Garbage collection of composite objects

Question

I have two classes

Class A { //constructor } Class B { private A a; public B() { a = new A(); } } 

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

I know that after setting b to null it will be immediately suitable for garbage collection? But what about an object of type A? Will it be available for garbage collection right after I set B to null ? Or will it be acceptable to collect garbage after B is a garbage collector ?

Theoretically, until B is garbage collected, does it still have a link to it? Thus, the SUN JVM compiler detects this immediately after setting b = null ;

Answer 1

GC is smart enough to track paths in object graphs to check if existing references to any object are available. It can even detect loops (for example, in your case, if b contained a link to a and a , contained a link to b .

Answer 2

After there are no hard links to b, there is no way to get to the link to a, so both are entitled to garbage collection, as far as I know.

Answer 3

As a rule, you don't care. This is not possible and you must let the trash can worry about everything else. As it becomes unavailable, it must be immediately removed for collection.

Answer 4

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

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

Answer 5

Object A will also be a suitable garbage collection as soon as you set ObjectB to null (* condition - ObjectA is not passed to any other living object). This condition is known as the “Isolation Island”.

Here is a good explanation of this situation. ( Here is another explanation from the SCJP book)