The InstanceKlass::_array_name was introduced to have a pointer to decrement the refcount in the symbol [L<classname>; so that the class got unloaded, the array name is unloaded also. At that time, InstanceKlass::release_C_heap_structures() was only called for classes in the SystemDictionary. Now all the _klasses in the ClassLoaderData are walked and release_C_heap_structures can be called on Array classes also.

At some point, maybe in permgen elimination, creating the array name called new_permanent_symbol so is never reclaimed.

If the Klass::_name Symbol refcount is decremented during unloading for both ArrayKlass and InstanceKlass, there's no need for InstanceKlass::_array_name field.