During -Xshare:dump, we run the following code to find all classes to be archived:

http://hg.openjdk.java.net/jdk/jdk/file/0348e9be94d1/src/hotspot/share/memory/metaspaceShared.cpp#l1582

  CollectClassesClosure collect_classes;
  ClassLoaderDataGraph::loaded_classes_do(&collect_classes);

However, CDS already maintains DumpTimeSharedClassTable that contains all possible classes that can be archived, and most of the CDS code iterates over this table (e.g., when writing the shared dictionaries). There's a potential risk that the ClassLoaderDataGraph may get out of sync DumpTimeSharedClassTable (unlikely but who knows), so it's better for all CDS code to exclusively use DumpTimeSharedClassTable.

NB., the above is only for -Xshare:dump (aka static dumping). We don't call ClassLoaderDataGraph::loaded_classes_do during -XX:ArchiveClassesAtExit (aka dynamic dumping).