AppletTalk.com Java discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  do self references cause memory leaks in Java? Goto page 1, 2  Next          AppletTalk.com Forum Index -> Java Help View previous topic :: View next topic   Author Message Novice Guest Posted: Mon Jul 14, 2003 7:25 pm    Post subject: do self references cause memory leaks in Java? Hi, I am doing some work with Java and C++ and am curious as to whether self references in Java cause memory leaks. For example: class Test{ Object foo =this; } If I instantiate Test, the reference count of any Test object will never become zero (as it always keeps a reference to itself) so long as the foo variable is never re-assigned. Does this present a problem for Java's garbage collection? Thanks, Novice Back to top Adam Maass Guest Posted: Mon Jul 14, 2003 7:37 pm    Post subject: Re: do self references cause memory leaks in Java? "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote: Quote: Hi, I am doing some work with Java and C++ and am curious as to whether self references in Java cause memory leaks. For example: class Test{ Object foo =this; } If I instantiate Test, the reference count of any Test object will never become zero (as it always keeps a reference to itself) so long as the foo variable is never re-assigned. Does this present a problem for Java's garbage collection? Garbage collection in Java does not use reference counting. Rather, most simple gc systems use a mark-and-sweep method. -- Adam Maass Back to top Novice Guest Posted: Mon Jul 14, 2003 7:57 pm    Post subject: Re: do self references cause memory leaks in Java? "Adam Maass" <adam.nospam.maass (AT) comcast (DOT) net> wrote Quote: "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote: Hi, I am doing some work with Java and C++ and am curious as to whether self references in Java cause memory leaks. For example: class Test{ Object foo =this; } If I instantiate Test, the reference count of any Test object will never become zero (as it always keeps a reference to itself) so long as the foo variable is never re-assigned. Does this present a problem for Java's garbage collection? Garbage collection in Java does not use reference counting. Rather, most simple gc systems use a mark-and-sweep method. Is there somewhere I can find this documented? The impression I get from the very high level documents I've read on java.sun.com's website is: "The Java runtime environment has a garbage collector that periodically frees the memory used by objects that are no longer referenced." So if an object references itself and never stops referencing itself that object will always have at least one reference. Thanks, Novice Back to top Novice Guest Posted: Mon Jul 14, 2003 8:03 pm    Post subject: Re: do self references cause memory leaks in Java? In fact I found this article: http://java.sun.com/docs/books/performance/1st_edition/html/JPAppGC.fm.html on the sun website and it says that: "Circular strong references don't necessarily cause memory leaks." This statement doesn't seem to indicate that all objects which are no longer referenced (except by themselves) will be removed/deleted. In fact, it goes on further to say: "although the VM might not actually collect these objects for an indefinite amount of time" Which to me says that the VM may not remove/delete these objects from memory until the VM is shut-down. Is there any reason to believe otherwise? Thanks, Novice Back to top Novice Guest Posted: Mon Jul 14, 2003 9:01 pm    Post subject: Re: do self references cause memory leaks in Java? | Quote: The VM might not collect any object for an indefinite amount of time. So yes, uncollected objects may remain in memory until the VM is shut down. This to me is a problem - I mean server programs would eventually run out of memory if they made use of classes with circular dependencies... Can anyone provide any clarification on this? Or do most people know to try to avoid circular dependencies? Thanks, Novice Back to top Adam Maass Guest Posted: Mon Jul 14, 2003 9:27 pm    Post subject: Re: do self references cause memory leaks in Java? "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote: Quote: "Adam Maass" <adam.nospam.maass (AT) comcast (DOT) net> wrote: | | "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote: | > Hi, I am doing some work with Java and C++ and am curious as to whether | self | > references in Java cause memory leaks. | | > For example: | | > class Test{ | > Object foo =this; | > } | | > If I instantiate Test, the reference count of any Test object will never | > become zero (as it always keeps a reference to itself) so long as the foo | > variable is never re-assigned. | | > Does this present a problem for Java's garbage collection? | | | Garbage collection in Java does not use reference counting. Rather, most | simple gc systems use a mark-and-sweep method. | Is there somewhere I can find this documented? OK, this isn't exactly crystal-clear from the specs, but here's the definition of an object's lifecycle: http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#74701 And here's the definition of the term "reachable," which is part of the lifecycle definition: http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#44762 The short answer is that an object that refers to itself may be garbage collected if there are no references to it from any live thread. Java garbage collection does not use (a naive implementation of) reference-counting. -- Adam Maass Back to top Bent C Dalager Guest Posted: Mon Jul 14, 2003 10:15 pm    Post subject: Re: do self references cause memory leaks in Java? In article <bev5h3$18a$1 (AT) knot (DOT) queensu.ca>, Novice <cassidy (AT) cs (DOT) queensuDOTca> wrote: Quote: This to me is a problem - I mean server programs would eventually run out of memory if they made use of classes with circular dependencies... The VM will always do garbage collection - and a complete garbage collection if necessary - before reporting an out of memory condition. The case of not having had anything collected until program shutdown will only occur for programs that did not run out of memory during execution. Quote: Can anyone provide any clarification on this? Or do most people know to try to avoid circular dependencies? Circular dependencies aren't a problem. If you're using an experimental VM, of course, you might want to make a point out of checking its documentation on this point. I believe some older VMs did conservative GC, which is a bit of a mess. Cheers Bent D -- Bent Dalager - [email]bcd (AT) pvv (DOT) org[/email] - http://www.pvv.org/~bcd powered by emacs Back to top Phillip Lord Guest Posted: Tue Jul 15, 2003 9:17 am    Post subject: Re: do self references cause memory leaks in Java? Quote: "Novice" == Novice <cassidy (AT) cs (DOT) queensuDOTca> writes: Novice> | The VM might not collect any object for an indefinite Novice> amount of |time. So yes, uncollected objects may remain in Novice> memory until the VM is shut |down. Novice> | Novice> This to me is a problem - I mean server programs would Novice> eventually run out of memory if they made use of classes Novice> with circular dependencies... Novice> Can anyone provide any clarification on this? Novice> Or do most people know to try to avoid circular Novice> dependencies? No. The JVM does not have to collect objects until it feels it wants to. Maybe never. However it is required to free all available memory before it throws an out of memory error. So, for instance, a JVM might choose not to GC until it runs short of memory. This makes sense, as GC is expensive. Why do it if you don't need to. If you are running out of memory, of course, this is a different issue. Then you do need to. In short, don't worry about circular dependencies. They will not cause the GC problems. And generally don't worry about when objects that are our of scope, are actually GC'd. Its an implementation issue, and it will differ between JVM's. Cheers Phil Back to top Ian deSouza Guest Posted: Tue Jul 15, 2003 2:24 pm    Post subject: Re: do self references cause memory leaks in Java? I think it might be worthwhile talking about the mark-and-sweep GC that you had previously mentioned. Mark-and-sweep involves the GC first looking at the main program and looking for object references there, and "marking" them as valid. It then looks at the references that those hold, marking them, and so on. After it completes the marking, it then deletes the objects that weren't marked, (since it has has a complete list of all the new'ed objects), first calling finalize on the objects before removing the memory. Mark-and-sweep been in place since at least 1.1 (maybe before).. "Adam Maass" <adam.nospam.maass (AT) comcast (DOT) net> wrote Quote: "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote: "Adam Maass" <adam.nospam.maass (AT) comcast (DOT) net> wrote: | | "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote: | > Hi, I am doing some work with Java and C++ and am curious as to whether self | > references in Java cause memory leaks. | | > For example: | | > class Test{ | > Object foo =this; | > } | | > If I instantiate Test, the reference count of any Test object will never | > become zero (as it always keeps a reference to itself) so long as the foo | > variable is never re-assigned. | | > Does this present a problem for Java's garbage collection? | | | Garbage collection in Java does not use reference counting. Rather, most | simple gc systems use a mark-and-sweep method. | Is there somewhere I can find this documented? OK, this isn't exactly crystal-clear from the specs, but here's the definition of an object's lifecycle: http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#74701 And here's the definition of the term "reachable," which is part of the lifecycle definition: http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#44762 The short answer is that an object that refers to itself may be garbage collected if there are no references to it from any live thread. Java garbage collection does not use (a naive implementation of) reference-counting. -- Adam Maass Back to top Roedy Green Guest Posted: Wed Jul 16, 2003 2:16 am    Post subject: Re: do self references cause memory leaks in Java? On Mon, 14 Jul 2003 17:01:33 -0400, "Novice" <cassidy (AT) cs (DOT) queensuDOTca> wrote or quoted : Quote: This to me is a problem - I mean server programs would eventually run out of memory if they made use of classes with circular dependencies... You are imagining Java gc works by reference counting where circular references are a problem. See http://mindprod.com/jgloss/reference.html and http://mindprod.com/jgloss/garbagecollection.html for how Java's mark sweep or generational gc works. -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. Back to top Doug Pardee Guest Posted: Wed Jul 16, 2003 4:47 am    Post subject: Re: do self references cause memory leaks in Java? "Steve Horsley" <steve.horsley1 (AT) virgin (DOT) NO_SPAM.net> wrote: Quote: A full Garbage Collection will only run when the VM doesn't have enough memory to create a new Object when required. A short-lived program or a program that dives into a tight loop without creating and discarding many objects may never trigger a full GC. This is only true for certain environments. The JVM specification declares that the behavior of the garbage collector is not defined, and the JVM implementor has a free hand. The only requirements are that there be a garbage collector, and that it make its best effort to reclaim memory before throwing the OutOfMemoryError. Most of us are using Sun JVMs, and Sun has documented their GCs fairly well. It's worth the trip to the Sun Web site to read the facts, rather than relying on conjecture. Modern Sun JVMs use multiple heaps, with separate garbage collectors on each. These typically DO run on an as-needed basis, but in many cases a "full GC" is never triggered even in a program that creates and discards many objects. On the other hand, if you're using RMI, the Distributed Garbage Collector will force a "full GC" on a regular basis. By default, the DGC triggers a full garbage collection every 60 seconds. Quote: This is why finalize() cannot be relied on to clean up resources - the program may terminate normally without the finalizers ever running. Again, not *necessarily* true. It is true for the default situation, but if you call System.runFinalizersOnExit(true) at some point, the finalizers will always be executed. Heh... unless someone calls System.runFinalizersOnExit(false) after you've called it with "true" :-) Back to top Doug Pardee Guest Posted: Wed Jul 16, 2003 5:15 am    Post subject: Re: do self references cause memory leaks in Java? [email]ian_desouza (AT) yahoo (DOT) com[/email] (Ian deSouza) wrote: Quote: I think it might be worthwhile talking about the mark-and-sweep GC that you had previously mentioned. [snip] Mark-and-sweep been in place since at least 1.1 (maybe before).. Historically, Sun JVMs have not used mark-and-sweep GCs. The older Sun JVMs used mark-and-compact. The difference is that mark-and-sweep leaves a checkerboarded heap. Modern Sun JVMs (since 1.2.something) use multiple heaps, divided into two generations for user objects and a third generation for system objects. The two user generations use separate garbage collectors. The young generation uses a single-pass "copying" garbage collector: all live objects are copied from Eden and the current survivor space into the new survivor space and possibly the tenured generation, and then Eden and the current survivor space are declared empty, and the new survivor space becomes the current survivor space. The tenured generation uses mark-and-compact. As of 1.4.1, Sun's JVMs have added some additional GC options, one of which is a low-latency, mostly concurrent, mark-and-sweep GC for the tenured generation. Its operation is a six-phase process that is a *bit* more complicated than an ordinary mark-and-sweep. See the FAQ reference below. A couple of useful references: http://java.sun.com/docs/hotspot/gc1.4.2/faq.html http://java.sun.com/docs/hotspot/gc1.4.2/ Back to top S. Balk Guest Posted: Wed Jul 16, 2003 8:44 am    Post subject: Re: do self references cause memory leaks in Java? Quote: Hi, I am doing some work with Java and C++ and am curious as to whether self references in Java cause memory leaks. For example: class Test{ Object foo =this; } If I instantiate Test, the reference count of any Test object will never become zero (as it always keeps a reference to itself) so long as the foo variable is never re-assigned. Does this present a problem for Java's garbage collection? why don't you stress-test this little class by generating trillions of objects, and watch the GC-log Back to top Roedy Green Guest Posted: Wed Jul 16, 2003 8:19 pm    Post subject: Re: do self references cause memory leaks in Java? On Wed, 16 Jul 2003 10:44:22 +0200, "S. Balk" <s.balk (AT) hccnet (DOT) n0spam.nl> wrote or quoted : Quote: watch the GC-log how would he to that? -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. Back to top Steve Horsley Guest Posted: Wed Jul 16, 2003 9:37 pm    Post subject: Re: do self references cause memory leaks in Java? On Tue, 15 Jul 2003 21:47:11 -0700, Doug Pardee wrote: Quote: "Steve Horsley" <steve.horsley1 (AT) virgin (DOT) NO_SPAM.net> wrote: This is why finalize() cannot be relied on to clean up resources - the program may terminate normally without the finalizers ever running. Again, not *necessarily* true. It is true for the default situation, but if you call System.runFinalizersOnExit(true) at some point, the finalizers will always be executed. Heh... unless someone calls System.runFinalizersOnExit(false) after you've called it with "true" runFinalizerdOnExit() is deprecated as "inherently unsafe". 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