JVM GC and unused references












0














Let's say there are following four objects in a Java program:




  • main

  • a

  • b

  • c


The dependencies are as follows:



main ==> a ==> b ==> c



In other words, main uses a, a uses b, b uses c.



Now, value of object a changes from reference to b to null. What it means is that there are no active references to b, making it available for garbage collection:



main ==> a =/=> b ==> c



What could happen now:



A) In laymans terms, b can be garbage collected at any time. My assumption is that c will become elligible for gargabe collection only after b is garbage collected.



B) However, I imagine this could be a case that JVM could somehow distinguish between references to c from GC-eligible and non-GC-eligible objects, and therefore mark c as eligible at the same time as b.



Is the behaviour determined by Java Language Specification, or any JVM-related specification? Or is it left for the JVM implementation to decide?






java garbage-collection jvm







share|improve this question


















  • 1




    Some related questions: stackoverflow.com/questions/31451346/…, stackoverflow.com/questions/8812635/…, and stackoverflow.com/questions/10587868/…
    – Slaw
    Nov 12 '18 at 13:23






  • 1




    All that a garbage collector cares for, is that main and a are still reachable. Everything else is garbage. Despite its misleading name, a garbage collector never looks at the garbage.
    – Holger
    Nov 12 '18 at 16:03
















0














Let's say there are following four objects in a Java program:




  • main

  • a

  • b

  • c


The dependencies are as follows:



main ==> a ==> b ==> c



In other words, main uses a, a uses b, b uses c.



Now, value of object a changes from reference to b to null. What it means is that there are no active references to b, making it available for garbage collection:



main ==> a =/=> b ==> c



What could happen now:



A) In laymans terms, b can be garbage collected at any time. My assumption is that c will become elligible for gargabe collection only after b is garbage collected.



B) However, I imagine this could be a case that JVM could somehow distinguish between references to c from GC-eligible and non-GC-eligible objects, and therefore mark c as eligible at the same time as b.



Is the behaviour determined by Java Language Specification, or any JVM-related specification? Or is it left for the JVM implementation to decide?






java garbage-collection jvm







share|improve this question


















  • 1




    Some related questions: stackoverflow.com/questions/31451346/…, stackoverflow.com/questions/8812635/…, and stackoverflow.com/questions/10587868/…
    – Slaw
    Nov 12 '18 at 13:23






  • 1




    All that a garbage collector cares for, is that main and a are still reachable. Everything else is garbage. Despite its misleading name, a garbage collector never looks at the garbage.
    – Holger
    Nov 12 '18 at 16:03














0












0








0







Let's say there are following four objects in a Java program:




  • main

  • a

  • b

  • c


The dependencies are as follows:



main ==> a ==> b ==> c



In other words, main uses a, a uses b, b uses c.



Now, value of object a changes from reference to b to null. What it means is that there are no active references to b, making it available for garbage collection:



main ==> a =/=> b ==> c



What could happen now:



A) In laymans terms, b can be garbage collected at any time. My assumption is that c will become elligible for gargabe collection only after b is garbage collected.



B) However, I imagine this could be a case that JVM could somehow distinguish between references to c from GC-eligible and non-GC-eligible objects, and therefore mark c as eligible at the same time as b.



Is the behaviour determined by Java Language Specification, or any JVM-related specification? Or is it left for the JVM implementation to decide?






java garbage-collection jvm







share|improve this question













Let's say there are following four objects in a Java program:




  • main

  • a

  • b

  • c


The dependencies are as follows:



main ==> a ==> b ==> c



In other words, main uses a, a uses b, b uses c.



Now, value of object a changes from reference to b to null. What it means is that there are no active references to b, making it available for garbage collection:



main ==> a =/=> b ==> c



What could happen now:



A) In laymans terms, b can be garbage collected at any time. My assumption is that c will become elligible for gargabe collection only after b is garbage collected.



B) However, I imagine this could be a case that JVM could somehow distinguish between references to c from GC-eligible and non-GC-eligible objects, and therefore mark c as eligible at the same time as b.



Is the behaviour determined by Java Language Specification, or any JVM-related specification? Or is it left for the JVM implementation to decide?






java garbage-collection jvm




java garbage-collection jvm






share|improve this question













share|improve this question











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asked Nov 12 '18 at 13:16









automatictester

1,084622




1,084622








  • 1




    Some related questions: stackoverflow.com/questions/31451346/…, stackoverflow.com/questions/8812635/…, and stackoverflow.com/questions/10587868/…
    – Slaw
    Nov 12 '18 at 13:23






  • 1




    All that a garbage collector cares for, is that main and a are still reachable. Everything else is garbage. Despite its misleading name, a garbage collector never looks at the garbage.
    – Holger
    Nov 12 '18 at 16:03














  • 1




    Some related questions: stackoverflow.com/questions/31451346/…, stackoverflow.com/questions/8812635/…, and stackoverflow.com/questions/10587868/…
    – Slaw
    Nov 12 '18 at 13:23






  • 1




    All that a garbage collector cares for, is that main and a are still reachable. Everything else is garbage. Despite its misleading name, a garbage collector never looks at the garbage.
    – Holger
    Nov 12 '18 at 16:03








1




1




Some related questions: stackoverflow.com/questions/31451346/…, stackoverflow.com/questions/8812635/…, and stackoverflow.com/questions/10587868/…
– Slaw
Nov 12 '18 at 13:23




Some related questions: stackoverflow.com/questions/31451346/…, stackoverflow.com/questions/8812635/…, and stackoverflow.com/questions/10587868/…
– Slaw
Nov 12 '18 at 13:23




1




1




All that a garbage collector cares for, is that main and a are still reachable. Everything else is garbage. Despite its misleading name, a garbage collector never looks at the garbage.
– Holger
Nov 12 '18 at 16:03




All that a garbage collector cares for, is that main and a are still reachable. Everything else is garbage. Despite its misleading name, a garbage collector never looks at the garbage.
– Holger
Nov 12 '18 at 16:03












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Thanks to this link, I believe option B is correct:
https://www.dynatrace.com/resources/ebooks/javabook/how-garbage-collection-works/
Once b and c are no longer reachable from GC roots, they are both collectible at the same time.






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    Thanks to this link, I believe option B is correct:
    https://www.dynatrace.com/resources/ebooks/javabook/how-garbage-collection-works/
    Once b and c are no longer reachable from GC roots, they are both collectible at the same time.






    share|improve this answer


























      0














      Thanks to this link, I believe option B is correct:
      https://www.dynatrace.com/resources/ebooks/javabook/how-garbage-collection-works/
      Once b and c are no longer reachable from GC roots, they are both collectible at the same time.






      share|improve this answer
























        0












        0








        0






        Thanks to this link, I believe option B is correct:
        https://www.dynatrace.com/resources/ebooks/javabook/how-garbage-collection-works/
        Once b and c are no longer reachable from GC roots, they are both collectible at the same time.






        share|improve this answer












        Thanks to this link, I believe option B is correct:
        https://www.dynatrace.com/resources/ebooks/javabook/how-garbage-collection-works/
        Once b and c are no longer reachable from GC roots, they are both collectible at the same time.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 12 '18 at 19:12









        automatictester

        1,084622




        1,084622






























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