public void init(CloseableVerificationKeyResolver verificationKeyResolver, AccessTokenValidator accessTokenValidator) {
        this.verificationKeyResolver = verificationKeyResolver;
        this.accessTokenValidator = accessTokenValidator;

        try {
            verificationKeyResolver.init();
        } catch (Exception e) {
            throw new KafkaException("The OAuth validator configuration encountered an error when initializing the VerificationKeyResolver", e);
        }

        isInitialized = true;
    }
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