private void handleValidatorCallback(OAuthBearerValidatorCallback callback) {
        checkInitialized();

        OAuthBearerToken token;

        try {
            token = accessTokenValidator.validate(callback.tokenValue());
            callback.token(token);
        } catch (ValidateException e) {
            log.warn(e.getMessage(), e);
            callback.error("invalid_token", null, null);
        }
    }
Because a {@link CloseableVerificationKeyResolver} instance can spawn threads and issue
HTTP(S) calls ({@link RefreshingHttpsJwksVerificationKeyResolver}), we only want to create
a new instance for each particular set of configuration. Because each set of configuration
may have multiple instances, we want to reuse the single instance.
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/OAuthBearerValidatorCallbackHandler.java