Weak, soft and Phantom references

Question

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.

Answer 1

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.