Java Does Nested Class Instance Prevent GC of Outer Class

Question

Assume Outer is instantiated somewhere, and after a while there are no references to it. Does the implicit reference in inner to the Outer instance prevent GC from running, or does the is it a "special" reference?

public class Outer {
    private class Inner extends Something {}

    private Inner inner = new Inner();
}

If an object tree is not reached form GC root it will be the subject of GC. In your sample it is. stackoverflow.com/questions/6366211/what-are-the-roots/… — vrudkovsk
– vrudkovsk, Commented Apr 13, 2015 at 5:46

jdphenix · Accepted Answer · 2015-04-13 05:57:33Z

3

If the instance of Outer is not reachable from a GC root, the reference to an Inner instance in your example code will not stop the garbage collector from freeing the memory the Outer used.

Consider this diagram:

Stack of main thread           public static void main(String[] args) {
         |                       Outer outer = new Outer();
         v                     
       Outer<--\
         |\    |
         v \---/
       Inner 

Stack of main thread               outer = new Outer();
         |
         v          
       Outer<--\
         |\ new|
         v \---/
       Inner 

       Outer<--\                   // a reference to the Outer from the Inner
         |\ old|                   // doesn't change the fact that the Outer
         v \---/                   // can't be reached from a GC root.
       Inner                       // Thus the old Outer is eligible for collection (dead)

edited Apr 13, 2015 at 5:57

answered Apr 13, 2015 at 5:46

jdphenix

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3 Comments

Eliezer Over a year ago

Just to clarify, since there are no references to Outer besides for one from "inside" itself, it is eligible for GC?

jdphenix Over a year ago

Not quite, it is eligible for GC under the assertion you made in your question "and after a while there are no references to it [Outer]"

Eliezer Over a year ago

Yes, that's what I meant. I should have been more explicit.

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