GC Process with reference

1. Start at a root.
2. Trace and mark strongly-referenced objects.
3. Optionally clear soft references.
4. Trace and mark softly-referenced objects.
5. Clear weak references.
6. Enqueue finalizable objects.
7. Repeat steps 1 through 5 for the queue.
8. Possibly enqueue phantom references.
9. The remaining objects are dead.
10. Repeat.