I developing a game using andengine. In J2me game when exit , i made all object as null
ie:
Image img;
Sprite s1;
When exit application ,
img=null;
s1=null;
In android i will use System.gc() or i need to make all texture, textureRegion and sprite as make as null, when exit appliaction ?
i think you should not call System.gc() explicitly. Android OS takes care of that.
"Calling System.gc() from your app is like providing electricity connection from your home to light up your complete society's lights"
I mean it slows down your app to clean all the garbages of the system.......
N_JOY.
Java garbage collection should take care of that. You don't need to do that.
However I would close open connections, file handles, etc..
System.gc() is just a hint to the JVM that garbage collection is suggested, however Java is running it at its own will.
In Android in the presence of a garbage collector, it is never good practice to manually call the GC. A GC is organized around heuristic algorithms which work best when left to their own devices. Calling the GC manually often decreases performance.
Occasionally, in some relatively rare situations, one may find that a particular GC gets it wrong, and a manual call to the GC may then improves things, performance-wise. This is because it is not really possible to implement a "perfect" GC which will manage memory optimally in all cases. Such situations are hard to predict and depend on many subtle implementation details. The "good practice" is to let the GC run by itself; a manual call to the GC is the exception, which should be envisioned only after an actual performance issue has been duly witnessed.
It's better to spend more effort in avoiding the unnecessary creation of objects (like creation of objects inside loops)..
Look at the Question Garbage collector in Android
Related
I have two activities in my android application. When I switch from first activity to second activity, gc starts and makes second activity to lag until it completes. I decided to make a splash screen (loading screen) that will not close until gc finishes but I do not know how to get gc status pro-grammatically. Is there any class of it? Please let me know how can I get this scenario!
To begin with, in Android, garbage collection is organized by the ART - Android Runtime or DVM - Dalvik Virtual Machine (on older devices). As ART/Dalvik are essentially specialized versions of JVM, they have similar approach to GC, hence it is solely managed by the system and not by the user.
Hence, you don't get to control the garbage collection in Android.
Indeed, you can call System.gc(), but it's nor guaranteed nor recommended to do. You are expected to completely forget about garbage collection process and leave it to the system.
While you cannot control it, you are still responsible to manage the memory and prevent excessive memory usage as much as possible. A few tips, you should consider:
Release bulky objects (remove hard references pointing to them) as soon as you're done working with them;
Utilize multithreading to your needs, threads will work in parallel and faster (especially on multi-core processors);
Optimize your algorithms, even basic list iterations could potentially slow the process and leak memory if done incorrectly
Thank you guys for answers. After some working i found what the problem in code.
I was executing this async class in while loop with new instance. So it keeps memory increasing and after two hours it starts hangs or when activity switched gc executes.
I think the answer by #Serj sums it up quite good. Maybe you find a workaround to get the GC triggered if you keep the instance of your old activity, and thus have it still being referenced, until your splash screen is set up. Then you remove the last references and hope for the GC to be called - but yet it could happen that it will get called later. It's a good question how to see the status of the GC, maybe you can read out the memory and see if its filled or not?
The best advice is refactoring and using objects only in scopes in which they are needed.
This question already has answers here:
How to force garbage collection in Java?
(25 answers)
Closed 8 years ago.
I have a complex java application running on a large dataset. The application performs reasonably fast but as time goes it seems to eat lots of memory and slow down. Is there a way to run the JVM garbage collector without re-starting the application?
No, You cant force garbage collection.
Even using
System.gc();
You can just make a request for garbage collection but it depends on JVM to do it or not.
Also Garbage collector are smart enough to collect unused memory when required so instead of forcing garbage collection you should check if you are handling objects in a wrong way.
If you are handling objects in a wrong way (like keeping reference to unnecessary objects) there is hardly anything JVM can do to free the memory.
From Doc
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.
Open Bug regarding System.gc() documentation
The documentation for System.gc() is extremely misleading and fails to
make reference to the recommended practise of never calling
System.gc().
The choice of language leaves it unclear what the behaviour would be
when System.gc() is called and what external factors will influence
the behaviour.
Few useful link to visit when you think you should force JVM to free up some memory
1. How does garbage collection work
2. When does System.gc() do anything
3. Why is it bad practice to call System.gc()?
All says
1. You dont have control over GC in Java even System.gc() dont guarantee it.
2. Also its bad practise as forcing it may have adverse effect on performance.
3. Revisit your design and let JVM do his work :)
you should not relay on System.gc() - if you feel like you need to force GC to run it usually means that there is something wrong with your code/design. GC will run and clear your unused objects if they are ready to be created - please verify your design and think more about memory management, look as well for loops in object references.
The
System.gc()
call in java, suggest to the vm to run garbage collection. Though it doesn't guarantee that it will actually do it. Nevertheless the best solution you have. As mentioned in other responses jvisualvm utility (present in JDK since JDK 6 update 7), provides a garbage functionality as well.
EDIT:
your question open my appetite for the topic and I came across this resource:
oracle gc resource
The application performs reasonably fast but as time goes it seems to eat lots of memory and slow down.
These are a classic symptoms of a Java memory. It is likely that somewhere in your application there is a data structure that just keeps growing. As the heap gets close to full, the JVM spends an increasing proportion of its time running the GC in a (futile) attempt to claw back some space.
Forcing the GC won't fix this, because the GC can't collect the data structure. In fact forcing the GC to run just makes the application slower.
The cure for the problem is to find what is causing the memory leak, and fix it.
Performance gain/drop depends how often you need garbage collection and how much memory your jvm has and how much your program needs.
There is no certainity(its just a hint to the interpreter) of garbage collection when you call System.gc() but at least has a probability. With enough number of calls, you can achieve some statistically derived performance multiplier for only your system setup.
Below graph shows an example program's executions' consumptions and jvm was given only 1GB(no gc),1GB(gc),3GB(gc),3GB(no gc) heaps respectively to each trials.
At first, when jvm was given only 1GB memory while program needed 3.75GB, it took more than 50 seconds for the producer thread pool to complete their job because having less garbage management lead to poor object creation rate.
Second example is about %40 faster because System.gc() is called between each production of 150MB object data.
At third example, jvm is given 3GB memory space while keeping System.gc() on. More memory has given more performance as expected.
But when I turned System.gc() off at the same 3GB environment, it was faster!
Even if we cannot force it, we can have some percentage gain or drain of performance trying System.g() if we try long enough. At least on my windows-7 64 bit operating system with latest jvm .
Garbage collector runs automatically. You can't force the garbage collector.
I do not suggest that you do that but to force the garbage collector to run from within your java code you can just use all the available memory, this works because the garbage collector will run before the JVM throws OutOfMemoryError...
try {
List<Object> tempList = new ArrayList<Object>();
while (true) {
tempList.add(new byte[Integer.MAX_VALUE]);
}
} catch (OutOfMemoryError OME) {
// OK, Garbage Collector will have run now...
}
My answer is going to be different than the others but it will lead to the same point.
Explain:
YES it is possible to force the garbage collector with two methods used at the same time and in the same order this are:
System.gc ();
System.runFinalization ();
this two methods call will force the garbage collector to execute the finalise() method of any unreachable object and free the memory. however the performance of the software will down considerable this is because garbage runs in his own thread and to that one is not way to controlled and depending of the algorithm used by the garbage collector could lead to a unnecessary over processing, It is better if you check your code because it must be broken to you need use the garbage collector to work in a good manner.
NOTE: just to keep on mind this will works only if in the finalize method is not a reassignment of the object, if this happens the object will keep alive an it will have a resurrection which is technically possible.
Is there a way to "clean up" objects and other variables you create? Or are they automatically disposed of or do I have this whole concept wrong? What is the proper way to go about doing this? I am trying to avoid the GC as much as possible.
The only way to cleanup in an GC language with no memory management is the GC . You can force GC but its not recommended , the GC is pretty good , to be more proactive set objects to null for the GC to clean up.
Addition:
Also try to make objects as local as possible , that way they are GCed as they scope out.
Calling System.gc() will force Garbage Collection to happen.
There is a system counting references to objects you create. If you are looping a lot and creating lots of objects you will create periods of time where they pile up. The system will collect the garbage when your processor is not doing anything, or it will wait till you need more free memory before collection occurs. If you have been processing for some time, you will experience hiccups in your performance due to Garbage Collection happening during your processes.
Please view this page and search for "Garbage Collection"
http://developer.android.com/guide/practices/design/performance.html
NOTE: Anything created with an Application Context will live until the end of the application execution. Anything created with an Activity Context will live until the end of the activity. This two situations can cause memory leaks!
For a more complete answer specific to Android:
Make sure you review the application lifecycle for android. It will help you avoid activity leaks in Android.
For the most part they are cleaned up as long as you do not maintain a reference to the object (variable). Something's like cursor's and bitmap's though need to be closed before they can be deleted to prevent memory leaks.
I don't think you have to worry about the GC as long as your object creation is not over the top. Note: GC is a part of java. You can't avoid it.
Addendum 1: If you really are that worried about it, you could reuse variables. That way you keep object creation to a minimum, but in so doing you will lose that variable and will be unable to store a wide range of data.
Android's activities have onDestroy() method. You can use this method to close open connections or dialogs or close some pending tasks.
You could also read about Java GC to get a more proper understanding of it. I would recommend SCJP book, Garbage collection chapter. It explains well when an object becomes eligible for garbage collection.
What conditions would prevent the JVM from running a FULL Garbage Collection when the CPU is at 5% to 8% load?
I am seeing a constant shallow GC cycle, but not able to tune the JVM to want to run FULL GC.
Where can I go to find the conditions that the JVM says "I am too busy to run".
When I was studying for my SCJP certification a lot of emphasis was made on
"You can not do anything to force the
GC to run at any given time, you can
just give hints to it"
The whole idea of having an automatic GC is precisely not having to worry about how or when it runs to clean up free memory for you. So, there is no way to actually change when or how GC does actually run... you would have to re-implement one JVM to do what you want.
There are just so many factors involved in this, there may be other, more elegant solutions for this.
It depends entirely on the garbage collector algorithm that you're using in your particular JDK. About all you can guarantee about garbage collection is that if the JVM throws an OutOfMemoryError, the garbage collector made its best effort to collect every unreachable/weakly reachable object. Even System.gc() doesn't guarantee anything, a no-op is a completely legal implementation.
Hence in that light I don't know if your question has any weight. If you truly believe that you need to tweak the garbage collector, it would help if you posted the problems you're seeing, and the profiling data that leads to believe that poor GC performance is the problem.
Outside of this, the garbage collector should be treated like a black box. The logic behind its implementation is surprisingly complex, and there's a very good chance it knows better than you what it ought to be doing at any given time. 99 times out of 100, trying to force the garbage collector to behave in a particular way will lower performance, not increase it.
It's not that it's to busy to run, but it does simply not need extra memory.
After reading this question, I was reminded of when I was taught Java and told never to call finalize() or run the garbage collector because "it's a big black box that you never need to worry about". Can someone boil the reasoning for this down to a few sentences? I'm sure I could read a technical report from Sun on this matter, but I think a nice, short, simple answer would satisfy my curiosity.
The short answer: Java garbage collection is a very finely tuned tool. System.gc() is a sledge-hammer.
Java's heap is divided into different generations, each of which is collected using a different strategy. If you attach a profiler to a healthy app, you'll see that it very rarely has to run the most expensive kinds of collections because most objects are caught by the faster copying collector in the young generation.
Calling System.gc() directly, while technically not guaranteed to do anything, in practice will trigger an expensive, stop-the-world full heap collection. This is almost always the wrong thing to do. You think you're saving resources, but you're actually wasting them for no good reason, forcing Java to recheck all your live objects “just in case”.
If you are having problems with GC pauses during critical moments, you're better off configuring the JVM to use the concurrent mark/sweep collector, which was designed specifically to minimise time spent paused, than trying to take a sledgehammer to the problem and just breaking it further.
The Sun document you were thinking of is here: Java SE 6 HotSpot™ Virtual Machine Garbage Collection Tuning
(Another thing you might not know: implementing a finalize() method on your object makes garbage collection slower. Firstly, it will take two GC runs to collect the object: one to run finalize() and the next to ensure that the object wasn't resurrected during finalization. Secondly, objects with finalize() methods have to be treated as special cases by the GC because they have to be collected individually, they can't just be thrown away in bulk.)
Don't bother with finalizers.
Switch to incremental garbage collection.
If you want to help the garbage collector, null off references to objects you no longer need. Less path to follow= more explicitly garbage.
Don't forget that (non-static) inner class instances keep references to their parent class instance. So an inner class thread keeps a lot more baggage than you might expect.
In a very related vein, if you're using serialization, and you've serialized temporary objects, you're going to need to clear the serialization caches, by calling ObjectOutputStream.reset() or your process will leak memory and eventually die.
Downside is that non-transient objects are going to get re-serialized.
Serializing temporary result objects can be a bit more messy than you might think!
Consider using soft references. If you don't know what soft references are, have a read of the javadoc for java.lang.ref.SoftReference
Steer clear of Phantom references and Weak references unless you really get excitable.
Finally, if you really can't tolerate the GC use Realtime Java.
No, I'm not joking.
The reference implementation is free to download and Peter Dibbles book from SUN is really good reading.
As far as finalizers go:
They are virtually useless. They aren't guaranteed to be called in a timely fashion, or indeed, at all (if the GC never runs, neither will any finalizers). This means you generally shouldn't rely on them.
Finalizers are not guaranteed to be idempotent. The garbage collector takes great care to guarantee that it will never call finalize() more than once on the same object. With well-written objects, it won't matter, but with poorly written objects, calling finalize multiple times can cause problems (e.g. double release of a native resource ... crash).
Every object that has a finalize() method should also provide a close() (or similar) method. This is the function you should be calling. e.g., FileInputStream.close(). There's no reason to be calling finalize() when you have a more appropriate method that is intended to be called by you.
Assuming finalizers are similar to their .NET namesake then you only really need to call these when you have resources such as file handles that can leak. Most of the time your objects don't have these references so they don't need to be called.
It's bad to try to collect the garbage because it's not really your garbage. You have told the VM to allocate some memory when you created objects, and the garbage collector is hiding information about those objects. Internally the GC is performing optimisations on the memory allocations it makes. When you manually try to collect the garbage you have no knowledge about what the GC wants to hold onto and get rid of, you are just forcing it's hand. As a result you mess up internal calculations.
If you knew more about what the GC was holding internally then you might be able to make more informed decisions, but then you've missed the benefits of GC.
The real problem with closing OS handles in finalize is that the finalize are executed in no guaranteed order. But if you have handles to the things that block (think e.g. sockets) potentially your code can get into deadlock situation (not trivial at all).
So I'm for explicitly closing handles in a predictable orderly manner. Basically code for dealing with resources should follow the pattern:
SomeStream s = null;
...
try{
s = openStream();
....
s.io();
...
} finally {
if (s != null) {
s.close();
s = null;
}
}
It gets even more complicated if you write your own classes that work via JNI and open handles. You need to make sure handles are closed (released) and that it will happen only once. Frequently overlooked OS handle in Desktop J2SE is Graphics[2D]. Even BufferedImage.getGrpahics() can potentially return you the handle that points into a video driver (actually holding the resource on GPU). If you won't release it yourself and leave it garbage collector to do the work - you may find strange OutOfMemory and alike situation when you ran out of video card mapped bitmaps but still have plenty of memory. In my experience it happens rather frequently in tight loops working with graphics objects (extracting thumbnails, scaling, sharpening you name it).
Basically GC does not take care of programmers responsibility of correct resource management. It only takes care of memory and nothing else. The Stream.finalize calling close() IMHO would be better implemented throwing exception new RuntimeError("garbage collecting the stream that is still open"). It will save hours and days of debugging and cleaning code after the sloppy amateurs left the ends lose.
Happy coding.
Peace.
The GC does a lot of optimization on when to properly finalize things.
So unless you're familiar with how the GC actually works and how it tags generations, manually calling finalize or start GC'ing will probably hurt performance than help.
Avoid finalizers. There is no guarantee that they will be called in a timely fashion. It could take quite a long time before the Memory Management system (i.e., the garbage collector) decides to collect an object with a finalizer.
Many people use finalizers to do things like close socket connections or delete temporary files. By doing so you make your application behaviour unpredictable and tied to when the JVM is going to GC your object. This can lead to "out of memory" scenarios, not due to the Java Heap being exhausted, but rather due to the system running out of handles for a particular resource.
One other thing to keep in mind is that introducing the calls to System.gc() or such hammers may show good results in your environment, but they won't necessarily translate to other systems. Not everyone runs the same JVM, there are many, SUN, IBM J9, BEA JRockit, Harmony, OpenJDK, etc... This JVM all conform to the JCK (those that have been officially tested that is), but have a lot of freedom when it comes to making things fast. GC is one of those areas that everyone invests in heavily. Using a hammer will often times destroy that effort.