }

}

参考：

• Upload files from Java client to a HTTP server
• Sending HTTP POST Request In Java