RestTemplate 返回值设置MediaType时的问题

这是client端的请求

        RestTemplate restTemplate = new RestTemplate();
        String url = "http://localhost:8902/tttt234";

        try {
            ResponseEntity res = restTemplate.exchange(url, HttpMethod.POST, null, JSONObject.class);
            JSONObject json = res.getBody();
            System.out.println(json);
        } catch (Exception e) {
            e.printStackTrace();
        }

这是Server端的方法

    @PostMapping(value = "/tttt234")
    private ResponseEntity xxxxx() {

        JSONObject jsonObject = new JSONObject();
        jsonObject.put("name", "123");
        jsonObject.put("data", "{}");
    //  return ResponseEntity.ok().contentType(MediaType.APPLICATION_JSON).body(jsonObject);
        return ResponseEntity.ok().contentType(MediaType.APPLICATION_JSON).body(jsonObject.toString());
    }

client端就会报错

org.springframework.web.client.RestClientException: Error while extracting response for type [class com.alibaba.fastjson.JSONObject] and content type [application/json]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize value of type `int` from String "{"data":"{}","name":"123"}": not a valid `int` value; nested exception is com.fasterxml.jackson.databind.exc.InvalidFormatException: Cannot deserialize value of type `int` from String "{"data":"{}","name":"123"}": not a valid `int` value
 at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 1]

如果server端在最后的返回的地方不使用toString方法，client端就不会报错，这是什么原理？