ExceptionHandlingHttpService.java

public final class ExceptionHandlingHttpService extends SimpleDecoratingService<HttpRequest, HttpResponse> {
    @Value(staticConstructor = "of")
    private static class ExceptionHandler {
        Class<? extends Throwable> exceptionType;
        BiFunction<HttpRequest, Throwable, HttpResponse> exceptionHandler;
    }

    private final ExceptionHandler defaultExceptionHandler =
            ExceptionHandler.of(Throwable.class, ExceptionHandlingHttpService::error);

    private final List<ExceptionHandler> exceptionHandlers = ImmutableList.of(
            // bad request
            ExceptionHandler.of(IllegalArgumentException.class, this::badRequest),
            // not found
            ExceptionHandler.of(ResourceNotFoundException.class, this::notFound),
            ExceptionHandler.of(PageNotFoundException.class, this::notFound),

            // Network error.
            ExceptionHandler.of(IOException.class, this::serverError)
    );
    //...
    private HttpResponse notFound(HttpRequest request, Throwable ex) {
        log.warn("Not found, request:{}", request.path(), ex);
        return HttpResponse.of(HttpStatus.NOT_FOUND);
    }
    //...
    private HttpResponse handleException(HttpRequest req, Throwable thrown) {
        Throwable rootCause = Throwables.getRootCause(thrown);
        return exceptionHandlers.stream()
                                .filter(handler -> rootCause.getClass()
                                                            .isAssignableFrom(handler.getExceptionType()))
                                .findFirst()
                                .orElse(defaultExceptionHandler)
                                .getExceptionHandler().apply(req, rootCause);
    }

    @Override
    public HttpResponse serve(ServiceRequestContext ctx, HttpRequest req) {
        final HttpResponse res;
        try {
            res = delegate().serve(ctx, req);
        } catch (Exception e) {
            return handleException(req, e);
        }

        return HttpResponse.from(res.aggregate().handle((message, thrown) -> {
            if (thrown != null) {
                return handleException(req, thrown);
            }
            return message.toHttpResponse();
        }));
    }
}