In Java, at some point in code I want to free memory taken by a huge HashMap<Integer, ArrayList<Integer>> object. Is it enough to point it to null like below:
Map<Integer, ArrayList<Integer>> complexObject = new HashMap<Integer, ArrayList<Integer>>(1000000);
...
complexObject = null;
?
You cannot explicitly de-allocate Java objects. But you can do the following:
Remove all references to the item you no longer need. You can do this by setting your only reference to an object to null.
Call System.gc() to "suggest" to the JVM to run the garbage collector, which deallocates no-longer used objects, although it's not guaranteed that calling this method will actually run the garbage collector.
Setting its reference to null will mark it available to the garbage collector next time it decides to run if there are indeed no more references to said object laying around anywhere. When the GC decides to run however, is not set in stone.
This is a big point where Java is different from C, but don't worry. 99.9% of the time you can trust the GC has your back.
There is no guarantee of freeing memory. GC will run and pickup nulls so that's all you can do yes.
I just read this article, there is something about freeing memory in it, too.
Check it out, nulling does not always help you.
Related
I have a Java ArrayList with references to other objects stored within the List.
If I mark the List as null then when it is garbage collected, will all the stored items in it also get claimed by GC (assuming there are no other references to them)?
thanks,
Jakao
If you know it (can access it from code / have a reference to it anywhere) it´s there (and won´t ever be collected), if you don´t it might be gone. When is it gone? None of your concern, that´s the point of having a garbage collector.
Assuming there are no other references to them, they will be GC.
In java object reference is an abstract concept, you should not worry about how the JVM manages object storage, but if you interested in memory managment in java, I suggest you deepen the arguments of the weak and soft reference and memory pools.
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.
Here is my code:
LinkedList <Mono> list = new LinkedList ();
list.add(new Mono (2, 2));
list.add(new Mono (1, -1));
list.remove (1);
Now, when the second item in the list is removed, is the object destroyed? IE, it undergoes garbage collection?
EDIT for new question:
Yes, the object will be eligible for garbage collection when there are no strong references remaining. However the JVM will try to do clean up garbage in big batches, so it could actually get collected at any arbitrary later time (or never if the JVM terminates before GC gets around to it)
Old answer:
Class unloading is a rare event, and will generally not happen in a timely manner (if at all.)
Specifically, even after it becomes eligible for collection it won't get collected along with "normal" new objects like your Mono instances - it's often in a special different pool (PermGen in the Oracle JVM)
You should assume that once a class is loaded it will stay loaded forever. Once you get into web applications in containers this is not always true, but anyone that has worked in those environments can tell you generally how well it (doesn't) work.
Garbage Collection in Java is generally non-deterministic insofar as when it will occur and which eligible objects the GC will evict ("free") when a GC cycle does occur.
The only reliable rule is thus:
An object will remain available (will not be GC'ed/freed/evicted) as long as it is strongly-reachable. (See "The Reachability Lifecycle of Objects" in Chapter 9 of Inside the Java Virtual Machine for what this means -- very good reading in general, if a tad dated.)
In the posted code the remove will result in the second object new Mono(1, -1) being eligible for reclamation as there are no longer any strong references to it (the object is no longer strongly-reachable). The actual eviction (if it occurs) will happen "sometime later" and may not even be the next GC cycle. Using finalizers (ick) further complicates the matter.
Note that an object is never guaranteed to be GC'ed (the JVM may just terminate [ab]normally, for instance) and the exact semantics of particular GC implementation can differ and still be a conforming virtual machine -- it all comes down to reachability.
Happy coding.
Do you mean when the Object is unloaded?
An empty list is still a list, so it'll stay in memory. It is cleared when you go:
list=somethingElse;
That is assuming that you don't assign anything else to be list.
As far as the class definition itself, it should stay in memory forever. Class definitions are in the permanent generation.
As a side note. list cannot be garbage collected at that point. Because you can add things to it after if you clear it.
the second object will be elligible for gargabe collection after removing it from the list, since there will be no more references to it .. hence out of scope .
hope this helped
Lots of answers, so here's my spin. Think about "scope", a concept in computer languages that describes where and when you can access a named bit of memory.
Here's your original code, with your purported removal of the second list member added:
LinkedList <Mono> list = new LinkedList ();
list.add(new Mono (2, 2));
list.add(new Mono (1, -1));
list.remove (1);
list.remove (2);`
At the point of list.remove(2), the object "list" can still be referenced. Sure, it's empty, but then you might decide to add a new Mono to it. You can because "list" is still in scope, so "list" is not reclaimed.
Compare to this:
{
LinkedList <Mono> list = new LinkedList ();
list.add(new Mono (2, 2));
list.add(new Mono (1, -1));
list.remove (1);
list.remove (2);`
}
After the closing brace, "list" can no longer be referenced. "list" was declared inside that scope, and when the scope was exited, "list" was removed from the namespace along with the scope itself. At that point, garbage collection could happen, since no one could possibly use "list" again.
Keep an eye on the scope of your objects. If you want to influence garbage collection, keep the scope of your objects limited to the region where they are used. As it happens, that is good programming style too.
I guess people got confused with the terminology you used. I believed you are asking if your Mono object will be "deleted"/"garbage collected".
Let's take a look at the remove(1) that you are invoking...
This is the remove function that you are calling as defined in java.util.LinkedList:
public E remove(int index) {
return remove(entry(index));
}
The function above calls the following (look at my comments in the code):
private E remove(Entry<E> e) {
if (e == header)
throw new NoSuchElementException();
E result = e.element;
e.previous.next = e.next; //Preceding element refers now to the one after the element to be removed;
e.next.previous = e.previous; //Next element refers now to the one before the one to be removed;
e.next = e.previous = null; //Element to be removed doesn't refer to anything anymore;
e.element = null;
size--;
modCount++;
return result;
}
After the function you call terminates, there is no way to refer to your Mono(1, -1) anymore. That Mono Object is not accessible anymore. This means that it will become eligible for Garbage Collection. Keep in mind that "eligible" might mean that it never be garbage collected... More on GC here.
The simple answer is that it should not matter to you when a Java object is garbage collected. The only thing that you need to know is that it will get garbage collected before your program runs out of memory ... provided that the object is unreachable.
The complicate answer includes other things:
The garbage collector typically runs at a time that you can't predict.
You can call System.gc() to suggest that the JVM runs the GC run now, but:
the JVM may ignore this hint, and
it is generally a bad idea to do this. (Running the GC is expensive, and your application has insufficient information to know when it is best to do this from an efficiency standpoint.)
Any particular run of the GC is not guaranteed to reclaim all unreachable objects. The GC has a lot of "smarts" that are aimed at making GC as efficient as possible, or reducing "pause" times. One of the "smarts" is to not GC the entire heap every time.
There are no guarantees that the GC will run at all, or that it will run before the JVM is shutdown.
Because of the above, it is a bad idea to write an application so that it depends on a specific object being reclaimed / deleted at a specific time.
(The one thing that should concern you in memory management is storage leaks; e.g. when your application keeps references to unwanted objects that prevent those objects from ever becoming unreachable. But that's not what your question is about.)
The class "Mono" cannot be unloaded since there are still references to it. The type of the list refers to it and there is still one element in the list.
I suppose you did not mean to ask whether the class is unloaded, but whether the instance is "unloaded". Each instance of a class, each object, is allocated on the heap. When the object is no longer in use, the space that it occupies in the heap can be reclaimed. This does not happen immediately however. All JVM implementations that I know use a garbage collector to clean up the memory. To really simplify things here: when there is no more space free to create a new object on the heap, the garbage collector kicks in and will check which parts of the heap are still in use. The parts that are no longer in use, can be reused for new objects.
So, the memory from an object that is no longer in use, will only be reclaimed when the garbage collector kicks in. And this is something that cannot be predicted.
Do you mean is the instance of Mono eligible for garbage collection or is the instance of list eligible for garbage collection?
The instance of mono will be eligible for garbage collection when it is removed (assuming that the code has not created over references to it.
The list is NOT eligible for garbage collection just because it is emptied. An empty list cannot be garbage collected because it is a valid object that can read and written to again.
An as others have pointed out. We are talking about eligible for garbage collection. The garbage collector does not necessarily run immediately.
In Java, to unload an object from the heap, is it sufficient to simply write myObject = null; and the GC will take care of it from there?
EDIT : Ok let me explain my use case, since everyone is assuming that I shouldn't explicitly null objects, I shouldn't worry about it, etc. That's missing the point. I am serializing an object, and am "consuming" a field of this object before I serialize it in order to save disk space. And before you jump down my throat for this, too, I cannot declare this field transient because I am including this field in the object sometimes, but not others.
Does setting an object to null have any effect on the GC?
In some modern VMs, actively setting a reference to null hinders the garbage collector. You should just forget about that.
For knowing when an object is garbage collected, look at the java.lang.ref package - although I can honestly say that in 16 years of Java programming, I've never needed to know when an object is garbage collected.
Can you elaborate on why you think you need this?
No; all references to that object must be lost/nulled. In practice this is something you shouldn't worry about.
Your object will be de-allocated when it is no longer used. Just be aware that any references left to the object will keep the object on the heap and simply assigning null to any single reference will not cause the underlying object to magically go away.
No, and no. myObject = null; will only help if there are no other references to the object, and in most cases it's superfluous because local objects go out of scope at the end of each method.
As for when objects are actually deallocated, that's completely up to the GC. What you can do is add a finalize method that will be called just before the object is deallocated, but this is problematic as well and should not be relied on.
I have read about making an object explicitly garbage collected, that in many ways. So i wanted to know some of the methods of making it explicitly garbage collected
There is no way for explicit garbage collection.
You can "politely ask" the virtual machine to do garbage collection by calling:
System.gc();
but it is not guaranteed it will.
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.
and "best effort" might be to postpone the garbage collection.
For how to make objects elligible for garbage collection read Effective Java, Chapter 2
You can explicitly make an object eligible for garbage collection by setting all references to it to null.
This won't call the garbage collector itself, but when the collection does start, it will collect this object.
You're best off letting the Java garbage collector doing it all automatically by itself. It has been optimized to the point where it'll be better and more efficient than anything you'd ever want to probably do.
System.gc is to tell the JVM to execute the garbage collector explicitly.
But as for as some methods which i know to make an object explicitly garbage collected are:
Assigning its reference to point to null
EX: Animal a = new Animal();
a = null
Assigning its reference to point to another object.
EX: Animal a1 = new Animal();
a1 = new Animal();
You shouldn't worry about forcing a GC unless you're writing, say, a tool like VisualVM yourself.
NetBeans, IntelliJ, VisualVM, and a great many others can all force a GC, not hint, but really force. Using JVMTI you can force a GC.
BUT once again you probably do NOT want to do that.
You force a GC (not hint) using JVMTI's ForceGarbageCollection.
That said, you probably really DO NOT want to do that (it really bares repeating).
Authoritative info on the subject if you really want to know (but you probably don't) how to force, for a fact, a GC:
http://java.sun.com/javase/6/docs/platform/jvmti/jvmti.html#ForceGarbageCollection