public void clear() {
        strongReferencesLock.lock();
        try {
            strongReferences.clear();
        } finally {
            strongReferencesLock.unlock();
        }
        // throw out garbage collected values
        processQueue();
        map.clear();
    }
Creates a new entry, but wraps the value in a SoftValue instance to enable auto garbage collection.
lang/src/main/java/org/apache/shiro/lang/util/SoftHashMap.java