最简单的java获取微信小程序openid的方法，不需要jar包

//appid 开发者appid

//secret开发者appid对应的**（好像）

//js_code code

@RequestMapping("getUserOpenId")
 @ResponseBody
 public String getUserOpenId(String appid, String js_code, String secret) {
//  System.out.println("appid"+appid);
//  System.out.println("secret"+secret);
//  System.out.println("js_code"+js_code);
  
  String requestUrl = "https://api.weixin.qq.com/sns/jscode2session";
 
  BufferedReader in = null;
  String result = "";
  try {
   String urlNameString = requestUrl+"?appid="+appid+"&js_code="+js_code+"&secret="+secret+"&grant_type=authorization_code";
   URL realUrl = new URL(urlNameString);
   
   URLConnection connection = realUrl.openConnection();
   //connection.setRequestProperty("accept", "*/*");
   //connection.setRequestProperty("connection", "Keep-Alive");
   //connection.setRequestProperty("Content-Type", "application/json; charset=utf-8");
   //connection.setRequestProperty("user-agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1)");
   
   connection.connect();
   Map<String, List<String>> map = connection.getHeaderFields();
   
//   for (String key : map.keySet()) {
//    System.out.println(key+"--->"+map.get(key));
//    for (String a : map.get(key)) {
//     System.out.println(key+"====="+a);
//    }
//   }
   in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
   
   String line;
   while((line = in.readLine()) != null){
    result += line;
   }
   return result;
  } catch (Exception e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
  return null;
 }