The problem
After the front end passes JSON format data to the back end, SpringMVC reports

org.springframework.http.converter.HttpMessageNotReadableException:
     * JSON parse error: Can not deserialize instance of java.lang.String out of START_OBJECT token;
     * nested exception is com.fasterxml.jackson.databind.JsonMappingException:
     * Can not deserialize instance of java.lang.String out of START_OBJECT token
     *  at [Source: java.io.PushbackInputStream@6822f8aa; line: 1, column: 88] (through reference chain: com.xxx.XXXDto["employees"])

plan
To solve this problem, we can use one of the Uniform Exception Handling methods in Spring

@ExceptionHandler({HttpMessageNotReadableException.class, JsonMappingException.class, HttpMediaTypeNotSupportedException.class})
    @ResponseBody
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String,Object> exceptionHandler(Exception ex){
        Map<String,Object> map = new HashMap<>(3);
        try{
           map.put("code","400");
           map.put("msg",ex.getMessage());
           return map;
        }catch (Exception e){
            log.error("exception handler error",e);
            map.put("code","400");
            map.put("msg",e.getMessage());
            return map;
        }
    }

Solve problems encountered in the process
The above method can handle the problem of JSON formatting errors, but the data content returned to the front end is in the following format

JSON parse error: Can not deserialize instance of java.lang.String out of START_OBJECT token;  nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token  at [Source: java.io.PushbackInputStream@6822f8aa; line: 1, column: 88] (through reference chain: com.xxx.XXXDto["callBackUrl"])

We can’t see what the problem is at first sight, and even if we are familiar with the error, we can’t immediately find out where the problem is, because we need to process the above data and then send it back to the front end. Right
Finally solve the problem
We debug the source code found, Change is abnormal in springframework org. Springframework. HTTP. Converter. Json. AbstractJackson2HttpMessageConverter
thrown inside abnormal is
JsonMappingException extends JsonProcessingException , Thrown in for springmvc HttpMessageNotReadableException, as a result, we only need to add corresponding processing logic unified exception handling, can be returned to the front

@ExceptionHandler({HttpMessageNotReadableException.class, JsonMappingException.class, HttpMediaTypeNotSupportedException.class})
    @ResponseBody
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String,Object> exceptionHandler(Exception ex){
        Map<String,Object> map = new HashMap<>(3);
        try{
            if(ex instanceof HttpMessageNotReadableException
                    && ex.getMessage().indexOf("JSON parse error:")>-1){
                map.put("code","400");
                String message=ex.getMessage();
                int beginIndex=message.indexOf("XXXDto[\"");
                int endIndex=message.indexOf("\"])",beginIndex);
                message="parameter"+message.substring(beginIndex+22,endIndex)+" format error";
                map.put("msg",message);
            }else{
                map.put("code","400");
                map.put("msg",ex.getMessage());
            }
            return map;
        }catch (Exception e){
            log.error("exception handler error",e);
            map.put("code","400");
            map.put("msg",e.getMessage());
            return map;
        }
    }