incomplete Object information when serialize object with Gson

[EvanAatrox]

    @PostMapping("/message/send?access_token={access_token}")
    MessageResponseDTO send(@RequestParam("access_token") String accessToken, @RequestBody AbsMessage message);

Such as the code, I need to call it,the interface has different performance depends on the Object passed.
However AbsMessage is a abstract class,when I call it need to pass an object which instance of AbsMessage.
But the reuest body doesn't conatin any field from the runtime sub class whic I passed. All fields are from the abstract class.
I find the cause at last.

  GsonEncoder#encode(Object object, Type bodyType, RequestTemplate template)
  @Override
  public void encode(Object object, Type bodyType, RequestTemplate template) {
    template.body(gson.toJson(object, bodyType));
  }

It serializes object with the type which declared in the function,not runtime type of the passed object.
If gson.toJson(object),It appears normal.

[EvanAatrox]

It shows the bug above the photos.

[kdavisk6]

If possible, please provide a minimal reproducible example to help diagnose the issue.

[EvanAatrox]

    @Data
    static abstract class Message {
        String toUser;

        public Message(String toUser) {
            this.toUser = toUser;
        }
    }

    @Getter
    @Setter
    @EqualsAndHashCode(callSuper = true)
    public static class SimpleTextMessage extends Message {
        String content;
        Integer type;

        public SimpleTextMessage(String toUser, String content, Integer type) {
            super(toUser);
            this.content = content;
            this.type = type;
        }
    }

    @Test
    void gsonTest() {
        Gson gson = new GsonBuilder().create();
        SimpleTextMessage message = new SimpleTextMessage("测试用户", "测试消息", Integer.valueOf(1));
        var res = gson.toJson(message, Message.class);
        // the final result only include toUser prop,bus loss content type props.
        // result: {"toUser":"测试用户"}
        System.out.println(res);
     }

Here are two classes,just like the lib feign-gson,it serializes object with the type declared,but not runtime real type.
This leads to a situation the json string not contains the props from the runtime real subclasses.
Thanks to your reply.

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