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I'm reading this article and I can't really understand how the finalizable objects (objects which override the finalize method) takes at least 2 GC cycles before it can be reclaimed.
It takes at least two garbage collection cycles (in the best case) before a finalizeable object can be reclaimed.
Can someone also explain in detail how is it possible for a finalizable object to take more than one GC cycle for reclamation?
My logical argument is that when we override finalize method, the runtime will have to register this object with the garbage-collector (so that GC can call finalize of this object, which makes me think that GC will have reference to all the finalizable objects). And for this, GC will have to keep a strong reference to the finalizable object. If that is the case then how this object became a candidate for reclamation by GC in the first place? I reach a contradiction by this theory.
PS: I understand that overriding finalize is not the recommended approach and this method is deprecated since Java 9.
You are right in that the garbage collector needs a reference to finalizable objects. Of course, this particular reference must not be considered when deciding whether the object is still reachable before the finalization. This implies special knowledge about the nature of this reference to the garbage collector.
When the garbage collector determines that an object is eligible for finalization, the finalizer will run, which implies that the object becomes strongly reachable again, at least as long as the finalizer is executed. After its finalization, the object must become unreachable again and this must be detected, before the object’s memory can be reclaimed. That’s why it takes at least two garbage collection cycles.
In case of the widely used Hotspot/OpenJDK environment (and likely also in IBM’s JVM), this is implemented by creating an instance of a special, non-public subclass of Reference, a Finalizer, right when an object, whose class has a non-trivial finalize() method, is created. Like with weak & soft references, these references are enqueued by the garbage collector when no strong reference to the referent exist, but they are not cleared, so the finalizer thread can read the object, making it strongly reachable again for the finalization. At this point, the Finalizer is cleared, but also not referenced anymore, so it would get collected like an ordinary object anyway, so by the next time the referent becomes unreachable, no special reference to it exists anymore.
For objects whose class has a “trivial finalizer”, i.e. the finalize() method inherited by java.lang.Object or an empty finalize() method, the JVM will take a short-cut and not create the Finalizer instance in the first place, so you could say, these objects, which make the majority of all objects, behave as if their finalizer did already run, right from the start.
Though you got your answer (which is absolutely correct), I want to add a small-ish addendum here. In general, references are of two types : strong and weak. Weak References are WeakReference/SoftReference/PhantomReference and Finalizer(s).
When a certain GC cycle traverses the heap graph and sees one of these weak references, it treats it in a special way. When it first encounters a dead finalizer reference (let's consider this being the first GC cycle), it has to resurrect the instance. finalize is an instance method, and it needs an actual instance to be invoked. So a GC first saw that this Object is dead, only to revive it moments later, to be able to call finalize on it. Once it calls that method on it, it marks the fact that it has already been called; so when the next cycle happens, it can be actually be GC-ed.
It would be incorrect to call this the second GC.
For example G1GC does partial clean-up of the heap (young and mixed), so it might not even capture this reference in the next cycle. It might not fall under its radar, as simple as that.
Other GCs, like Shenandoah, have flags that control on which iteration to handle these special references (ShenandoahRefProcFrequency, 5 by default).
So indeed there is a need for two cycles, but they do not have to be subsequent.
As far as I understand, GC starts with some set of initial objects (stack, static objects) and recursively traverses it building a graph of reachable objects. Then it marks the memory taken by these objects as occupied and assumes all the rest of the memory free.
But what if this 'free' memory contains an object with finalize method? GC has to call it, but I don't see how it can even know about objects that aren't reachable anymore.
I suppose GC can keep track of all 'finalizable' objects while they are alive. If so, does having finalizable objects make garbage collecting more expensive even when they are still alive?
Consider the Reference API.
It offers some references with special semantics to the GC, i.e Weak, Soft, and Phantom references. There’s simply another non-public type of special reference, for objects needing finalization.
Now, when the garbage collector traverses the object graph and encounters such a special reference object, it will not mark objects reachable through this reference as strongly reachable, but reachable with the special semantics. So if an object is only finalizer-reachable, the reference will be enqueued, so that one (or one of the) finalizer thread(s) can poll the queue and execute the finalize() method (it’s not the garbage collector itself calling this method).
In other words, the garbage collector never processes entirely unreachable objects here. To apply a special semantic to the reachability, the reference object must be reachable, so the referent can be reached through that reference. In case of finalizer-reachability, Finalizer.register is called when an object is created and it creates an instance of Finalizer in turn, a subclass of FinalReference, and right in its constructor, it calls an add() method which will insert the reference into a global linked list. So all these FinalReference instances are reachable through that list until an actual finalization happens.
Since this FinalReference will be created right on the instantiation of the object, if its class declares a non-trivial finalize() method, there is already some overhead due to having a finalization requirement, even if the object has not collected yet.
The other issue is that an object processed by a finalizer thread is reachable by that thread and might even escape, depending on what the finalize() method does. But the next time, this object becomes unreachable, the special reference object does not exist anymore, so it can be treated like any other unreachable object.
This would only be a performance issue, if memory is very low and the next garbage collection had to be performed earlier to eventually reclaim that object. But this doesn’t happen in the reference implementation (aka “HotSpot” or “OpenJDK”). In fact, there could be an OutOfMemoryError while objects are pending in the finalizer queue, whose processing could make more memory reclaimable. There is no guaranty that finalization runs fast enough for you’re purposes. That’s why you should not rely on it.
But what if this 'free' memory contains an object with finalize
method? GC has to call it, but I don't see how it can even know about
objects that aren't reachable anymore.
Let's say we use CMS garbage collector. After it successfully marked all live objects in a first phase, it will then scan memory again and remove all dead objects. GC thread does not call finalize method directly for these objects.
During creation, they are wrapped and added to finalizer queue by JVM (see java.lang.ref.Finalizer.register(Object)). This queue is processed in another thread (java.lang.ref.Finalizer.FinalizerThread), finalize method will be called when there are no references to the object. More details are covered in this blog post.
If so, does having finalizable objects make garbage collecting more
expensive even when they are still alive?
As you can now see, most of the time it does not.
The finalise method is called when an object is about to get garbage collected. That means, when GC determines that the object is no longer being referenced, it can call the finalise method on it. It doesn't have to keep track of objects to be finalised.
According to javadoc, finalize
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
So the decision is based on reference counter or something like that.
Actually it is possible not to have this method called at all. So it may be not a good idea to use it as destructor.
I have some objects in my code which are not invoked in everyloop,But they are useful in future when some forced termination from loop,unpredictable error etc happens, So how to make sure that this objects are not ever collected by garbage collector.
For example : I have one class so I dont want GC to perform any Garbage collection on this class
If the objects are valuable and expensive to create, you should hang on to them in a field in your class. You could create a Map that would act sort of like a cache and grab them out of the Map when you're ready to use them again.
If they are useful in the future you have a reference to these objects somewhere. This is enough to ensure that they will never be garbage collected.
Keep a strong reference to the object, Doing this will always ensure that your object will not be GCed. I would also leave such things to the Garbage collector which I think is smarter than us(with all due respect) when it comes to memory management
You should have a look at the scope of your variables. as long as they are in scope they should not be garbage collected.
Garbage Collector Simply Avoids The Object Which Have Any Reference In The Code Further. So If You Want Any Object To Never Get Collected By G.C. Until The Execution Of Your Code. Just Have A Global Reference To That Object.
I want to know what all variables are garbage collected in my program and the order of garbage collection. Is there a simple way to do that:
class GarbageUtility {
public static void main(String args[]) {
int a =10;
int b = a;
int c = a + b;
System.out.println(a);
}
}
you may have a look at com.sun.management.GarbageCollectionNotificationInfo, see http://docs.oracle.com/javase/7/docs/jre/api/management/extension/com/sun/management/GarbageCollectionNotificationInfo.html
also, this post presents a way to make a gc detector for hashmaps, http://java.dzone.com/articles/letting-garbage-collector-do-c
I want to know what all variables are garbage collected in my program and the order of garbage collection. Is there a simple way to do that:
No there isn't1.
Variables are not garbage collected2. Objects are garbage collected ... or at least, they may be garbage collected.
Most of the variables in you program have type int and int is not an object / reference type. They won't even be affected by the garbage collector.
Your program won't even compile ...
Footnotes:
There are a couple of ways that you can use to infer that an object is being garbage collected:
If you declare a finalize() method, that will be called when the GC detects that the object has no strong, soft or weak references to is ... and garbage collection is imminent. You can get a similar effect with Reference objects and their associated queues. Note however that this may also change the lifetime of the respective objects ... causing them to live longer than they otherwise might.
You might be able to detect that an object is dues to be garbage collected via the external debugger or profiler agents. (I'm not sure about this. I actually think that unreachable objects are invisible to the agents.)
Actually, it is a little more complicated than that.
Variables don't have an independent lifetime. They are always part of "something else" ... and the "something else" can be garbage collected ... in some cases:
We can divide variables into 3 kinds:
Stack variables (that is method parameters and locals) are not stored in space that is managed by the GC. Any object whose reference is in a (live) stack variable won't be garbage collected.
Instance variables (instance fields declared by a class) are part of the respective object. When the object is garbage collected, the variables "go away".
Static variables (static fields declared by a class) normally stay around for the lifetime of the application. However, there are circumstances where a class may be garbage collected, and if that happens its static variables go away at the same time.
Note that GarbageCollectionNotificationInfo tells you that the GC has collected a certain heap, and gives some basic statistics such as how long the GC took and what space was available in the heap before and after.
I don't see how that helps you tell whether specific objects have been garbage collected and when it happened.
I am confuse in between system.gc() and finalize() method of java.
We can't force to collect garbage object to JVM. We are allow to write both methods in our java code then if both are used for garbage collection, then what is point in providing two methods for garbage collection by java?
Please tell me the exact working of both methods and internally how it works?
System.gc() kindly asks the sytem to perform a garbage collection. Javadoc says:
Runs the garbage collector.
You can not control how "hard" the garbage collector will work. How the garbage collector work internally is VM-specific and a research topic on its own. But there are usually "full" garbage collection and some smaller "incremental" collection going on. So consider System.gc as a request, but there's not guaranteed that garbage collection of your object will happen.
Object.finalize() can be overriden to specify what to do when a given object is garbage collected (actually what to do just before it is garbage collected). Javadoc says:
Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object.
Classical use of finalizer are to de-allocate system resources when an object is garbage collected, e.g. release file handles, temporary files, etc.
Do not use finalizer to perform actions when the JVM exits. For this purpose use a shutdown hook that you register with Runtime.getRuntime().addShutdownHook(Thread).
System.gc() forces the garbage collector to run, while the Finalize() method of your object defines what garbage collector should do when collecting this specific object.
Roughly speaking, it is like this:
class MyClass {
public UnmanagedResource resource;
void Finalize() {
FreeUnmanagedResource(resource);
}
}
MyClass[] data = new MyClass[1000];
for(int i=0; i<1000; i++) {
data[i] = new MyClass();
}
//do some work
data = null;
System.gc(); //Note that it doesn't guarantee you that all MyClass instances will be actually collected at this point.
system.gc() method notifies the JVM that the garbage collector can run now to clear the memory by deleting unused objects. As per the java 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.
finalize() method will not trigger garbage collector instead it will be called while the garbage collector about the destroy the object. It provides the instructions to clear the object properly.
The answers here are great, just wanted to elaborate small point about the finalize() method: you should never use it.
It's execution is not guaranteed at all and eventually the usage of finalize() adds performance penalty.
As written in Effective Java by Joshua Bloch:
Finalizers are unpredictable, often dangerous, and generally
unnecessary. never do anything time-critical in a finalizer. never
depend on a finalizer to update critical persistent state.
And:
Oh, and one more thing: there is a severe performance penalty for
using finalizers. On my machine, the time to create and destroy a
simple object is about 5.6 ns. Adding a finalizer increases the time
to 2,400 ns. In other words, it is about 430 times slower to create
and destroy objects with finalizers.
Prefer using the following resource terminating try-finally pattern, which are guaranteed to run:
Bar bar = new Bar(...);
try {
// Your code here
} finally {
bar.releaseResource(); // You implementation. For example close connection
}
The System.gc() method garbage collects the objects that are created by a new keyword, whereas the finalize() method is used when we want to garbage collect the objects that are not created using a new keyword. So, I guess this ans. your Q
Garbage collection is used to reclaim the allocated memory of object and Finalize() is called by garbage collector before reclaiming the memory Finalize() method mostly used to do some operation before deleting the object
Garbage collector is invoked by calling the System.gc() method on any class. But it does not guarantees that all the nullified objects will be garbage collected before jvm shuts down. So basically gc() method is garbage collecting only those objects which are created using new keyword. Objects are created by any other logic than this will be garbage collected by explicitly calling finalize() method before gc calls the garbage collector.
There are many classes that contain the finalize() method, I'll assume you mean the one in the Object class. System.gc() is what you call when you want Java's garbage compiler to run. It will run by itself every few minutes, but you can ask it to go whenever you want. When the garbage collector runs, it calls the finalize() method on each object that has no more reference. Basically, System.gc() cleans up memory and uses finalize() to get rid of the individual objects.