[JAVA] I want to control the default error message of Spring Boot

Purpose

I want to control the default error message returned when an error occurs in the REST API created by Spring Boot.

background

Suppose you have prepared the following RestController.

MyRestController.java

package demo;

import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class MyRestController {
    
    @RequestMapping(value="/test/{id}")
    private String hello(@PathVariable String id){
        if ("0000".equals(id)) {
            throw new MyCustomException(); //Expected Exception
        }
        
        return "Hello world!!";
    }

    @RequestMapping(value="/test-null")
    private String hello(){
        String s = null;
        s.equals("hoge"); //Inadvertent NPE outbreak
        
        return "Hello world!!";
    }
}

Spring Boot will then return the following error message by default.

$ curl http://localhost:8080/test/0000 #Expected MyCustomError
{"timestamp":1526654632290,"status":500,"error":"Internal Server Error","exception":"my.spring.MySpringBootSample.MyCustomError","message":"No message available","path":"/test/0000"}%
$ curl http://localhost:8080/test-null #NPE that I accidentally put out
{"timestamp":1526654636310,"status":500,"error":"Internal Server Error","exception":"java.lang.NullPointerException","message":"No message available","path":"/test-null"}%
$ curl http://localhost:8080/wrong-path #Wrong path
{"timestamp":1526654639289,"status":404,"error":"Not Found","message":"No message available","path":"/wrong-path"}%

However, it's too cool and not good for security to see the NullPointerException that happened accidentally to the user. I'm also exposed to using Spring Boot.

Controlling possible errors

By preparing a class with @ControllerAdvice annotation, it is possible to control error messages across RestController.

MyControllerAdvice.java

package demo;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

@ControllerAdvice
public class MyControllerAdvice {

    /**
     *Handler when MyCustomException occurs
     * 
     * @return response
     */
    @ExceptionHandler(MyCustomException.class)
    public ResponseEntity<String> handleControllerException() {
        return new ResponseEntity<String>("customized message for MyCustomError.", HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

$ curl http://localhost:8080/test/0000
customized message for MyCustomError.%

Control against unexpected errors

In addition to the above, if you create an ExceptionHandler that receives Exception.class, it is possible to return a custom error message uniformly for all unexpected errors that occur in RestController.

MyControllerAdvice.java

package demo;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

@ControllerAdvice
public class MyControllerAdvice {

    /**
     *Handler when MyCustomException occurs
     * 
     * @return response
     */
    @ExceptionHandler(MyCustomException.class)
    public ResponseEntity<String> handleControllerException() {
        return new ResponseEntity<String>("customized message for MyCustomError.", HttpStatus.INTERNAL_SERVER_ERROR);
    }
    
    /**
     *Handler when an Exception occurs
     * 
     * @return response
     */
    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleControllerException2() {
        return new ResponseEntity<String>("customized message for any unexpected exception.", HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

However, this method does not control errors before entering RestController, such as incorrect paths.

$ curl http://localhost:8080/test/0000
customized message for MyCustomError.%
$ curl http://localhost:8080/test-null
customized message for any unexpected exception.%
$ curl http://localhost:8080/wrong-path #Default error is returned for wrong path
{"timestamp":1526655182855,"status":404,"error":"Not Found","message":"No message available","path":"/wrong-path"}%

When I looked it up, it was [Official](https://docs.spring.io/spring-boot/docs/1.5.3.RELEASE/reference/html/boot-features-developing-web-applications.html#boot-features -error-handling) has the following description.

Spring Boot provides an /error mapping by default that handles all errors in a sensible way, and it is registered as a ‘global’ error page in the servlet container.

The point is that every error is mapped under / error by default. If you want to control it as you like, realize ErrorController, so prepare the following class.

MyErrorController.java

package demo;

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

@RestController
public class MyErrorController implements ErrorController {

    @RequestMapping("/error")
    public ResponseEntity<String> handleError() {
        return new ResponseEntity<String>("customized message for any unhandled error.", HttpStatus.INTERNAL_SERVER_ERROR);
    }

    @Override
    public String getErrorPath() {
        return "/error";
    }
}

Then, the default error message disappeared.

$ curl http://localhost:8080/test/0000
customized message for MyCustomError.%
$ curl http://localhost:8080/test-null
customized message for any unexpected exception.%
$ curl http://localhost:8080/wrong-path
customized message for any unhandled error.%