概要分析：

1.你的项目要支持https才能实现https接口的调用;（这里提供搭建https项目网址）
2.和传统http请求方式有所不同，HTTPS，即安全的超文本传输协议，采用了SSL技术，被广泛使用以保证Web应用系统的安全性。访问Web应用的编程接口大多封装了SSL，使得访问HTTPS和访问HTTP一样简单。但是很多中、小型应用系统或基于局域网、校园网的应用系统所使用的证书并不是由权威的认证机构发行或者被其验证，直接使用这些编程接口将不能访问HTTPS。所以需要添加证书和信任请求完成验证才能访问接口。

package com.example.demo;
import org.springframework.stereotype.Controller;
import org.springframework.util.StringUtils;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

import javax.net.ssl.*;
import javax.servlet.http.HttpServletRequest;
import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.security.cert.X509Certificate;
import java.util.HashMap;
import java.util.Map;

@Controller
public class Mains {


    @RequestMapping(value="/test",method = RequestMethod.GET)
    @ResponseBody
    public void test() {
        Map<String, String> params = new HashMap<>();
        params.put("firstName", "Mickey");
        params.put("lastName", "Mouse");
        //要发送的POST请求url?Key=Value&amp;Key2=Value2&amp;Key3=Value3的形式
        //application/json;charset=UTF-8  application/x-www-form-urlencoded;charset=UTF-8
        sendPost("https://localhost:8888/tests",map2Url(params),"application/x-www-form-urlencoded;charset=UTF-8","POST");
    }

    //添加主机名验证程序类，设置不验证主机
    private final static HostnameVerifier DO_NOT_VERIFY = new HostnameVerifier() {
        public boolean verify(String hostname, SSLSession session) {
            return true;
        }
    };

    //添加信任主机
    private static void trustAllHosts() {
        // 创建不验证证书链的信任管理器 这里使用的是x509证书
        TrustManager[] trustAllCerts = new TrustManager[]{new X509TrustManager() {
            public java.security.cert.X509Certificate[] getAcceptedIssuers() {
                return new java.security.cert.X509Certificate[]{};
            }

            public void checkClientTrusted(X509Certificate[] chain, String authType) {
            }

            public void checkServerTrusted(X509Certificate[] chain, String authType) {
            }
        }};
        // 安装所有信任的信任管理器
        try {
            SSLContext sc = SSLContext.getInstance("TLS");
            sc.init(null, trustAllCerts, new java.security.SecureRandom());
            //HttpsURLConnection通过SSLSocket来建立与HTTPS的安全连接，SSLSocket对象是由SSLSocketFactory生成的。
            HttpsURLConnection.setDefaultSSLSocketFactory(sc.getSocketFactory());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    //这里可以用main方法测试
    public static void main(String[] args) {
        Map<String, String> params = new HashMap<>();
        params.put("firstName", "Mickey");
        params.put("lastName", "Mouse");
        //要发送的POST请求url?Key=Value&amp;Key2=Value2&amp;Key3=Value3的形式
        //application/json;charset=UTF-8  application/x-www-form-urlencoded;charset=UTF-8
        sendPost("https://localhost:8888/tests",map2Url(params),"application/x-www-form-urlencoded;charset=UTF-8","POST");
    }

    /**
     *  发送post 数据
     * @param urls
     * @return
     */
    public static String sendPost(String urls, String param, String contentType, String method) {
        StringBuffer sb=new StringBuffer();
        DataOutputStream out = null;
        BufferedReader responseReader = null;
        InputStream in1 = null;
        try {
            trustAllHosts();
            // 创建url资源
            URL url = new URL(urls);
            // 建立http连接
            HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();
            conn.setHostnameVerifier(DO_NOT_VERIFY);
            // 设置不用缓存
            conn.setUseCaches(false);
            // 设置允许输出
            conn.setDoOutput(true);
            // 设置允许输入
            conn.setDoInput(true);
            // 设置传递方式
            conn.setRequestMethod(method);
            System.out.println(conn.getRequestMethod());
            // 设置维持长连接
            conn.setRequestProperty("Connection", "Keep-Alive");
            // 设置文件字符集:
            conn.setRequestProperty("Charset", "UTF-8");
            // 转换为字节数组
//            byte[] data = (param).getBytes();
//            // 设置文件长度
//            conn.setRequestProperty("Content-Length", String.valueOf(data.length));
            // 设置文件类型:
            conn.setRequestProperty("Content-Type", contentType);
            // 开始连接请求
            conn.connect();
            out = new DataOutputStream(conn.getOutputStream());
            // 写入请求的字符串
            out.writeBytes(param);
            out.flush();

            System.out.println(conn.getResponseCode());

            // 请求返回的状态
            if (HttpURLConnection.HTTP_OK == conn.getResponseCode()) {
                System.out.println("连接成功");
                // 请求返回的数据
                in1 = conn.getInputStream();
                String readLine;
                responseReader = new BufferedReader(new InputStreamReader(in1,"UTF-8"));
                while((readLine=responseReader.readLine()) != null){
                    sb.append(readLine).append("\n");
                }
            } else {
                System.out.println("error++");
            }
        } catch (Exception e) {

        } finally {
            try {
                if (null != responseReader)
                    responseReader.close();
                if (null != in1)
                    in1.close();
            } catch(Exception e) {}
            try {
                out.close();
            } catch(Exception e) {}
        }

        return sb.toString();

    }

    /**
     * map转url参数
     */
    public static String map2Url(Map<String, String> paramToMap) {
        if (null == paramToMap || paramToMap.isEmpty()) {
            return null;
        }
        StringBuffer url  = new StringBuffer();
        boolean      isfist = true;
        for (Map.Entry<String, String> entry : paramToMap.entrySet()) {
            if (isfist) {
                isfist = false;
            } else {
                url.append("&");
            }
            url.append(entry.getKey()).append("=");
            String value = entry.getValue();
            if (!StringUtils.isEmpty(value)) {
                try {
                    url.append(URLEncoder.encode(value, "UTF-8"));
                } catch (UnsupportedEncodingException e) {
                    e.printStackTrace();
                }
            }
        }
        return url.toString();
    }

    @RequestMapping(value="/tests",method = RequestMethod.POST)
    @ResponseBody
    //如果post发送的参数过多，这里提供一个思路，就是序列化或者转成一个指定格式发送后，这边用一个参数解析
    public void tests(HttpServletRequest request, @RequestParam(required = false) String firstName,@RequestParam(required = false) String lastName){
        try {
//            //接收请求参数 如果是application/json;charset=UTF-8，用这种方式接收参数
//            InputStreamReader reader=new InputStreamReader(request.getInputStream());
//            BufferedReader buffer=new BufferedReader(reader);
//            String data = buffer.readLine();
//            System.out.println(data);

            //如果是application/x-www-form-urlencoded，直接在参数上指定map的键即可
            System.out.println(firstName+"  "+lastName);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

注：这里我使用的是springboot项目，将代码复制到controller直直接可以调用，记得更改host和ip即可。