Implement Best-Practice Error Message in Spring REST Controller

Question

I am writing a server-side REST application for a mobile app. I have been trying to setup an exception handler which follows the explanation here, where instead of showing some HTTP error page, the client receives a JSON object similar to this one:

{
    "status": 404,
    "code": 40483,
    "message": "Oops! It looks like that file does not exist.",
    "developerMessage": "File resource for path /uploads/foobar.txt does not exist.  Please wait 10 minutes until the upload batch completes before checking again.",
    "moreInfo": "http://www.mycompany.com/errors/40483"
}

I have modeled my exception on those detailed in the guide, and they seem to be working well (the custom errors are being shown in the console). But I got stuck at this point, because I don't know where I'm supposed to put the bean configuration.

Given that I have all my exception handlers, resolvers, etc., I thought I'd try go around it differently. At this point I would still get Spring's Whitelabel error page when I entered an invalid HTTP request, but this time with my custom error messages from my exceptions. So I figured if I tried to implement my own ErrorHandler as explained here, I might be able to construct the JSON objects using Gson or something, instead of the way the previous article went about it.

I tried to get a bare minimum ErrorHandler working:

package com.myapp.controllers;

import org.springframework.boot.autoconfigure.web.ErrorController;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.servlet.http.HttpServletRequest;

@RestController
public class ErrorMessageController implements ErrorController {

    private static final String ERROR_PATH = "/error";

    @Override
    public String getErrorPath(){
        return ERROR_PATH;
    }

    @RequestMapping(value = ERROR_PATH)
    public String renderErrorPage(HttpServletRequest request){

        String errorPage = (String) request.getAttribute("javax.servlet.error.status_code");

        return errorPage;
    }

}

So I expected to get something like a solitary 404 appearing on the webpage. But instead I'm getting a Tomcat error page:

Why is this? I'd appreciate any help.

Dimitri Mestdagh · Accepted Answer · 2017-09-06 13:25:34Z

1

This happens because request.getAttribute("javax.servlet.error.status_code") should be an Integer and you're casting it as a String. This causes an error during the error handling, which pops up the default Tomcat error handler.

If you cast it as an int, it will work:

@RequestMapping(value = ERROR_PATH)
public int renderErrorPage(HttpServletRequest request){

    int errorPage = (int) request.getAttribute("javax.servlet.error.status_code");

    return errorPage;
}

Alternatively, if you just want to return certain JSON structure, you could use @ExceptionHandler methods in stead of implementing an ErrorController.

For example, let's say you have the following controller:

@GetMapping
public String getFoo() throws FileNotFoundException {
    throw new FileNotFoundException("File resource for path /uploads/foobar.txt does not exist");
}

If you want to handle all FileNotFoundExceptions in a particular way, you could write a method with the @ExceptionHandler annotation:

@ExceptionHandler(FileNotFoundException.class)
@ResponseStatus(HttpStatus.NOT_FOUND)
public ErrorResponse notFound(FileNotFoundException ex) {
    return new ErrorResponse(HttpStatus.NOT_FOUND.value(), 40483, "Oops! It looks like that file does not exist.", ex.getMessage(), "http://www.mycompany.com/errors/40483");
}

In this case, ErrorResponse is a POJO containing the fields you want. If you want to re-use this for all your controllers, you can put this in a @ControllerAdvice.

edited Sep 6, 2017 at 13:25

answered Sep 6, 2017 at 13:20

Dimitri Mestdagh

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2 Comments

Luke Collins Over a year ago

So where do I put the @ExceptionHandler method? In a new RestController?

Dimitri Mestdagh Over a year ago

You can choose it. If it only applies to a single controller, then you can put that method in the same controller class. If it applies to all controllers, then you create a new simple class and just annotate it with @ControllerAdvice and nothing else.

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