@Override
    public void handle(Callback[] callbacks) throws IOException, UnsupportedCallbackException {
        if (!configured())
            throw new IllegalStateException("Callback handler not configured");
        for (Callback callback : callbacks) {
            if (callback instanceof OAuthBearerTokenCallback)
                handleCallback((OAuthBearerTokenCallback) callback);
            else if (callback instanceof SaslExtensionsCallback)
                handleCallback((SaslExtensionsCallback) callback, Subject.getSubject(AccessController.getContext()));
            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/OAuthBearerSaslClientCallbackHandler.java