Hi I have one doubt about phantom reference. What I understand the finalize method is called just before when object are going for garbage collection. But some time if object are not eligible for garbage collection then finalize method will not execute.
Now talking about phantom reference when this finalize method will called.
Is finalize always called in phantom reference.
I am very much confuse about this. Please help me.
Finalizers are never guaranteed to be called, whether there is a phantom reference or not. Don't rely on finalizers for any critical part of your code because there is no guarantee that they will be called in a timely manner or in fact at all.
Many people advocate that you simply should never use finalizers at all because they are incredibly difficult to use correctly.
When object becomes available only through phantom reference then after the first GC finalize() method is invoked and after the second GC the reference is enqueued. If after that phantom reference is cleaned (or becomes unavailable itself) then the memory is cleared after the third GC.
Finalize will always be called, but not neccessarely, when you expect it. It may happen, that the call will only be made at the JVM shutdown (assuming you don't simply kill the program). You should not rely on finalize() in order to do significant work. But it is also good practice to implement a usefull finalize() and include a call to super.finalize() too.
Related
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.
Java allow to write:
new PhantomReference(new Object(), null)
At this case new Object() will be collected?
As I understand, phantom reference is alternative of finalize() method usage.
And after appearing reference in queue, I need to do some additional actions and then run clear()
java doc stays:
It is possible to create a phantom reference with a null queue, but
such a reference is completely useless: Its get method will always
return null and, since it does not have a queue, it will never be
enqueued
What does mean if it will never be enqueued?
As I understand it means that after finalize method invocation rerference will not be added to the referenceQueue. Thus it may lead to:
1. object memory will be cleared at once
2. Object memory will not be cleared
which case correct?
Well, as you noticed yourself, a PhantomReference is not automatically cleared. This implies that as long as you keep a strong reference to the PhantomReference, the referent will stay phantom reachable. As the documentation says: “An object that is reachable via phantom references will remain so until all such references are cleared or themselves become unreachable.”
However, considering when an object is unreachable (now I’m talking about the “phantom references themselves”) can lead to many surprises. Especially as it’s very likely that the reference object, not providing useful operations, will not be subsequently touched anymore.
Since the PhantomReference without a queue will never be enqueued and its get() method will always return null, it is indeed not useful.
So why does the constructor allows to construct such a useless object? Well, the documentation of the very first version (1.2) states that it will throw a NullPointerException if the queue is null. This statement persists until 1.4, then Java 5 is the first version containing the statement that you can construct a PhantomReference without a queue, despite being useless. My guess is, that it always inherited the super class’ behavior of allowing a null queue, contradicting the documentation, and it was noticed so late, that the decision was made to stay compatible and adapt the documentation rather than changing the behavior.
The question, even harder to answer, is why a PhantomReference isn’t automatically cleared. The documentation only says that a phantom reachable object will remain so, which is the consequence of not being cleared, but doesn’t explain why this has any relevance.
This question has been brought up on SO, but the answer isn’t really satisfying. It says “to allow performing cleanup before an object is garbage collected”, which might even match the mindset of whoever made that design decision, but since the cleanup code can’t access the object, it has no relevance whether it is executed before or after the object is reclaimed. As said above, since this rule depends on the reachability of the PhantomReference object, which is subject to optimizing code transformations, it might be even the case that the object is reclaimed together with the PhantomReference instance before the cleanup code completes, without anyone noticing.
I also found a similar question on the HotSpot developer mailing list back in 2013 which also lacks an answer.
There is the enhancement request JDK-8071507 to change that behavior and clear PhantomReferences just like the others, which has the status “fixed” for Java 9, and indeed, its documentation now states that they are cleared like any other reference.
This, unfortunately implies that the answer at the beginning of my post will be wrong starting with Java 9. Then, new PhantomReference(new Object(), null) will make the newly created Object instance immediately eligible for garbage collection, regardless of whether you keep a strong reference to the PhantomReference instance or not.
i want to call destructor, when i close object, but i dont know how or when,.. I try whit finalize but its not calling. I dont know why
There is example code, i try with debugger but its help me..
class tralala(){
int i=0;
tralala(){
i=i+1;}
protected void finalize( ){
i=i-1;
}}
But it isnt call i dont know why. If anyone have got a suggestion plis help me. I am in the stuck at the moment, but i am not shure if java has got destructors.
Method finalize( ) "сalled by the garbage collector on an object when garbage collection determines that there are no more references to the object". So it will be called after garbage collector observe it to delete, but not immediately when object leave the scope.
I rly can't understand what do you mean as "close object"? You should find some other architecture solution to do what you want: maybe just call your method or raise some custom event with supplied handler when object leave scope ("close object").
I think you are trying to implement similar behavior like destructors. As destructors are not available into Java, I think you are trying to do with finalize.
But the finalize method is called when an object is about to get garbage collected. That can be at any time after it has become eligible for garbage collection.
Note that it's entirely possible that an object never gets garbage collected (and thus finalize is never called). This can happen when the object never becomes eligible for gc (because it's reachable through the entire lifetime of the JVM) or when no garbage collection actually runs between the time the object become eligible and the time the JVM stops running (this often occurs with your programs).
Read more about When is the finalize() method called in Java? and Why not to use finalize() method in java
And read How to implement object counter in Java SO QA for your solution. This is used for object counter. You can use same logic.
There is no destructors in java
class Tralala(){
int i=0;
Tralala(){
i=i+1;}
protected void reset( ){
i=i-1;
}}
call reset method when ever you want to destruct.
As a side note:There are lots of corrections in your code like naming conventions, formatting and also providing modifiers.
And I'm not getting, when finalize called, your instance is no more. Why you are resetting at that time,Since after that NO MORE OPERATIONS ON THAT OBJECT.
JAVA does not have destructors. JAVA has Garbage collectors. Garbage Collector runs as a thread and automatically destroys instances which do not have any active references.
it depends on your definition of closing the class.
you could implement the interface Closable so that eclipse could warn you about not closing the instance when needed.
it's different from finalize since you need to call it by yourself.
you could call close() in the finalize function in order to make sure it will get called when the instance is GC'ed .
In order to understand weak references in Java, I have had to consult the Java Language Specification. The following part, from section 12.6, puzzles me:
An unfinalized object has never had its finalizer automatically invoked;
a finalized object has had its finalizer automatically invoked. A finalizable
object has never had its finalizer automatically invoked, but the Java virtual
machine may eventually automatically invoke its finalizer.
So what is the formal difference between an unfinalized and a finalizable object ? From the quote it seems that if unfinalized and finalizable are to be different, then for an unfinalized object it must be the case that it is not true that the JVM may eventually invoke its finalizer. A little confusing or I still have some English semantics to study ;)
Link to the section in the Java spec: Implementing Finalization
The answer seems to lie in this line:
If the Java virtual machine detects that an unfinalized object has become finalizer-reachable or unreachable, it may label the object finalizable (G, H);
Unfinalized objects are not eligible for finalization yet. They are reachable. Finalizable objects are eligible to be finalized, so the JVM may do that when it chooses. In other words, "may" in the sense of "has permission to" not just in the sense of "it might happen."
The difference between an unfinalized and a finalizable object is that the finalizer on the second one could be automatically invoked at any time in the future, while the finalizer on the unfinalized object can't be automatically invoked, unless the object first becomes finalizable.
a unfinalized object will not get its finalizer automatically invoked by the JVM in this state
a finalizable object can eventually get its finalizer automatically invoked by the JVM
There is no guarantee that a GC will ever be performed or that finalize() will ever be called. It is highly likely that it will happen at some point.
When an object no longer has a strong reference to it, it can be garbage collected. Some time later a GC can be performed and the object is added to a finalization queue to have its finalize() method called. Once the method has been called it can be removed if there still is not strong reference to it.
In Java, finalize is called on an object (that overrides it) when it's about to be garbage collectioned, so when it's unreachable. But what if the finalizer makes the object reachable again, what happens then?
Then the object doesn't get garbage collected, basically. This is called object resurrection. Perform a search for that term, and you should get a bunch of interesting articles. As Jim mentioned, one important point is that the finalizer will only be run once.
The object will not be collected until it gets unreachable again.
According to the JavaDoc, finalize() will not be called again.
If you read the API description carefully, you'll see that the finalizer can make the object reachable again. The object won't be discarded until it is unreachable (again), but finalize() won't be called more than once.
Yeah, this is why you don't use finalizers (Well, one of the many reasons).
There is a reference collection that is made to do this stuff. I'll look it up and post it here in a sec, but I think it's PhantomReference.
Yep, PhantomReference:
Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.
It actually does another pass to check and make sure there are no more references to the object. Since it will fail that test on its second pass, you'll end up not freeing the memory for the object.
Because finalize is only called a single time for any given object, the next time through when it has no references, it will just free the memory without calling finalize. Some good information here on finalization.