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)
Related
I want to use WeakReferences as part of an (android) bitmap-cache to be able to check when a bitmap isn't used anymore.
My cache has a maximum-size that is smaller than the Java heap-space. When a new bitmap would overflow the cache, it should let go of bitmaps that aren't needed anymore.
My question: When does the get()-method of a WeakReference return null?
As soon as there are no more strong-references to the object? (and GC hasn't occurred yet)
Or when the GC has run and determined that their are no more strong-references to the object?
If 2. is true than I could run into the situation that my cache could fill up and GC hasn't recently run for some reason.
Then even if I had already let go of references AFTER the last GC run, WeakReference#get() would still return the object and my cache wouldn't clear it out.
The answer is it depends on what version of Android you're using. Somewhere in the 2.3 timeframe, Android changed its handling or weak references. Previously, it deleted them when the GC ran. As of some version of 2.3 (2.3.3?) it started deleting them immediately upon the last strong reference going away. So in modern versions of Android, weak references are useless.
Before this change, weak references were used for caching. They no longer work. The correct way now is to use an LRUCache. If you need to support older versions, use the support library to backport the LRU cache.
After some searching, I think the change was made in 3.0, not 2.3. Still, the solution is the same.
The WeakReference is cleared as soon as the GC has determined that the object is weakly reachable.
This is close to your second case. However, weak reachability requires not just the absence of strong references, but also the absence of soft references.
From the Java package documentation for java.lang.ref:
Soft and weak references are automatically cleared by the collector before being added to the queues with which they are registered, if any.
...
An object is weakly reachable if it is neither strongly nor softly reachable but can be reached by traversing a weak reference. When the weak references to a weakly-reachable object are cleared, the object becomes eligible for finalization.
To cache bitmaps use SoftReference rather than Weak. GC would clear WeakReference as soon as it looses all its Strong and Soft references which might destroy the purpose of caching. SoftReference are cleared only if there is low memory.And it is guaranteed that the GC would be run before throwing a OOME.
As stated in the other answers, a WeakReference will return null once the pointed object has no more strong/soft references and the GC has reclaimed memory.
On a more general rule, I don't think Weak/SoftReferences are a good thing in an application. It makes your mix concerns :
Your application is about business logic
The JVM and Dalvik are about memory management and code optimisation.
When you start to use Weak/Soft references, you introduce memory-managed concerns within your application, which makes it harder to develop/debug/understand.
You might want to have a fixed-size (number of elements or bitmap size) LRU cache instead.
Hope that helps !
It's assured that Garbage Collector destroys all the unwanted and unused objects,
what if we manually nullify the objects eg. List<String> = null ,
does this action makes any negative or positive performance effect?
I am on Java.
Thanks.
Not an expert on details of memory handling but I can share what I know. GC will collect whatever is not used. Thus when you eliminate the last reference to an object (by explicitly nullifying) you'll be marking it for garbage collection. This does not guarantee that it'll be collected immediately.
You can explicitly try and invoke GC but you'll see lots of people advising against it. My understanding is that the call to GC is unreliable at best. The whole point with GC and Java is that you as a programmer should not need to worry much about the memory allocation. As for performance, unless you have tight limitations for heap space, you shouldn't notice GC activity.
Garbage collection is a way in which Java recollects the space occupied by loitering objects. By doing so, it [Java] ensures that your application never runs out of memory (though we cannot be assured that the program will ever run out of memory).
It is suggested to leave it on JVM.
Read related : Does setting Java objects to null do anything anymore?
Explicit nulling makes little or no difference. Usually the GC can reliably detect when an object can no longer be reached, and can thus be GCd.
Particularly, nulling stack (i.e. inside methods) variables helps absolutely nothing. It's trivial to for the runtime to automatically detect when they will be needed and when not. nulling heap (i.e. inside classes) variables could in some rare instances help, but that's a rare exception, and probably does more harm (in code legibility/maintainability) than good.
Also note that nulling doesn't guarantee if, or when, an object will be GCd.
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Possible Duplicate:
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.
Read in some blog that GC in Android happens on main(UI) thread, this may create sluggishness in UI screen depending on the frequency of GC execution.
Hence I was wondering will it be a good idea if I manually release objects(by assigning null value) which has no further use for me.
This way we may avoid multiple execution of GC in the application.
Please share your thoughts.
Thanks,
sku
There's no such thing as "manually releasing objects" -- at least not in any way that's meaningful to GC. An object doesn't immediately get freed/collected/whatever when you lose all references to it; it just becomes eligible for collection. GC is what actually does the releasing of the object, and it does so when it feels like doing so.
The only real way to keep the GC from working so hard is to create fewer objects, particularly temporary objects. Less garbage == less collection.
Releasing (dereferencing) objects for which you have no further use is always a good idea. You can also use SoftReference, WeakReference and/or WeakHashMap to help the GC pick up stuff that you don't mind going away if the system needs space.
There's more information about Android's GC system here.
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