Spring Boot Exception Handler: User Does not exist

I'm developing a spring boot web app for users and one feature is for users to be able to reset their password. In order to do so, they must provide their email address and have a verification message sent to them where they will be able to reset their password. My question, though, is how do I make an error message specific to the situation where the email the user provides doesn't exist in the database? What I have so far is below in my GlobalExceptionHandler.java:

package bcoreHW.controllers;

import javax.servlet.http.HttpServletRequest;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.dao.DataIntegrityViolationException;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.servlet.ModelAndView;

@ControllerAdvice
public class GlobalExceptionHandler {

	@Value("${message.error.exception}")
	private String exceptionMessage;
	
	@Value("${message.user.nonexistent}")
	private String userNonexistentMessage;

	@ResponseStatus(value = HttpStatus.NOT_FOUND)
	@ExceptionHandler(value=Exception.class)
	public ModelAndView defaultErrorHandler(HttpServletRequest req, Exception e) {
		ModelAndView modelAndView = new ModelAndView();
		modelAndView.getModel().put("message", exceptionMessage);
		modelAndView.getModel().put("url", req.getRequestURL());
		modelAndView.getModel().put("exception", e);
		modelAndView.setViewName("app.exception");
		return modelAndView;
	}
}

And so the error I get as of now is the basic 404 HttpStatus.NOT_FOUND error, which I have simply saying "An error occurred." But of course I want this to be more specific so that the user knows more of the issue, and userNonexistentMessage will say "This user does not exist."

So, how do I add an exception handler for this case? Thanks!

A solution to handle errors in a flexible way can be complex, but you can start by creating an enum to define in a single place all the errors that your business logic can throw.

The Java enum supports fields, so you can associate each literal with the error message and the status code.

public enum ErrorCode {

	USER_NOT_FOUND(HttpStatus.NOT_FOUND, "This user does not exist"),
	USER_IS_LOCKED(HttpStatus.UNPROCESSABLE_ENTITY, "This user is locked");
	
	private HttpStatus httpStatus;
	private String message;
	
	private ErrorCode(HttpStatus httpStatus, String message) {
		this.httpStatus = httpStatus;
		this.message = message;
	}

	public HttpStatus getHttpStatus() {
		return httpStatus;
	}

	public String getMessage() {
		return message;
	}
}

Then you can create a custom API Exception that has an ErrorCode field, so you throw the Exception in your code passing the specific error in the Exception constructor.

Finally in the GlobalExceptionHandler you can add a method to handle your custom Exception, get the ErrorCode value contained in the Exception and from that ErrorCode obtain the error message, http status, or any other error-related information required to build your response. That way you can handle all errors in a single way in your GlobalExceptionHandler instead of having specific error messages injected.

As mentioned in the beginning error handling can be more sophisticated. Your exception could accept also dynamic arguments to interpolate the error message ("The user with email {} does not exist"), you can have separate business exceptions for each status code, you can add i18N support to have the error message in a properties file for each language instead of hard-coded in the error enum, etc. But I think implementing the ErrorCode enum and the custom Exception is a good starting point.

So I looked at the trace and found the following lines to be of interest for determining the error at hand:

2020-10-13 22:04:53.555 DEBUG 63687 --- [nio-8080-exec-8] o.s.web.servlet.DispatcherServlet        : POST "/forgotPassword", parameters={masked}
2020-10-13 22:04:53.556 DEBUG 63687 --- [nio-8080-exec-8] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped to bcoreHW.controllers.AuthController#forgotUserPassword(ModelAndView, SiteUser)
2020-10-13 22:04:53.654 DEBUG 63687 --- [nio-8080-exec-8] org.hibernate.SQL                        : select siteuser0_.user_id as user_id1_3_, siteuser0_.email as email2_3_, siteuser0_.enabled as enabled3_3_, siteuser0_.password as password4_3_, siteuser0_.role as role5_3_ from users siteuser0_ where siteuser0_.email=?
Hibernate: select siteuser0_.user_id as user_id1_3_, siteuser0_.email as email2_3_, siteuser0_.enabled as enabled3_3_, siteuser0_.password as password4_3_, siteuser0_.role as role5_3_ from users siteuser0_ where siteuser0_.email=?
2020-10-13 22:04:53.662 DEBUG 63687 --- [nio-8080-exec-8] .m.m.a.ExceptionHandlerExceptionResolver : Using @ExceptionHandler bcoreHW.controllers.GlobalExceptionHandler#invalidUserMessage(HttpServletRequest, Exception)
2020-10-13 22:04:53.662 DEBUG 63687 --- [nio-8080-exec-8] .m.m.a.ExceptionHandlerExceptionResolver : Resolved [java.lang.NullPointerException] to ModelAndView [view="app.exception"; model={message=Invalid user., url=http://localhost:8080/forgotPassword, exception=java.lang.NullPointerException}]
2020-10-13 22:04:53.662 DEBUG 63687 --- [nio-8080-exec-8] o.s.web.servlet.DispatcherServlet        : Using resolved error view: ModelAndView [view="app.exception"; model={message=Invalid user., url=http://localhost:8080/forgotPassword, exception=java.lang.NullPointerException}]
2020-10-13 22:04:53.663 DEBUG 63687 --- [nio-8080-exec-8] o.s.w.s.v.ContentNegotiatingViewResolver : Selected 'text/html' given [text/html, application/xhtml+xml, image/avif, image/webp, image/apng, application/xml;q=0.9, application/signed-exchange;v=b3;q=0.9, */*;q=0.8]
2020-10-13 22:04:53.663 DEBUG 63687 --- [nio-8080-exec-8] o.s.web.servlet.view.tiles3.TilesView    : View name 'app.exception', model {message=Invalid user., url=http://localhost:8080/forgotPassword, exception=java.lang.NullPointerException}
2020-10-13 22:04:53.768 DEBUG 63687 --- [nio-8080-exec-8] o.s.web.servlet.DispatcherServlet        : Completed 200 OK

You can see towards the end that there is a NullPointerException, and so I added the following to my controller shown in the question above:

	@ExceptionHandler(value=NullPointerException.class)
	public ModelAndView invalidUserMessage(HttpServletRequest req, Exception e) {
		ModelAndView modelAndView = new ModelAndView();
		modelAndView.getModel().put("message", invalidUserMessage);
		modelAndView.getModel().put("url", req.getRequestURL());
		modelAndView.getModel().put("exception", e);
		modelAndView.setViewName("app.exception");
		return modelAndView;
	}

And so now I avoid the overarching 404 error status and have a specific error message for an invalid user.