is there a way to check if an object can be fetched by the garbage collector?
Somewhere in my code I've got a reference to an object:
MyObject mo = myObject;
Then, via Eclipse Debugger, I get the objects memory location. Afterwards, I set the reference null:
mo = null;
Is there any way to check if the previously referenced object is now suitable for garbage collection or if there's somewhere another reference to it?
Thanks a lot,
Stefan
You cannot do this at runtime with an arbitrary object, and in fact it's not fully possible to do this deterministically. However, there are two options that may be suitable depending on your needs:
Take a heap dump after you set the reference to null, and then load it up in a heap analyzer tool such as jhat or a profiler that supports this. These tools should let you traverse the path from the GC roots and thus check if your object is still reachable or not.
Wrap the object in a PhantomReference with a given ReferenceQueue. When the reference is enqueued, you know that the object has been garbage collected. (Unfortunately, if the reference is unqueued it could be because the object is still reachable, or it could be because the GC just hasn't inspected the object yet. As with all GC-related questions, garbage collection is not a deterministic process!)
On the whole though, I agree that the best option is to be aware of memory leak issues and design your application to avoid them. If you do have a memory leak it should be obvious enough, and you can then focus your energies on finding the problem (again by dumping and analysing the heap for objects that are incorrectly reachable).
The steps above are relatively time-consuming, and shouldn't be something that you do after every change just to reassure yourself, but rather are tools you'd use to investigate a specific problem.
No. The only thing to do is to be careful and keep in mind that memory leaks can exist in Java when writing your application.
The only you can do, is to use tools to try to find where memory leaks come from when you noticed such a problem. I would strongly recommend Memory Analyzer for this purpose.
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Forcing Garbage Collection in Java?
Can I Force Garbage Collection in Java by any means?
System.gc() is just a suggestion.It's useless.
When I know for sure that some resources won't be used any more,why can't I force to clean them?
Just like delete() in C++ and free() in C?
When there are lots of resources that can't be reused,this can really suck the performance.All that we can do is sleep().
Any solutions?Thanks
Nope, System.gc() is as close as you can get. Java isn't C or C++, the JVM manages memory for you, so you don't have that kind of fine grained control. If you set objects you're no longer using to null, or loose all references, they will get cleaned up. And the GC is pretty smart, so it should take good care of you.
That said, if you are on a unix box, and force a thread dump (kill -3), it'll pretty much force garbage collection.
You shouldn't be trying to force GC - if you are running low on memory then you have a memory leak somewhere. Forcing GC at that point won't help, because if you are holding a reference to the object then it still won't be garbage collected.
What you need to do is solve the real problem, and make sure you are not holding references to objects you are not using any more.
Some common culprits:
Holding lots of references in a large object graph that never get cleared up. Either set references to null when you don't need them any more, or better still simplify your object graph so it doesn't need all the extra long-term references.
Caching objects in a hashmap or something similar that grows huge over time. Stop doing this, or use something like Google's CacheBuilder to create a proper soft reference cache.
Using String.intern() excessively on large numbers of different strings over time.
References with larger scope than they need. Are you using an instance variable when it could be a local variable, for example?
There is no way to explicitly instruct the JVM to collect garbage. This is only performed when the system needs the resources.
The only two actions I'm aware of to potentially get the GC running are the following:
As you stated, attempt to "suggest" that GC now would be a good time by called System.gc().
Set any references you are not using to the null reference to make the elements eligible for collection.
On my second point, see the answer here: Garbage collector in java - set an object null. In essence, if you don't make the objects you don't need available for garbage collection (by losing the reference you have to it) then there is no reason for the garbage collector to run, because it's unaware of any available garbage.
In addition, it's important to consider why/how those objects in memory are affecting performance:
Are you getting lots of OutOfMemoryExceptions? This could be resolved by point #2 and by increasing the available heap space for the JVM.
Have you done measurements to see that more objects in the JVM's allocated heap space makes a difference in performance? Determining when you could let references to objects go earlier could help reduce these issues.
For a project for school I have to program different kind of algorithms. The problem is, I got a working algorithm. But I have to run it several times and after some time it gives me the following errors:
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
I know what the error means, but is it possible to let Java search for empty space during the run? I know it uses a lot of space which isn't used at some point. It sets a lot of object to null during the application run and create a lot of new ones, because of this it runs out of memory.
So concrete: is it possible to let the JVM free some space that is set to null? Or free some space in the time the program is running? I know I can set the JVM to more space, but sooner or later I will run to the same problem.
If you need my IDE (in case it is IDE specific) it is Eclipse.
Please google 'garbage collection'. Java is always looking to reuse space from objects that you aren't using. If you run out of memory, you either need to use -Xmx to configure for more memory, or you have to fix your code to retain fewer objects. You may find that a profiler like jvisualvm would help you find wasteful memory usage.
If you're using an Oracle/Sun JVM, I'd recommend that you download Visual VM 1.3.3, install all the plugins, and start it up. It'll show you what's happening in every heap generation, threads, CPU, objects, etc. It can tell you which class is taking up the most heap space.
You'll figure it out quickly if you have data.
I would use a memory profiler to determine where the memory is being used. Setting to null rarely helps. The GC will always run and free as much space as possible before you get an OOME.
Q: "is it possible to let the JVM free some space that is set to null? Or free some space in the time the program is running?"
A: Yes, use a call to System.gc() will do this, but this will not likely solve your problem as the system does this automatically from time to time. You need to find the object that is using all the memory and fix it in your code. Likely a list that is never cleared and only ever added to.
I actually encountered this issue while implementing a particularly complicated algorithm that required a massive data structure. I had to come and post a question on this website. It turned out I had to use a completely different type of object altogether in order to avoid the memory error.
Here is that question.
GC will reclaim 'unused' memory automatically, so yes, it is possible to free some space at runtime, but it's crucial to understand what's classified as possible to be reclaimed.
Basically an object's space can be reclaimed (garbage collected) if the object itself is unreachable - there are no references to it. When you say 'setting space to null' you're most likely removing just one link (reference) to the object by setting it to null. This will allow to reclaim the object only if that was the only link (reference)
Object First= new Object(); //first object
Object Second= new Object(); //second object
Object SecondPrim=Second; //second reference to second object
First=null;
// First memory will be reclaimed (sooner or later)
Second=null;
// there is still a reference to second object via SecondPrim
// second object will not be reclaimed
Hope this helps. As for checking what's exactly going on I would second advice to profile your program.
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
Is there any possibility that a object which is not referenced anywhere and still existing on heap. I mean is there a possibility that a unused object getting escaped from garbage collector and be there on the heap until the end of the application.
Wanted to know because if it is there, then while coding i can be more cautious.
If an object is no longer referenced, it does still exist on the heap, but it is also free to be garbage-collected (unless we are talking Class objects, which live in PermGen space and never get garbage-collected - but this is generally not something you need to worry about).
There is no guarantee on how soon that will be, but your application will not run out of memory before memory from those objects is reclaimed.
However, garbage collection does involve overhead, so if you are creating more objects than you need to and can easily create less, then by all means do so.
Edit: in response to your comment, if an object is truly not referenced by anything, it will be reclaimed during garbage collection (assuming you are using the latest JVM from Sun; I can't speak toward other implementations). The reason why is as follows: all objects are allocated contiguously on the heap. When GC is to happen, the JVM follows all references to "mark" objects that it knows are reachable - these objects are then moved into another, clean area. The old area is then considered to be free memory. Anything that cannot be found via a reference cannot be moved. The point is that the GC does not need to "find" the unreferenced objects. If anything, I would be more worried about objects that are still referenced when they are not intended to be, which will cause memory leaks.
You should know that, before a JVM throws an out-of-memory exception, it will have garbage collected everything possible.
If an instance is no longer referenced, it is a possible candidate for garbage collection. This means, that sooner or later it can be removed but there are no guaranties. If you do not run out of of memory, the garbage collector might not even run, thus the instance my be there until the program ends.
The CG system is very good at finding not referenced objects. There is a tiny, tiny chance that you end up keeping a weird mix of references where the garbage collector can not decide for sure if the object is no longer referenced or not. But this would be a bug in the CG system and nothing you should worry about while coding.
It depends on when and how often the object is used. If you allocate something then deallocate (i.e., remove all references to it) it immediately after, it will stay in "new" part of the heap and will probably be knocked out on the next garbage collection run.
If you allocate an object at the beginning of your program and keep it around for a while (if it survives through several garbage collections), it will get promoted to "old" status. Objects in that part of the heap are less likely to be collected later.
If you want to know all the nitty-gitty details, check out some of Sun's gc documentation.
Yes; imagine something like this:
Foo foo = new Foo();
// do some work here
while(1) {};
foo.someOp(); // if this is the only reference to foo,
// it's theoreticaly impossible to reach here, so it
// should be GC-ed, but all GC systems I know of will
// not Gc it
I am using definition of: garbage = object that can never be reached in any execution of the code.
Garbage collection intentionally makes few guarantees about WHEN the objects are collected. If memory never gets too tight, it's entirely possible that an unreferenced object won't be collected by the time the program ends.
The garbage collector will eventually reclaim all unreachable objects. Note the "eventually": this may take some time. You can somewhat force the issue with System.gc() but this is rarely a good idea (if used without discretion, then performance may decrease).
What can happen is that an object is "unused" (as in: the application will not use it anymore) while still being "reachable" (the GC can find a path of references from one of its roots -- static fields, local variables -- to the object). If you are not too messy with your objects and structures then you will not encounter such situations. A rule of thumb would be: if the application seems to take too much RAM, run a profiler on it; if thousands of instances of the same class have accumulated without any apparent reason, then there may be some fishy code somewhere. Correction often involves explicitly setting a field to null to avoid referencing an object for too long.
This is theoretically possible (there is no guarantee the GC will always find all objects), but should not worry you for any real application - it usually does not happen and certainly does not affect a significant chunk of memory.
In theory, the garbage collector will find all unused objects. There could, of course, be bugs in the garbage collector…
That said, "In theory there is no difference between theory and practice, in practice, there is." Under some, mostly older, garbage collectors, if an object definition manages to reach the permanent generation, then it will no longer be garbage collected under any circumstances. This only applied to Class definitions that were loaded, not to regular objects that were granted tenured status.
Correspondingly, if you have a static reference to an object, that takes up space in the "regular" object heap, this could conceivably cause problems, since you only need to hold a reference to the class definition from your class definition, and that static data cannot be garbage collected, even if you don't actually refer to any instances of the class itself.
In practice though, this is a very unlikely event, and you shouldn't need to worry about it. If you are super concerned about performance, then creating lots of "long-lived" objects, that is, those that escape "escape-analysis", will create extra work for the garbage collector. For 99.99% of coders this is a total non-issue though.
My advice - Don't worry about it.
Reason - It is possible for a non-referenced object to stay on the heap for some time, but it is very unlikely to adversely affect you because it is guaranteed to be reclaimed before you get an out of memory error.
In general, all objects to which there are no live hard references, will be garbage-collected. This is what you should assume and code for. However, the exact moment this happens is not predictable.
Just for completeness, two tricky situations [which you are unlikely to run into] come into my mind:
Bugs in JVM or garbage collector code
So called invisible references - they rarely matter but I did have to take them into account one or two times during the last 5 years in a performance-sensitive application I work on
I have a cache which has soft references to the cached objects. I am trying to write a functional test for behavior of classes which use the cache specifically for what happens when the cached objects are cleared.
The problem is: I can't seem to reliably get the soft references to be cleared. Simply using up a bunch of memory doesn't do the trick: I get an OutOfMemory before any soft references are cleared.
Is there any way to get Java to more eagerly clear up the soft references?
Found here:
"It is guaranteed though that all
SoftReferences will get cleared before
OutOfMemoryError is thrown, so they
theoretically can't cause an OOME."
So does this mean that the above scenario MUST mean I have a memory leak somewhere with some class holding a hard reference on my cached object?
The problem is: I can't seem to
reliably get the soft references to be
cleared.
This is not unique to SoftReferences. Due to the nature of garbage collection in Java, there is no guarantee that anything that is garbage-collectable will actually be collected at any point in time. Even with a simple bit of code:
Object temp = new Object();
temp = null;
System.gc();
there is no guarantee that the Object instantiated in the first line is garbage collected at this, or in fact any point. It's simply one of the things you have to live with in a memory-managed language, you're giving up declarative power over these things. And yes, that can make it hard to definitively test for memory leaks at times.
That said, as per the Javadocs you quoted, SoftReferences should definitely be cleared before an OutOfMemoryError is thrown (in fact, that's the entire point of them and the only way they differ from the default object references). It would thus sound like there is some sort of memory leak in that you're holding onto harder references to the objects in question.
If you use the -XX:+HeapDumpOnOutOfMemoryError option to the JVM, and then load the heap dump into something like jhat, you should be able to see all the references to your objects and thus see if there are any references beside your soft ones. Alternatively you can achieve the same thing with a profiler while the test is running.
There is also the following JVM parameter for tuning how soft references are handled:
-XX:SoftRefLRUPolicyMSPerMB=<value>
Where 'value' is the number of milliseconds a soft reference will remain for every free Mb of memory. The default is 1s/Mb, so if an object is only soft reachable it will last 1s if only 1Mb of heap space is free.
You can force all SoftReferences to be cleared in your tests with this piece of code.
If you really wanted to, you can call clear() on your SoftReference to clear it.
That said, if the JVM is throwing an OutOfMemoryError and your SoftReference has not been cleared yet, then this means that you must have a hard reference to the object somewhere else. To do otherwise would invalidate the contract of SoftReference. Otherwise, you are never guaranteed that the SoftReference is cleared: as long as there is still memory available, the JVM does not need to clear any SoftReferences. On the other hand, it is allowed to clear them next time it does a GC cycle, even if it doesn't need to.
Also, you can consider looking into WeakReferences since the VM tends to be more aggressive in clear them. Technically, the VM isn't ever required to clear a WeakReference, but it is supposed to clean them up next time it does a GC cycle if the object would otherwise be considered dead. If your are trying to test what happens when your cache is cleared, using WeakReferences should help your entries go away faster.
Also, remember that both of these are dependent on the JVM doing a GC cycle. Unfortunately, there is no way to guarantee that one of those ever happens. Even if you call System.gc(), the garbage collector may decide that it is doing just peachy and choose to do nothing.
In a typical JVM implementation (SUN) you need to trigger a Full GC more than once to get the Softreferences cleaned. The reason for that is because Softreferences require the GC to do more work, because for example of a mechanism that allows you to get notified when the objects are reclaimed.
IMHO using a lot of sofreferences in an application server is evil, because the developer has not much control over when they are released.
Garbage collection and other references like soft references are non deterministic this it's not really possible to reliable do stuff so that soft references are definitely cleared at that point so your test can judge how yourcache reacts. I would suggest you simulate the reference clearing in more definite way by mocking etc - your tests will be reproducable and more valuable rather than just Hopi g for the GC to clean up references. Using the latter approach is a really bad thing to do and willjust introduce additional problems rather than help you improve the quality of your cache and it's collaborating components.
From the documentation and my experience I'd say yes: you must have a reference somewhere else.
I'd suggest using a debugger that can show you all references to an object (such as Eclipse 3.4 when debugging Java 6) and just check when the OOM is thrown.
If you use eclipse, there is this tool named Memory Analyzer that makes heap dump debugging easier.
Does the cached object have a finalizer? The finalizer will create new strong references to the object, so even if the SoftReference is cleared the memory will not be reclaimed until a later GC cycle
If you have a cache which is a Map of SoftReferences and you want them cleared you can just clear() the map and they will all be cleaned up (including their references)