Is there any way to tell the Java VM (for this case, HotSpot or Zing) to no longer treat an objects as if there is a finalize method? Really, the whole class, I'm guessing, if at all possible.
I'm having problems with objects that have already had their resources freed but still put pressure on the VM that thinks it needs to call finalize on them.
This is in a library, so there is no way to change the class.
The use case is a library was written to clean up off-heap resources if you forgot or decided to let the GC do it for you. The problem is I get a lot of them hanging around.
Your only option would be to use an agent (-javaagent) that will override the finalize implementation with an empty one.
The JVM automatically creates a FinalReference object for each instance of a class with non-empty finalize (and this is how the JVM (Oracle) tracks the finalizable objects). You can't change his behavior.
The use case is a library was written to clean up off-heap resources if you forgot or decided to let the GC do it for you.
For that we use a Cleaner as ByteBuffer does. This is more light weight way of cleaning up resources.
EDIT Another option is you could
obtain the Finalizer.queue and remove elements yourself while holding the Finalizer.lock You could add a thread which periodically cleans up the queue of elements which don't need to be there.
you could replace this queue with your own implementation so it behaves differently. e.g. not add objects of a chosen class in the first place.
No, you can't do this. It's part of the JVM spec. Nor is it clear why you should do this; the VM will call finalize() if they have that method when they're cleaned up, regardless of what has happened previously. If you're calling finalize directly and triggering some kind of double finalize bug, then the answer is simple: "Don't do that"
Related
Does java collect the garbage-signed things as objects?
If yes, can i tell java to direct them to one of my hashtables(accepts objects right?) programmatically?
I am curious about this functionality.I know System.gc() is the command but how can i achieve first question? Can i?
myTrashBin=System.gc().getObjectList(); //???
If not, may be there could be a way to create this functionality by custom classes.
Last question: how can we override System.gc() ?
Thanks.
This isn't under your control. If your objects are unreachable then GC will collect these. System.gc() is nothing more than a hint, and can't be relied upon.
finalize() may be of interest, but read the answers to this question to understand limitations etc. PhantomReferences may also be of interest.
I think you can achieve something like that if you implement the Finalize method and writing the code there: maybe adding the object to a custom list
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object
http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#finalize%28%29
System.gc() issues a call for the garbage collector but that's all there is to it. It might rise its priority and it might collect your items sooner, but there is no guarantee, as the docs say:
Calling the gc method suggests that the Java Virtual Machine expend effort toward
recycling unused objects in order to make the
memory they currently occupy available for quick reuse. When control
returns from the method call, the Java Virtual Machine has made a best
effort to reclaim space from all discarded objects.
You're better off taking other approaches such as the finalize() method or managing a reference counter in your objects so when it hits zero you know it is elegible to be collected. Check this link out.
Java uses managed memory. This means the JVM manages it because you don't want to ;)
can i tell java to direct them to one of my hashtables(accepts objects right?) programmatically?
You can progammatically get all the objects which would be cleaned up if they are referenced via a WeakReference.
how can we override System.gc() ?
You can't. In fact its only a hint as its not guaranteed to do anything.
So, I recently discovered the finalize method in Java (not sure why I missed it before, but there it is). This seems like it could be the answer to a lot of the issues I'm working with, but I wanted to get a bit more information first.
Online, I found this diagram illustrating the process of garbage collection and finalize:
A couple of questions:
This takes place in a separate thread, correct?
What happens if I instantiate a new object during finalize? Is that allowed?
What happens if I call on a static method from finalize?
What happens if I establish a new reference to the object from within finalize?
I suppose I should explain why I'm interested. I work with LWJGL a lot, and it seems that if I could use finalize to cause Java objects to automatically clean up OpenGL resources, then I could do some really nice things in terms of an API.
finalize() is called by the Java Garbage Collector when it detects that no references to that particular object exists. finalize() is inherited by all Java objects through the Object class.
As far as I am aware you would have no difficulty making static method calls from a finalize() method I and you could establish a new reference to it from finalize() - however I would say this is poor programming practice.
You shouldn't rely on finalize() for clearing up, and it is better to clear up as you go. I prefer to use try, catch, finally for clearing up, rather than using finalize(). In particular, by using finalize() you will cause the JVM to hold onto all other objects that your finalizable object references, just in case it makes calls to them. This means your holding onto memory you might not need to use. More importantly, this also means you can cause the JVM to never end up disposing of objects, because they have to hold onto them incase another objects finalize method needs it e.g. a race condition.
Also, consider that it is entirely possible that GC won't be called. Therefore you can't actually guarantee that finalize() will ever be called.
Clear up resources as and when you are finished with them, and do not rely on finalize() to do it is my advice.
I don't think there are any guarantees about what thread will be used. New objects may be instantiated and static methods may be called. Establishing a new reference to your object will prevent it from being garbage collected, but the finalize method will not be called again--you don't want to do this.
Cleaning up resources is precisely what the finalize method is for, so you should be good there. A couple of warnings, though:
The method is not guaranteed to be called. If you have tied up resources that will not automatically be freed when your program stops do not depend on finalize.
When the method is called is not guaranteed. With memory tight, this will be sooner. With lots of free memory, it will be later if at all. This may suit you fine: with lots of memory you may not be all that concerned about freeing up the resources. (Though hanging on to them may interfere with other software running at the same time, in which case you would be concerned.)
My ususal solution is to have some kind of dispose method that does the clean up. I call it explicitly at some point if I can, and as soon as I can. Then I add a finalize method that just calls the dispose method. (Note that the dispose method must behave well when when called more than once! Indeed, with this kind of programming I might call dispose several times outside finalize, not being sure if previous calls were made successfully and yet wanting it to be called effectively as soon as possible.) Now, ideally, my resources are freed as soon as I no longer need them. However, if I lose track of the object with the resources, the finalize method will bail me out when memory runs short and I need the help.
First of all, remember there is no guarantee that finalization will even be run at all for all your objects. You can use it to free memory allocated in native code associated with an object, but for pure Java code most use cases are only to perform a "backup" mechanism of cleaning up resources. This means in most cases you should free resources manually and finalizers could act only a sort of helper to clean up if you forget to do it the standard way. However, you can't use them as the only or the main mechanism of cleanup. Even more generally, you shouldn't write any code whose correctness depends on finalizers being run.
Ad 1. As far as I know, there are no guarantees about what thread calls finalize(), though in practice this will probably be one of the GC threads.
Ad 2. Instantiating new objects is allowed. However, there are a number of pitfalls with handling object references in finalizers. In particular, if you store a hard reference to the object being finalized in some live object, you can prevent your about-to-be-garbage-collected object from being cleaned up. This kind of object resurrection may lead to exhausting your resources if it gets out of control. Also, watch out for exceptions in finalize() - they may halt the finalization, but there's no automatic way for your program to learn about them. You need to wrap the code in try-catch blocks and propagate the information yourself. Also, long execution time of finalizers may cause the queue of objects to build up and consume lots of memory. Some other noteworthy problems and limitations are described in this JavaWorld article.
Ad 3. There shouldn't be any issues with calling static methods from finalizers.
Ad 4. As mentioned in point 2, it is possible to prevent an object from being garbage collected (to resurrect it) by placing a reference to it in another live object during finalization. However, this is tricky behavior and probably not good practice.
To sum up, you can't rely on finalizers for cleaning up your resources. You need to handle that manually and finalizers in your case may at best be used as a backup mechanism to cover up after sloppy coding to some degreee. This means, unfortunately, your idea of making the API nicer by using finalizers to clean up OpenGL resources probably won't work.
I know that the GC collects objects that have no references pointing to the object in question, but what happens in the event of listener objects?
Suppose you have an AnimationDelegate that listens to data from a DataSupplier. When the DataSupplier recieves data and fires off the event to the AnimationDelegate, the delegate will then invalidate (/update/redraw etc...) a Graphic. Now say the screen is disabled, removed or, through various means, the graphic can no longer draw and is collected. The AnimationDelegate is still registered to the DataSupplier. How will the GC know to collect it? Should one unregister the delegate in the graphics finalize() method?
I'm afraid the answer won't fit the format :) Start with this article by Brian Goetz: he's a perfect person to read if you're interested in GC.
Basically, as soon as object is not reachable from active threads, it's collected. The actual algorithms vary even within one JVM, but the point stays the same: what's not reachable is a garbage. What's reachable is not a garbage. Easy.
GC will not collect the Graphic in your example, as it's reachable from AnimationDelegate, which in turn is reachable (via subscription) from DataSupplier which is supposed to be reachable from some active thread. So the answer will be: your assumptions are wrong; GC will not collect anything here.
To answer your question, unsubscribe everything you don't need.
As #rfeak rightfully says, finalize() is a big no-no. It's almost impossible to use it properly, and it's way too easy to use it wrong. That said,it's OK to use it as a backup solution when you need to free resources. But generally your application has to be able to work just fine even if finalize() never gets called.
It all depends on the JVM you're using and the GC. Most default GC from the JDK use the so called "tracing collectors", which simply start at a given root set of objects and trace all the objects reachable from that set. All the other objects in memory are seen as garbage and deleted. So circular references aren't really a problem unless one of the objects is reachable from the root set.
What is the root set of objects? Well if memory serves right roots can be found in: program registers, local variables in each thread's stack and static variables.
To see if your objects will be GC'd we would know more about the design of your application.
#Edit: Oh and I almost forgot: Memory Management in the JavaHotSpotâ„¢ Virtual Machine. This is a pretty good overview of how it all works.
It will only know if you have removed the references (nulled them out).
However, don't do this on finalize(). Finalize is bad bad bad. There should be other lifecycle methods available for cleaning up listener(observer) type objects.
By the way, observer pattern is notorious for creating memory leaks because the GC couldn't collect due to lingering references.
I'm creating a service that will run constantly, each day at a specified time it will run the main body of the program.
Essentially:
while(true){
run();
Thread.sleep(day);
}
After a while, I'm getting OutOfMemoryHeapExceptions.
After reading about this a little I'm thinking its because any objects created inside the run() method will never be garbage collected.
Therefore I have done something like:
public void run(){
Object a = new Object();
a.doSomething();
a= null; //Wasn't here before
}
My question is, will this solve my problem? I'm under the impression that once an object is null, the object it previously referenced will be garbage collected? Also is this a good idea? Or should I look at doing something else?
Thanks
Adding a = null will almost certainly be insufficient to fix the problem (since a is about to go out of scope anyway).
My advice would be to use a memory profiler to pinpoint what's leaking and where.
I personally use YourKit. It's very good, but costs money (you can get a free evaluation).
Another recently-released tool is Plumbr. I am yet to try it, but the blurb says:
Try out our Java agent for timely discovery of memory leaks. We'll tell you what is leaking, where the leak originates from and where the leaked objects currently reside - well before the OutOfMemoryError!
That might indeed help, in some circumstances the GC algorithm needs a little help to perform, but it doesn't guarantee to solve your problems, merely delay them.
My advice:
Simulate the same behavior with a lower time period, so you can force the error to happen.
Run it with a profiler and see where all that memory is going, and work from there.
Your impression is incorrect. Objects created inside the run() method will be garbage collected provided they 1) go out of scope, and 2)have released any native or remote system resources they are using.
What functionality are you actually performing inside your run() method call? Are you reading files, making database calls, writing to sockets? Without knowing the details its very difficult to provide a better suggestion.
No. You don't need to set the variable to null. The VM knows that you exit that scope and that the variable a no longer exists, so it automatically decrements the reference count and your object is elegible for garbage collection if it had no other references.
The error is somewhere else.
Setting references to null depends if your object is still in scope in a long time consuming process, though theoretically it will mark the reference as null you cannot guarantee when it will be garbage collected.
You need to check if your objects are being held in long scope somewhere in your code.
Found a nice explanation of setting references to null : Does setting Java objects to null do anything anymore?
In order to corner out your issue you need to profile your application.
Searching SO gave so many pointers on Garbage Collection that I have decided to just place the search string here:
https://stackoverflow.com/search?q=Java+Garbage+collection+and+setting+references+to+null
http://java.sun.com/docs/books/performance/1st_edition/html/JPAppGC.fm.html
Local variables should be collected by GC. So, you don't need to put obj=null;. Because Object is also stored in Heap area.
You should get a memory dump and analyze that using tools like JConsole JVisualVM.
The scope of the run() method is left before the Thread.sleep(day); and thus any variables inside that method are destroyed. After that a won't exist any more and thus the object referenced by that variable might be eligible for garbage collection provided there's no other reference to it.
Analyzing a memory dump should allow you to find any references to those object if they still exist.
It might as well not be those objects but others that are kept alive and which eat up the memory. That depends on what you're actually doing and might be hard to analyze here. Thus look out for huge object graphs in terms of memory usage.
For instance, we had a problem with database connections that were created frequently (XA recovery mechanism) and we thought they'd be destroyed once the method scope is left. However, the server put those connections into a static list and never cleared it and thus we ended up with no memory really soon. What helped us identify that case was analyzing a memory dump. :)
In the short term a pragmatic approach to keeping your application stable is to exit the JVM after each execution. Use a batch scheduler (e.g. cron on *nix, at on Windows) to execute your application just once every day. Any memory leaks will be cleaned up when the JVM exists for sure. However you may have to be careful you're not leaving database connections open, etc.
This will give you time to troubleshoot and fix the underlying memory leak issues while keeping your production code running and not requiring support staff to restart servers, etc.
I'm assuming you're not running out of memory on a single execution
I have a Swing browser application with a bug that as I add/remove to/from the GUI the memory isn't released for those objects and I'm trying to track down what is holding onto them. Problem is I don't know how to tell when something has actually be fully released from memory.
Is there a way to tell if an object has been released from memory? I'm used to Objective-C where there are several ways to tell.
Thanks
You can't really do it in Java. All the answers mentioning finalizers are really not what you're after.
The best you can do is enqueue a PhantomReference in a ReferenceQueue and poll until you get the reference.
final ReferenceQueue rq = new ReferenceQueue();
final PhantomReference phantom = new PhantomReference( referenceToObjectYouWantToTrack, rq );
You want to read Peter Kofler's answer here (it explains what a PhantomReference is):
Have you ever used Phantom reference in any project?
Very interesting read here:
http://www.kdgregory.com/index.php?page=java.refobj
Basically, I'm using a PhantomReference in a project where a very special kind of cache needs to be computed once, when the software is installed. To efficiently compute this (disk-based) cache, a gigantic amount of memory is needed (the more the better). I'm using a PhantomReference to track "nearly exactly" when that gigantic amount of memory is released.
There are multiple ways of detecting memory leaks. Here the three I'm currently thinking of:
Attaching a Profiler to your application (Netbeans or Eclipse TPTP should be useful here)
Making a heap dump and analyze (with Eclipse Memory Analyzer) what instances of a class are held in memory by which other instances.
Attaching VisualVM to track Garbage Collection status.
Edit:
As NoozNooz42 has pointed out, a PhantomReference can do everything a finalizer can, without the problems finalizers present. So, I encourage using PhantomReferences over extending finalize(). I am keeping my original post in tact, since I think Java programmers should at least know that finalize() exists.
Original Post:
Every Java class has a finalize() method that gets run when no other Objects hold a reference to that class. You can extend this method like so:
protected void finalize() throws Throwable {
try {
// Do whatever you want
} finally {
super.finalize();
}
}
By doing so, you can figure out if anything holds a reference to the Objects in question. Just make sure you always call super.finalize() if you use this approach.
Reference on finalization:
http://java.sun.com/developer/technicalArticles/javase/finalization/
Try YourKit. It's a Java profiler which can show you your memory usage and what the heck is going on. IIRC, the free trial can integrate with Eclipse and has all of the paid version's features (it just has a time limit).