@Nullable
    private ClientAuthentication clientAuthentication() {
        if (clientAuthenticationSupplier != null) {
            final ClientAuthentication clientAuthentication = clientAuthenticationSupplier.get();
            return requireNonNull(clientAuthentication, "clientAuthenticationSupplier.get() returned null");
        }
        return null;
    }
Implements Token introspection request/response flow, as per
<a href="https://datatracker.ietf.org/doc/html/rfc7662">RFC 7662</a>.
oauth2/src/main/java/com/linecorp/armeria/server/auth/oauth2/TokenIntrospection.java