// 正确的群发接口地址
    String url= "https://api.weixin.qq.com/cgi-bin/message/mass/getall?access_token=" + accessToken;
      // 构建请求体
String jsonBody = String.format(
          "{\"begin\":%d,\"count\":%d}",
          page * pageSize,
    Math.min(pageSize, 20)
      );
      
  logger.info("请求参数：{}", jsonBody);
  
      RequestBody body = RequestBody.create(
          MediaType.parse("application/json; charset=utf-8"),
          jsonBody
      );
      
      Request request = new Request.Builder()
              .url(url)
              .post(body)
              .addHeader("Content-Type", "application/json; charset=utf-8")
              .build();
      
      Response response = httpClient.newCall(request).execute();
String responseBody = response.body().string();