How strong ,soft ,weak ,phantom references are used in garbage collection

Hi
to all here.I have doubt that how garbage collection is deciding to cleaning up heap , and what are the roles of strong , soft , weak and phantom reference in garbage collection, i went throgh sun's java docs but i couldn't get any clear idea about those , please can anyone explain me with nice examples for which i will be really thankful.

See:
http://java.sun.com/developer/technicalArticles/ALT/RefObj/

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IterateOverReachableObjects and soft/weak/phantom references

Hi,
what are soft/weak/phantom references for the JVM TI call IterateOverReachableObjects? Are those reference-objects
1. ... "normal" objects having references and beeing traversed by this function
(OR)
2. ... interpreted as references with hiding the object nature
(OR)
3. ... ignored as they are ignored by the GC?
Thanks,
Robert

The reference_kind with JVMTI_REFERENCE_FIELD, the referrer_tag will be the value of the tagged Reference instance, and the referrer_index will be the index of the "referent" field. It is this field that is treated in a special way by the GC.

Soft, weak & phantom reference

Hi all :)
A short while ago, I began to study "java.lang.ref" package. It's not so easy to me. Also I've gotten many relative questions & answers from this forum, here is a problem confused me for so long time :( Please help me out, thanks!
import java.lang.ref.*;
import java.util.Hashtable;
public class TestMe {
    private Hashtable ht = new Hashtable();
    private ReferenceQueue rq = new ReferenceQueue();
    public TestMe() { }
    private void clearQueue() {
        SoftReference r;
        while ((r = (SoftReference)rq.poll()) != null) {
            Object obj = r.get();
            if (obj != null) {
                //* Here is the problem: why this always doesn't work?
                // using weak or phantom reference is same like soft reference here :(
                System.out.println("=> " + obj);
            ht.remove(r);
    public void put(Object key, Object obj) {
        clearQueue();
        ht.put(new SoftReference(key, rq), obj);
    public static void main(String[] args) {
        TestMe tm = new TestMe();
        int i=0;
        while(true) {
            tm.put(Integer.toString(i++), new String("wow"));

The reference object is put into the reference queue after all SoftReferences to the reference object have been cleared. Since you're trying to get the reference pointed to by the SoftReference after it's been queued, it is null.
If you want to store some information to use after the reference object has been queued, you need to subclass the SoftReference class and store the information in a field.

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Turn off push notifications every chance you get.
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Turn off your internet connection when you do not need it. The easiest way to do this is to swipe up from the bottom of you screen to get the control centre, and then touch the airplane to turn on airplane mode. You can repeat this sequence to turn it back on again when you need it. Most especially turn airplane mode on whenever you are sleeping your iPad for long periods. This will save battery life too. OR actually turn your iPad off - which means holding the power key down for several seconds until the red swipe bar appears, and then swipe to turn it off. If you go this route, note that it will take longer to turn on then it takes to wake from sleep.

How to find out which users are using SharePoint Designer to make changes in SharePoint 2010 site?

Hi there,
How to find out which users are using SharePoint Designer to make changes in SharePoint 2010 site?
Thanks.

You need to enable auditing on SharePoint server and it will let you know, if someone makes any critical changes for the same.
Please walk through this informative KB to enable auditing on SharePoint :https://support.office.com/en-za/article/Configure-audit-settings-for-a-site-collection-f5a346d0-ee0f-4412-a5e6-d9b5abaa1012
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Photoshop CS4- Does anyone know how to tell if two people are using same file?

Hi,
Thanks for your response. I'm not the solution in the link.
The problem that I am having at work it that several people in our department want to be able to update a file, but we want to be sure that no one else is currently editing that file. If someone has the file open, it's helpful to know the user's name so we can talk about when the next person can edit the file. This works in Excel files that we use, and I would love to find out how to do it in Photoshop and Illustrator.
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How to find that 2 references are pointing to same obejct??

how to find that 2 references are pointing to same obejct??
i feel that hashCode and equals will not work as far as i understood from javadoc

I wnat to know above because i want to check if ps in
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for (int i = 0; i < 10 ;i ++)
PreparedStatement ps = con.prepareStatement(sql);
ps.setInt(1,i);
ps.executeUpdate();}
Here's a better question. Why in the world did you put that statement in the loop in the first place, unless the variable 'sql' is modified within?

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Weak, soft and Phantom references

I have got some confusions about reference types in java. These are:-
1. How we create weak and soft references! Are they predefined classes and we extend them, or they are interfaces and we implement them, or they are the concepts we implement by following any particular pattern.
or the references which are gone out of scope are considered as weak references.
2. can we change the type of reference dynamically! as JVM does it.
for eg:-
While the data objects are actually in use (or they are in scope) they are immune because they are strongly referenced by the display windows, once you move to look at something else (or they go out of scope) they become eligable for removal.
3. There is one set of problem I am facing, could that be resolved by weak or soft references.
"I set variables at session level(if they are needed at more than one page."
"At the last page I remove it from session."
"If for example there are 4 page sequence, and user quits after three steps, then the garbage variables are left in the session."
"if I make them as weak reference(or soft reference) then would it be a solution. or will it increase any potential risk of loosing the variables in between the process."
"or is there any way out for my problem using ref package, or it is used only for special purposes, not for a senario like this."
"or I am bound to use request level instead of session level, in these type of problems(which I am presently doing to solve this problem, but in this case I have to to set same attribute 3 times.)"
please comment.
4. What happens when we create an instance of reference queue.
-->does it give us a reference to the main queue that is used by garbage collector. when destroying objects out of scope.
that is garbage collector removes the reference of from the object and makes it point to queue element.
--> or when object goes out of scope, at that time only the reference is removed from the object and is made to point to queue element.
--> and when we do obj=null; at that time also obj reference is made to point to queue element.
please comment. I am confused about how java handles references.
5. does reference queue takes all the junked references.
if yes!
then for eg. we are using a background thread and doing some work when instance of class A is cleared, and doing some other work when instance of class B is cleared up by garbage collector.
queue.remove will return the junked reference, now how will we be determining which object is junked, instance of class A or class B. As reference of object is junked we can't use instanceOf operator.
please comment.
Thanks.

I find the documentation a bit opaque myself.
You create, say, a Weak reference by creating an object of class WeakReference, or a subclass of same. The object referenced (the referent) is a separate object which you pass in the WeakReference's constructor.
As long as the referent exist then calling get on the WeakReference object will return it. Once you call get and put the resulting reference in an object you have a hard reference and you know that the referent won't disappear until you let go of it.
If the referent has been disposed of then get returns null.
Sometimes you want to do extra cleaning up when a reference is cleared. Then you need to subclass the Reference class you are using to add indentifying information about the object that just disappeared. Obviously this can't be a reference to the object itself, that would be a "Hard" reference and would prevent the object being cleared.
You then create a ReferenceQueue, which is an ordinary object you own. The Reference objects have a reference to this queue. What happens is the the GC, when it clears one of these Reference objects, adds the Reference to the queue it references. You run a background thread which takes these cleared references from the queue and, using the extra information you added when you subclassed them, does whatever cleaning up you need to do. A typical example is that you have a Map, with the Reference objects as values. Each reference is extended to store the key, so that the cleanup will remove cleared references from the map.
No, it's not suitable for session objects, because you can't depend on the object still being arround. You can only rely on session timeout. It's a basic problem with web services that you never really know if the user has gone away and abandoned you.

How to know what ActiveX references are still open

Okay, so I'm about the millionth person to post about this topic, but I feel like I know what I should be doing, but have hit a wall. I have read through pretty much everything I could find in the forums about references and ActiveX.
I am communicating with Excel via ActiveX, and everything works seemlessly, until I realized that if the user closes the Excel sheet (via an "exit" button on my FP), the "EXCEL.exe" process is still running. I've read enough to know that this probably points to a reference that I have not closed.
Problem is, I have poured over my code, and as far as I can tell, I have closed every reference, in the opposite order it was opened (as I read to do in a post earlier).
Aside from casting references to int's to see what is still considered "active" (as I mentioned earlier, all the ones I can find are closed, so this doesn't help me), is there a way to determine which ActiveX references are still open?
Even a way outside of LV? A Microsoft utility even or ...well anything! This is very annoying, and I know it should be quick and easy, I just have to know what is remaining open.
Or, am I missing something else that might be keeping EXCEL.exe running? (yes, I have an application quit and close the application reference)
Thanks for any help

Hi Will,
As you said it yourself closing the same number of references as you open and using Application Quit should be enough to close down Excel.exe.
Did you run your code in highlight execution to see how the behavior of your program changes when you hit the "Exit" button on your Front Panel?
If you can post the minimum amount of code that reproduces the error, I will be more than happy to take a look at it.
Best regards,
Message Edited by Kalin T on 12-07-2006 03:11 PM
Kalin T.
National Instruments
Attachments:
activex_excel.jpg ‏16 KB

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thanks!

me too, my file has 39 pages with many unuse symbols, now I want to delete those unuse.
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I cant find out from the where used list from function modules?
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How to find whether user exits are used in the query or not??

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Thanks
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Hi Maruthi,
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Best Wishes,
Mayank

How strong ,soft ,weak ,phantom references are used in garbage collection

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