Implement Best-Practice Error Message in Spring REST Controller - spring

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.

Answer 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.