ManagedBean and PropertyChangeListener

Question

I have two ManagedBeans (@SessionScoped and @ViewScoped):

@ManagedBean(name="sessionController")
@SessionScoped
public class SessionController implements Serializable{    
    private PropertyChangeSupport propertyChangeSupport = new PropertyChangeSupport(this);

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        propertyChangeSupport.addPropertyChangeListener(listener);
    }

    ...
}

@ManagedBean(name="viewController")
@ViewScoped
public class ViewController implements Serializable, PropertyChangeListener{  
    @ManagedProperty(value="#{sessionController}")
    private SessionController sessionController ;  
    ...

    @PostConstruct
    public void init() {
        sessionController.addPropertyChangeListener(this);
    }


    @Override
        public void propertyChange(PropertyChangeEvent evt) {
            ...
        }

    }

Can I use PropertyChangeListener in ViewController to know about changes in SessionController? Will there be any problems?

Answer 1

No you can't. To add PropertyChangeSupport to the @SessionScoped bean, you need to call

   propertyChangeSupport.addPropertyChangeListener("instanceofViewScopedBean")     

at a convenient spot (preferably the @PostConstructor). Note that you require an actual instance of the target bean (implementing PropertyChangeListener) to pass to the addPropertyChangeListener. As far as I can tell, You cannot get this from within the session bean, especially at bean initialization. Why? well it's viewscoped, it only lives as long as a page is being viewed.

Related to this restriction is the JSF policy that Managed Beans can only be injected into other beans of a narrower scope (using your case, only the session bean can be injected into the viewscoped bean). What you're looking to do amounts to pretty much the opposite, tying a viewscoped bean to a session scoped variable.

I presume you're trying to achieve a low-cost server-side push mechanism. Well, I don't know of another means to get this done apart from good ol' polling or comet pushing.