Can someone write about a scenario where you cannot do without Stateful beans

[Mustafa Garhi]

After reading a lot about Session Beans my concepts have gone for a toss.

Please provide a simple scenario where your application would need Stateful for sure.

So if my code is :

class MyEJBClient {

// part 1
MyStateLessEJB statelessEjb = getStatelessEJBInstance();
statelessEjb.setCount(10);
statelessEjb.getCount(); // Should fetch me 10

// part 2
MyStateFullEJB statefullEjb = getStatefullEJBInstance();
statefullEjb.setCount(10);
MyStateFullEJB newStatefullEjb = getStatefullEJBInstance();
newStatefullEjb .getCount(); // Should fetch me 0

}


So in part 1 i have two calls to a stateless ejb and i still get the correct state, and
in part 2 i have two calls to two different stateful ejbs and i do not get the current/correct state of the bean using stateful.

So what am i achieving using stateful EJBs?

[Jeanne Boyarsky]

Mustafa,
In example one, it only works because you happen to be getting the same bean without anyone calling it in between. What if you had multiple callers and they called setCount() before you call getCount(). The idea of stateless session beans is to not store state because it may not be there when you get back.

In example two, what does getStatefullEJBInstance() do? Depending on the logic, you may not be getting the same stateful bean.

In practice, I haven't needed stateful beans for anything. I have an HttpSession to store state in so I have no need to store it in the EJB layer.

[Mustafa Garhi]

Ok so JB,

How do you get around 'multiple setters for my count problem' using stateful without using HttpSession etc.

And what do you mean by getting back to my bean - Does it mean trying to use the same bean in some other servlet say?

I am so messed up with the difference between stateful and stateless that i guess i am not even asking the right questions.

Please bear with me

[Jeanne Boyarsky]

Mustafa Garhi wrote:How do you get around 'multiple setters for my count problem' using stateful without using HttpSession etc.

Store a reference to the bean in the caller. I don't know how to say without using the HttpSession. I wouldn't use a stateless bean if I had an HttpSession.

Mustafa Garhi wrote:
And what do you mean by getting back to my bean - Does it mean trying to use the same bean in some other servlet say?

Yes. Suppose you only have one bean. Caller A sets it to 5. Caller B sets it o 7. Caller A gets the bean. It is now 7. When caller A got the same bean back, the value had changed out from under it.

[Mustafa Garhi]

JB Considering the code again :

// part 1
MyStateLessEJB statelessEjb = getStatelessEJBInstance();
statelessEjb.setCount(10); // line 1
statelessEjb.getCount(); // line 2

Also assume there is no one else using this stateless.

Q. Is it possible that statelessEjb at line 1 and statelessEjb at line 2 are two different beans if i have a pool of say 10 for this stateless?

[Jeanne Boyarsky]

Mustafa Garhi wrote:JB Considering the code again :

// part 1
MyStateLessEJB statelessEjb = getStatelessEJBInstance();
statelessEjb.setCount(10); // line 1
statelessEjb.getCount(); // line 2

Also assume there is no one else using this stateless.

Q. Is it possible that statelessEjb at line 1 and statelessEjb at line 2 are two different beans if i have a pool of say 10 for this stateless?

I don't think so. Once you have a reference to the bean, it shoull be the same object. Unless your app server is doing load balancing behind the scenes.

[Vishwanath Krishnamurthi]

Hi Mustafa,

I thought I'd explain the concepts using a shopping cart example...

Say UserA, UserB and UserC are accessing a shopping website and adding items to the cart...

Method One: Using HttpSesison Only

Consider we have a plain Java class called ShoppingCart

If the sessions are maintained using HttpSession, for each user we would create a ShoppingCart object, add items to it, and store it as a session attribute.

Now note that the ShoppingCart object is in the web-tier.


Method Two: Using Stateful EJB and HttpSession

Consider the ShoppingCart class here is a Stateful EJB

For each user the ShoppingCart (stateful EJB) is created (so the object now is in the Business Tier) and there is a HttpSession attribute holding a reference to this object.

So when userA adds an item, the ShoppingCart reference is taken from the session and the item is added...
likewise for UserB

Wrong Method: Using Stateless EJB and HttpSession

Consider ShoppingCart to be a Stateless EJB here..

And the rest of the implementation is as above...
Though the reference in the HttpSession points to the same object every time, there is no guarantee that, that object was specific to one user. UserA, userB and UserC may all given the same object ! And hence the data is corrupted.

Why would one need a Stateful EJB at all.. ? It would look like MethodOne (using just HttpSession alone) suits most situations..

Yes. But Stateful EJBs help when the user has logged in using a different channel say WAP phone etc, and wants to know the state (Think of a railway ticket transaction scenario) i.e to maintain channel independent session

Also Stateful EJBs provides transaction service, security service, RMI, etc...

so when these features are required, Stateful EJBs would be very useful !

HTH,
Vishwa

[Mustafa Garhi]

Vishwa you mentioned "UserA, userB and UserC may all be given the same object"

Now let me write some code that helps me ask a doubt from that statement :

Servlet I

doPost() {
@EJB
ShoppingCart myStateless;
// some code
myStateless.addToCart(item);
myStateless.incrementItemCount();
// some more code
session.setAttribute("myStateless",myStateless);
}

Servlet II (that executes after Servlet I)

doPost() {
ShoppingCart myStateless = session.getAttribute("myStateless");
myStateless.addToCart(item);
myStateless.incrementItemCount();
// more code
session.setAttribute("myStateless",myStateless);
}

Considering userA is executing all the above, so for userB to be getting the same object,
the object should have gone back to the pool right? But it would not because we never let it go, using the session.
So how could the situation you mentioned occur bro?

[Vishwanath Krishnamurthi]

Hi bro,

Mustafa Garhi wrote: But it would not because we never let it go, using the session.

No! Holding the reference for an Stateless bean object in Session-scope does not prevent the bean from returning to the pool...

A Stateless Session Bean is returned to pool when a single method invocation gets over....

Or say, stateless session beans don't remember the 'state' beyond a method invocation

So considering this part:

UserA thread:
----------------------

doPost() { 
@EJB 
ShoppingCart myStateless;  // lets call the object that myStateless references as ObjectX
// some code 
myStateless.addToCart(item); // You are changing an instance variable of ObjectX. 

// now with this method runs and completes, the ObjectX goes to the pool

//Now UserB thread may start  (read UserB thread code)

myStateless.incrementItemCount(); 
// some more code 
session.setAttribute("myStateless",myStateless); 
}  

UserB thread:
--------------------

doPost() { 
@EJB 
ShoppingCart myStateless;  // now again this might get ObjectX

// some code 
myStateless.addToCart(item); // UserB is changing that instance variable here !!!! 

// the rest of the code
}  

HTH,
Vishwa

[Mustafa Garhi]

Vishwa,

Thanks a ton man.

One more doubt though to strengthen the concepts. Consider :

MyStateLessEJB statelessEjb = getStatelessEJBInstance();
statelessEjb.setCount(10); // line 1
statelessEjb.getCount(); // line 2

Is it possible that statelessEjb at 1 and statelessEjb at 2 are two different beans from the pool?

I ask this as you mentioned the state is not remembered across method calls right?

[Ralph Jaus]

Hi Mustafa,

what do you mean by the following statement ?

MyStateLessEJB statelessEjb = getStatelessEJBInstance();

Usually you don't get direct access to an ejb instance because that's handled by the ejb container. If you use something like

@EJB
ShoppingCart myStateless;

then myStateless will reference a proxy object and a method call like myStateless.addToCart(item); will result in a communication between the proxy object and the ejb container. Then the container can choose any instance of the stateless bean from the pool that finally performs addToCart method.

In contrast, if ShoppingCart would be a stateful session bean then the ejb container would use the same bean instance for each method invoked on myStateless.

Is it possible that statelessEjb at 1 and statelessEjb at 2 are two different beans from the pool?

It's possible that the methods calls in lines 1, 2 are performed by different bean instances from the pool.

[Mustafa Garhi]

RJ,

Thanks man. I think i have understood things to a good extent after that handle/proxy related explainations.

I just loved the way all of you have answered this thread .. Thank you all !!

I might have more doubts as i keep exploring EJBs so not marking this one resolved

[Jeanne Boyarsky]

Mustafa Garhi wrote:I might have more doubts as i keep exploring EJBs so not marking this one resolved

Unless those doubts are specifically about writing a scenario where you cannot do without stateful beans, please start a new thread.

[Mustafa Garhi]

Yes bro absolutely.