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ridepublic void addApplicationListener(ApplicationListener<?> listener) {    synchronized (this.retrievalMutex) {        // Explicitly remove target for a proxy, if registered already,        // in order to avoid double invocations of the same listener.        Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);        if (singletonTarget instanceof ApplicationListener) {            this.defaultRetriever.applicationListeners.remove(singletonTarget);