code: public V retrieve(K key) {
          Ref<V> ref = map.get(key);
          return ref == null ? null : ref.get();
      }
docs: Retrieve the {@link ClassLoader} (if any) stored with the given {@link ClassLoaderIdentity}.
  
  @return <code>null</code> if no {@link ClassLoader} has been stored or the {@link ClassLoader} has been collected.