@Override
    public void handle(Callback[] callbacks) throws UnsupportedCallbackException {
        if (!configured())
            throw new IllegalStateException("Callback handler not configured");
        for (Callback callback : callbacks) {
            if (callback instanceof OAuthBearerValidatorCallback) {
                OAuthBearerValidatorCallback validationCallback = (OAuthBearerValidatorCallback) callback;
                try {
                    handleCallback(validationCallback);
                } catch (OAuthBearerIllegalTokenException e) {
                    OAuthBearerValidationResult failureReason = e.reason();
                    String failureScope = failureReason.failureScope();
                    validationCallback.error(failureScope != null ? "insufficient_scope" : "invalid_token",
                            failureScope, failureReason.failureOpenIdConfig());
                }
            } else if (callback instanceof OAuthBearerExtensionsValidatorCallback) {
                OAuthBearerExtensionsValidatorCallback extensionsCallback = (OAuthBearerExtensionsValidatorCallback) callback;
                extensionsCallback.inputExtensions().map().forEach((extensionName, v) -> extensionsCallback.valid(extensionName));
            } else
                throw new UnsupportedCallbackException(callback);
        }
    }
Return true if this instance has been configured, otherwise false

@return true if this instance has been configured, otherwise false
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/unsecured/OAuthBearerUnsecuredValidatorCallbackHandler.java