private static void ensureResourcesInitialized() {
        if (RESOURCES.get() == null) {
            RESOURCES.set(new HashMap<Object, Object>());
        }
    }
Returns the value bound in the {@code ThreadContext} under the specified {@code key}, or {@code null} if there
is no value for that {@code key}.

@param key the map key to use to lookup the value
@return the value bound in the {@code ThreadContext} under the specified {@code key}, or {@code null} if there
is no value for that {@code key}.
@since 1.0
core/src/main/java/org/apache/shiro/util/ThreadContext.java