public Object removeAttribute(Object key) {
        Map<Object, Object> attributes = getAttributes();
        if (attributes == null) {
            return null;
        } else {
            return attributes.remove(key);
        }
    }
Determines if this session is expired.

@return true if the specified session has expired, false otherwise.
core/src/main/java/org/apache/shiro/session/mgt/SimpleSession.java