ExceptionHandler

Additionally you can implement a ExceptionHandler; a generic hook for handling exceptions that occurs during the execution of an HTTP request.

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()
    }
}

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

If you want to respond 200 OK with 0 (stock level) as the response body when the OutOfStockException is thrown, you could register a ExceptionHandler:

@Produces
@Singleton
@Requires(classes = {OutOfStockException.class, ExceptionHandler.class})
public class OutOfStockExceptionHandler implements ExceptionHandler<OutOfStockException, HttpResponse> {
    @Override
    public HttpResponse handle(HttpRequest request, OutOfStockException exception) {
        return HttpResponse.ok(0);
    }
}

@Produces
@Singleton
@Requires(classes = [OutOfStockException.class, ExceptionHandler.class])
class OutOfStockExceptionHandler implements ExceptionHandler<OutOfStockException, HttpResponse> {
    @Override
    HttpResponse handle(HttpRequest request, OutOfStockException exception) {
        HttpResponse.ok(0)
    }
}

@Produces
@Singleton
@Requirements(
        Requires(classes = [OutOfStockException::class, ExceptionHandler::class])
)
class OutOfStockExceptionHandler : ExceptionHandler<OutOfStockException, HttpResponse<*>> {
    override fun handle(request: HttpRequest<*>, exception: OutOfStockException): HttpResponse<*> {
        return HttpResponse.ok(0)
    }
}