How to throw an exception back in JSON in Spring Boot

Prateek Narendra :

I have a Request Mapping -

  @RequestMapping("/fetchErrorMessages")
  public @ResponseBody int fetchErrorMessages(@RequestParam("startTime") String startTime,@RequestParam("endTime") String endTime) throws Exception
  {
      if(SanityChecker.checkDateSanity(startTime)&&SanityChecker.checkDateSanity(endTime))
      {
          return 0;
      }
      else
      {
          throw new NotFoundException("Datetime is invalid");
      }
  }

If the startTime and endTime are invalid, I want to throw a 500 error but return the exception string in JSON. However, I get a HTML Page instead saying

Whitelabel Error Page

This application has no explicit mapping for /error, so you are seeing this as a fallback.

Wed Dec 20 10:49:37 IST 2017
There was an unexpected error (type=Internal Server Error, status=500).
Datetime is invalid

I instead wanted to return 500 with a JSON

{"error":"Date time format is invalid"}

How do I go about this?

Ataur Rahman Munna :

Suppose you have a custom Exception class NotFoundException and its implementations something like this:

public class NotFoundException extends Exception {

    private int errorCode;
    private String errorMessage;

    public NotFoundException(Throwable throwable) {
        super(throwable);
    }

    public NotFoundException(String msg, Throwable throwable) {
        super(msg, throwable);
    }

    public NotFoundException(String msg) {
        super(msg);
    }

    public NotFoundException(String message, int errorCode) {
        super();
        this.errorCode = errorCode;
        this.errorMessage = message;
    }


    public void setErrorCode(int errorCode) {
        this.errorCode = errorCode;
    }

    public int getErrorCode() {
        return errorCode;
    }

    public void setErrorMessage(String errorMessage) {
        this.errorMessage = errorMessage;
    }

    public String getErrorMessage() {
        return errorMessage;
    }

    @Override
    public String toString() {
        return this.errorCode + " : " + this.getErrorMessage();
    }
}

Now you want to throw some exception from controller. If you throw a exception then you must catch it from a standard Error Handler class, say for example in spring they provide @ControllerAdvice annotation to apply to make a class Standard Error Handler. When it is applied to a class then this spring component (I mean the class you annotated) can catch any exception thrown from controller. But We need to map exception class with proper method. So we defined a method with your exception NotFoundException handler something like below.

@ControllerAdvice
public class RestErrorHandler {

    @ExceptionHandler(NotFoundException.class)
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ResponseBody
    public Object processValidationError(NotFoundException ex) {
        String result = ex.getErrorMessage();
        System.out.println("###########"+result);
        return ex;
    }
}

You want to sent http status to internal server error(500), so here we used @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR). Since you used Spring-boot so you do not need to make a json string except a simple annotation @ResponseBody can do that for you automagically.