// Phase2: remove all those references whose referents are reachable.

inline void process_phase2(DiscoveredList&      refs_list,

                             BoolObjectClosure* is_alive,

                             OopClosure*        keep_alive,

                             VoidClosure*       complete_gc) {

    // complete_gc is ignored in this case for this phase

    pp2_work(refs_list, is_alive, keep_alive);

    // ...

}

// Traverse the list and remove any Refs that are not active, or

// whose referents are either alive or NULL.

void ReferenceProcessor::pp2_work(

                             DiscoveredList&    refs_list,

                             BoolObjectClosure* is_alive,

                             OopClosure*        keep_alive

) {

  DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

  while (iter.has_next()) {

    iter.load_ptrs();

    if (iter.is_referent_alive()) {

      // The referent is reachable after all.

      // Remove Reference object from list.

      iter.remove();

      // Update the referent pointer as necessary: Note that this

      // should not entail any recursive marking because the

      // referent must already have been traversed.

      iter.make_referent_alive();

      iter.move_to_next();

    } else {

      iter.next();

    }

  }

}

// Traverse the list and process the referents, by either

// clearing them or keeping them (and their reachable

// closure) alive.

void ReferenceProcessor::process_phase3(

                                   DiscoveredList&    refs_list,

                                   bool               clear_referent,

                                   BoolObjectClosure* is_alive,

                                   OopClosure*        keep_alive,

                                   VoidClosure*       complete_gc

) {

  ResourceMark rm;

  DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

  while (iter.has_next()) {

    iter.update_discovered();

    iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));

    if (clear_referent) {

        // NULL out referent pointer

        // 将Reference的referent字段置为null，之后会被GC回收

        iter.clear_referent();

    } else {

        // keep the referent around

        // 标记引用的对象为存活，该对象在这次GC将不会被回收

        iter.make_referent_alive();

    }

    iter.next();

  }

  // Remember to update the next pointer of the last ref.

  iter.update_discovered();

  // Close the reachable set

  complete_gc->do_void();

}