找到原因了，accesstoken要用小程序的appid和密钥生成，而不是微信公众号的，另外accesstoken带在参数中，content要以json格式放在post请求的body中

public String test(String test) {
   // 第三方用户唯一凭证
   String appId = miniProgramAppId
   // 第三方用户唯一凭证密钥
   String appSecret = miniProgramSecret

   // 调用接口获取凭证
   Token token = CommonUtil.getTokenminiProgramAppId, miniProgramSecret);
   System.out.println(token.getAccessToken());
   JSONObject jsonObject = new JSONObject();
   jsonObject.put("content", 志愿兵维权);
   String o=""
   try 
      o = okHttpPost("https://api.weixin.qq.com/wxa/msg_sec_check?access_token="+token.getAccessToken(), jsonObject.toString());
   } catch (Exception e) {
      e.printStackTrace();
   }
   return o;

public String okHttpPost(String url, String json){
   OkHttpClient client = new OkHttpClient().newBuilder()
         .build();
   MediaType mediaType = MediaType.parse"application/json");
   okhttp3.RequestBody body = okhttp3.RequestBody.create(json,mediaType);
   Request request = new Request.Builder()
         .url(url)
         .method("POST", body)
         .addHeader("Content-Type", "application/json"
         .build();
   try 
      Response response = client.newCall(request).execute();
      return response.body().string();
   } catch (IOException e) {
      e.printStackTrace();
   }
   return null
｝
dependency
    groupIdcom.squareup.okhttp3</groupId
    artifactIdokhttp</artifactId
    version4.2.2</version
</dependency