6.17.4.2 Custom Exception Handler

Imagine your e-commerce app throws an OutOfStockException when a book is out of stock:

public class OutOfStockException extends RuntimeException {
}

class OutOfStockException extends RuntimeException {
}

class OutOfStockException : RuntimeException()

Along with BookController:

@Controller("/books")
public class BookController {
    @Produces(MediaType.TEXT_PLAIN)
    @Get("/stock/{isbn}")
    Integer stock(String isbn) {
        throw new OutOfStockException();
    }
}

@Controller("/books")
class BookController {
    @Produces(MediaType.TEXT_PLAIN)
    @Get("/stock/{isbn}")
    Integer stock(String isbn) {
        throw new OutOfStockException()
    }
}

@Controller("/books")
class BookController {
    @Produces(MediaType.TEXT_PLAIN)
    @Get("/stock/{isbn}")
    internal fun stock(isbn: String): Int? {
        throw OutOfStockException()
    }
}

The server returns a 500 (Internal Server Error) status code if you don’t handle the exception.

To respond with 400 Bad Request as the response when the OutOfStockException is thrown, you can register a ExceptionHandler:

@Produces
@Singleton
@Requires(classes = {OutOfStockException.class, ExceptionHandler.class})
public class OutOfStockExceptionHandler implements ExceptionHandler<OutOfStockException, HttpResponse> {
    private final ErrorResponseProcessor<?> errorResponseProcessor;
    public OutOfStockExceptionHandler(ErrorResponseProcessor<?> errorResponseProcessor) {
        this.errorResponseProcessor = errorResponseProcessor;
    }
    @Override
    public HttpResponse handle(HttpRequest request, OutOfStockException e) {
        return errorResponseProcessor.processResponse(ErrorContext.builder(request)
                .cause(e)
                .errorMessage("No stock available")
                .build(), HttpResponse.badRequest()); (1)
    }
}

@Produces
@Singleton
@Requires(classes = [OutOfStockException, ExceptionHandler])
class OutOfStockExceptionHandler implements ExceptionHandler<OutOfStockException, HttpResponse> {
    private final ErrorResponseProcessor<?> errorResponseProcessor
    OutOfStockExceptionHandler(ErrorResponseProcessor<?> errorResponseProcessor) {
        this.errorResponseProcessor = errorResponseProcessor
    }
    @Override
    HttpResponse handle(HttpRequest request, OutOfStockException e) {
        errorResponseProcessor.processResponse(ErrorContext.builder(request)
                .cause(e)
                .errorMessage("No stock available")
                .build(), HttpResponse.badRequest()) (1)
    }
}

@Produces
@Singleton
@Requirements(
    Requires(classes = [OutOfStockException::class, ExceptionHandler::class])
)
class OutOfStockExceptionHandler(private val errorResponseProcessor: ErrorResponseProcessor<Any>) :
    ExceptionHandler<OutOfStockException, HttpResponse<*>> {
    override fun handle(request: HttpRequest<*>, exception: OutOfStockException): HttpResponse<*> {
        return errorResponseProcessor.processResponse(
                ErrorContext.builder(request)
                    .cause(exception)
                    .errorMessage("No stock available")
                    .build(), HttpResponse.badRequest<Any>()) (1)
    }
}