How does Java Garbage Collection work with Circular References?

Question

From my understanding, garbage collection in Java cleans up some objects if nothing else is 'pointing' to that object.

My question is, what happens if we have something like this:

class Node {
    public object value;
    public Node next;
    public Node(object o, Node n) { value = 0; next = n;}
}

//...some code
{
    Node a = new Node("a", null), 
         b = new Node("b", a), 
         c = new Node("c", b);
    a.next = c;
} //end of scope
//...other code

a, b, and c should be garbage collected, but they are all being referenced by other objects.

How does the Java garbage collection deal with this? (or is it simply a memory drain?)

Answer 1

Java's GC considers objects "garbage" if they aren't reachable through a chain starting at a garbage collection root, so these objects will be collected. Even though objects may point to each other to form a cycle, they're still garbage if they're cut off from the root.

See the section on unreachable objects in Appendix A: The Truth About Garbage Collection in Java Platform Performance: Strategies and Tactics for the gory details.