The HotSpot VM maintains three hash tables to track class loading. The
SystemDictionary contains loaded classes, which maps a class name/class loader pair to a klassOop. The SystemDictionary contains both class name/initiating loader pairs and class name/defining loader pairs. Entries are currently only
removed at a safepoint. Safepoints are described in more detail in the “VM Operations and Safepoints” section later in the chapter. The
PlaceholderTable contains classes that are currently being loaded. It is used for ClassCircularityError checking and for parallel class loading for class loaders that support multithreaded class loading. The
LoaderConstraintTable tracks constraints for type safety checking. These hash tables are all guarded by a lock; in the HotSpot VM it is called the SystemDictionary_lock. In general, the load class phase in the HotSpot VM is serialized using
the Class loader object lock.