Spring session bean shared between HttpSession-s

Question

I'm trying to undestand session beans in Spring. The book I'm reading says, about them, that:

the bean is created when needed and stored in the javax.servlet.http.HttpSession. When the session is destoyed so is the bean instance.

I tried with the following example:

The bean:

package com.at.test.web;

public class Cart {
    public static int dummy = 0;
    public Cart() {
        System.out.println("Cart::<init> with hashCode " + hashCode());
    }
}

The bean definition:

<beans:bean id="cartBean" class="com.at.test.web.Cart" scope="session">
    <apo:scoped-proxy/>
</beans:bean>

The controller:

@Controller
public class HomeController {
    @Autowired 
    private Cart cart;

    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String home(HttpSession session, Model model) {
      System.out.println("Cart is: " + cart.hashCode() 
          + " ; dummy = " + (cart.dummy++) 
          + " (" + cart.getClass().getCanonicalName() + ")" 
          + "; session is: " + session.hashCode());
      return "home.jsp";
  }
}

This is what happens on Tomcat startup:

Cart::<init> with hashCode 970109301

I think Spring need this instance in order to create the CGLIB proxies. Anyway, I'm not that sure.

After the startup, I use two different browsers to have two different HttpSession. The result, when the controller is invoked, is:

Cart is: 578093288 ; dummy = 0 (com.at.test.web.Cart$$EnhancerByCGLIB$$2eb8334f); session is: 1013723725
Cart is: 578093288 ; dummy = 1 (com.at.test.web.Cart$$EnhancerByCGLIB$$2eb8334f); session is: 1060682497

The cart instance seems to be shared between HttpSession-s, but I was expecting two instances of Cart.

The same if I let the bean implements an interface, use annotation-driven approch and component scan:

public interface ICart {}

--

@Component
@Scope(value="session", proxyMode=ScopedProxyMode.INTERFACES)
public class Cart implements ICart {
    public static int dummy = 0;
    public Cart() {
        System.out.println("Cart::<init> with hashCode " + hashCode());
    }
}

Am I missing something? Did I misunderstood the meaning of session bean?

Answer 1

There's a lot of proxying and delegation going on.

This field

@Autowired 
private Cart cart;

where Cart is session scoped will be proxied as you can see in your logs

com.at.test.web.Cart$$EnhancerByCGLIB$$2eb8334f

But this proxy is not wrapping a Cart object. It is wrapping a SimpleBeanTargetSource which takes care of getting a Cart bean from the BeanFactory.

Your bean definition attaches a SessionScope object which is responsible for checking your HttpSession through a static ThreadLocal field in RequestContextHolder. When request is made to get a bean from the BeanFactory, it will delegate the action to the SessionScope object which will check the HttpSession, use the one there if it exists or create and register a new one if it doesn't.

You don't notice this with your test because

cart.hashCode()

delegates to the SimpleBeanTargetSource object (incorrectly, if you ask me). But if you did

cart.toString();

you would see the difference as that actually reaches the underlying Cart object.

---

Depending on the scope and proxying strategy you use, all this may be different, but the end goal is still achieved.

Answer 2

public class HomeController {
  @Autowired 
  private Cart cart; <-- Proxy

The Cart instance that gets injected into the HomeController instance is only a proxy that delegates method calls to the "real" instance. The Cart class itself has no own method or state yet, so you won't notice any difference between sessions, of course.