ExceptionHandling.md

These are the three annotation that helps us.

• @ResponseStatus
• @ExceptionHandler
• @ControllerAdvice

Default Exception Handling

If we call the /product API with an invalid id the service will throw a NoSuchElementFoundException runtime exception and we’ll get the following response:

{
  "timestamp": "2020-11-28T13:24:02.239+00:00",
  "status": 500,
  "error": "Internal Server Error",
  "message": "",
  "path": "/product/1"
}

We can see that besides a well-formed error response, the payload is not giving us any useful information. Even the message field is empty, which we might want to contain something like “Item with id 1 not found”.

Spring Boot provides some properties with which we can add the exception message, exception class, or even a stack trace as part of the response payload:

server:
  error:
  include-message: always
  include-binding-errors: always
  include-stacktrace: on_trace_param
  include-exception: false

Now if we call the /product API again with an invalid id we’ll get the following response:

{
  "timestamp": "2020-11-29T09:42:12.287+00:00",
  "status": 500,
  "error": "Internal Server Error",
  "message": "Item with id 1 not found",
  "path": "/product/1"
} 

Moving on! The status and error message - 500 - indicates that something is wrong with our server code but actually it’s a client error because the client provided an invalid id.

ResponseStatus

As the name suggests, @ResponseStatus allows us to modify the HTTP status of our response. It can be applied in the following places:

@ResponseStatus(value = HttpStatus.NOT_FOUND)
public class NoSuchElementFoundException extends RuntimeException {
  ...
}

{
  "timestamp": "2020-11-29T09:42:12.287+00:00",
  "status": 404,
  "error": "Not Found",
  "message": "Item with id 1 not found",
  "path": "/product/1"
} 

ExceptionHandler

The @ExceptionHandler annotation gives us a lot of flexibility in terms of handling exceptions. For starters, to use it, we simply need to create a method either in the controller itself or in a @ControllerAdvice class and annotate it with @ExceptionHandler:

 @ExceptionHandler(NoSuchElementFoundException.class)
  @ResponseStatus(HttpStatus.NOT_FOUND)
  public ResponseEntity<String> handleNoSuchElementFoundException(
      NoSuchElementFoundException exception
  ) {
    return ResponseEntity
        .status(HttpStatus.NOT_FOUND)
        .body(exception.getMessage());
  }

ControllerAdvice

Controller advice classes allow us to apply exception handlers to more than one or all controllers in our application:

Global Exception Handling

We use ResponseEntityExceptionHandler class for this. We need to extend this class.
We need to also add @ControllerAdvice.

@ControllerAdvice is to implement logic common to all the controllers.Its a specialization of Component for classes that declare exception methods to be shared across multiple controller classes.

Ok, we just create a custom class that extends ResponseEntityExceptionHandler and implement a method called handleException. But this method is final so we just change the name as handleAllExceptions.
So, what we want to do in this method is that ne create an ExceptionResponse class with defined fields that represents common messages.

The class looks like this. We just add methods for custom exception that we need to handle @ControllerAdvice public class CustomExceptionHandler extends ResponseEntityExceptionHandler {

    @ExceptionHandler(Exception.class)
    public final ResponseEntity<Object> handleAllExceptions
            (Exception ex, WebRequest request) throws Exception {

        ExceptionResponse exceptionResponse = new ExceptionResponse().setTimestamp(new Date())
                .setMessage(ex.getMessage())
                .setMessage(request.getDescription(false));

        return new ResponseEntity(exceptionResponse, HttpStatus.INTERNAL_SERVER_ERROR);
    }

    @ExceptionHandler(UserNotFoundException.class)
    public final ResponseEntity<Object> handleUserNotFoundExceptions
            (UserNotFoundException ex, WebRequest request) throws Exception {

        ExceptionResponse exceptionResponse = new ExceptionResponse().setTimestamp(new Date())
                .setMessage(ex.getMessage())
                .setMessage(request.getDescription(false));

        return new ResponseEntity(exceptionResponse, HttpStatus.NOT_FOUND);
    }
}