Garbage collection of Composed Objects

Question

I have two classes

Class A
{
    //constructor
}

Class B
{
    private A a;

    public B()
    {
        a = new A();
    }
}

Suppose I use an object of B [say b] in my code and after I end up using it, I set it to null. I know that the object of B is now available for garbage collection.

I know that after setting b to null, it will be immediately eligible for garbage collection? But what about the object of type A? Will it be available for garbage collection immediately after I set B to null? Or will it be eligible for garbage collection after B is garbage collected?

Theoretically, until B is garbage collected, a still has a reference to it? So will the SUN JVM compiler detect this immediately after setting b = null;

Answer 1

Once there are no hard references to b there is no way to reach its reference to a, so both are eligible for garbage collection as far as I know.

Answer 2

Generally, you shouldn't care. It's not reachable, and you should let the garbage collection worry about the rest. Since it becomes unreachable, it should be eligeble for collection immediately.

Answer 3

There are also some flags you can use when running a Java program (at least with the Sun JVM) that will give some debugging information about what goes on during garbage collection. Try the -verbose:gc option.

http://download.oracle.com/javase/6/docs/technotes/tools/solaris/java.html

Answer 4

The GC is smart enough to follow paths in graphs of objects in order to check if the existing references to some object are reachable. It can even detect cycles (like, in your case, if b held a reference to a and a held a reference to b.

Answer 5

Object A will also become eligible garbage collection as soon as you set ObjectB to null (* condition is ObjectA is not referred by any other live object). This condition is known as "Island of Isolation".

Here is a nice explanation of this situation. (Here is one more explanation from SCJP book)