Json parse error cannot deserialize instance of

I’m trying to send a JSON to a POST bodyRequest service to use this information as a Java ArrayList or something like that. When I try to do it, I receive a deserialize error

This is my JSON:

{information: [ {fields: “Periods Offered”, tables: “Courses”,
columns: “Academic Level*”}, {fields: “Default Grading Basis*”,
tables: “Courses”, columns: “Default Offering Percentage”}, {fields:
“Allowed Locations”, tables: “Courses”, columns: “Allowed to Offer”}
] }

This is my POST bodyRequest service

@ResponseBody
@PostMapping("/dataMapping/update/table")

public ResponseEntity<Object> updateDataMappingTable(@RequestBody List<UpdateDataMapping> information) {

    try {
        String update = this.dataMappingService.update(information);
        return new ResponseEntity<Object>(update, HttpStatus.OK);
    } catch (Exception e) {
        Response response = new Response();
        response.setError(e.getMessage());
        return new ResponseEntity<Object>(response, HttpStatus.OK);
    }
}

My UpdateDataMapping class looks like:

public class UpdateDataMapping {

    public String fields;
    public String tables;
    public String columns;

    public UpdateDataMapping() {

    }

    public UpdateDataMapping(String fields, String tables, String columns) {
    
        this.fields = fields;
        this.tables = tables;
        this.columns = columns;
    }

    public String getFields() {
        return fields;
    }

    public void setFields(String fields) {
        this.fields = fields;
    }

    public String getTables() {
        return tables;
    }

    public void setTables(String tables) {
        this.tables = tables;
    }

    public String getColumns() {
        return columns;
    }

    public void setColumns(String columns) {
        this.columns = columns;
    }
}

This is my error:

.w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize instance of java.util.ArrayList<com.kastech.model.UpdateDataMapping> out of START_OBJECT token; nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of java.util.ArrayList<com.kastech.model.UpdateDataMapping> out of START_OBJECT token
at [Source: (PushbackInputStream); line: 1, column: 1]]

If someone can help me I will appreciate it.

Answer

Your JSON example shows that your array is delivered inside an information object. You are seeing your current stack trace because the Jackson is unable to find an appropriate way to deserialize your request payload.

Either, you can change your request JSON to simply be a list

[ {fields: "Periods Offered", tables: "Courses", columns: "Academic Level*"}, {fields: "Default Grading Basis*", tables: "Courses", columns: "Default Offering Percentage"}, {fields: "Allowed Locations", tables: "Courses", columns: "Allowed to Offer"} ]

or you can have a POJO represent that information object

class UpdateMappingList {
    List<UpdateDataMapping> information;
    ...
}

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: [email protected]; 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: [email protected]; 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;
        }
    }