Garbage collectors have no problem with reference cycles.

A GC works by finding all objects that are reachable (referenced by some other object or a root), then discarding all other objects. So a cyclic data structure is completely discarded as soon as there are no live references to any of its objects.

As usual, Wikipedia has lots more information: http://en.wikipedia.org/wiki/Garbage_collection_(computer_sc...

GC roots are generally things like: static/global variables, active stack frames (function parameters and local variable), CPU registers.

BDW GC is a tracing collector. It doesn't need to have any special handle of cycles--either a cycle is reachable from the root set, in which case it is marked and considered alive, or it is not, and is never even seen by the collector before being reclaimed.