private Authorizer<HttpRequest> authorizer() {
        if (authorizer == null) {
            throw new IllegalStateException("no " + Authorizer.class.getSimpleName() + " was added.");
        }
        return authorizer;
    }
Returns a newly-created decorator that decorates an {@link HttpService} with a new
{@link AuthService} based on the {@link Authorizer}s added to this builder.
core/src/main/java/com/linecorp/armeria/server/auth/AuthServiceBuilder.java