Is there a bean implementation that is like a "Stateful Singleton Bean"?

[madchedar0]

I'm migrating code to JBoss 7.1.1... and I have some services which used to be singleton services.  Basically there is just one instance of the service floating around in the Application Server and it is holding/updating a state that other parts of the system might ask for.  Let's call it "ObjectManager"

I would like to have the ObjectManager be created at startup of the Application Server.  It should be a singleton.  It should be stateful.

@Singletons are by definition Stateless so that's out.

@Stateful is good but you can't specify that the ObjectManager be created on start up can you?

Are there any acceptable solutions developers use to have a "Stateful Singleton" in their system?

Here's one idea I had:

Maybe you create an "ObjectManagerInitializer" bean that is a singleton... and all it does is create one ObjectManager bean?  Is that a good approach?

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.EJB;
import javax.ejb.Singleton;
import javax.ejb.Startup;
import org.apache.log4j.Logger;

@Singleton
@Startup
public class ObjectManagerInitializer {
    
    @EJB
    ObjectManagerLocal dpm; // local view of the ObjectManager bean
    
    public ObjectManagerInitializer(){} 
}

And ObjectManager would look something like the following:

@Stateful
@Local(ObjectManagerLocal.class)
@Remote(ObjectManagerRemote.class)
public class ObjectManager
        implements MessageListener, Serializable, DisplayPersistenceManagerLocal,
        DisplayPersistenceManagerRemote {

   // code

}

What do you guys think?  Is there a better way to do this?

Thanks

[wdfink]

Why you think a Singleton is stateless?

A singleton is a singleton, you might have a state and concurency can be managed by the container or by bean, you can annotate the whole bean or methods.

One other thing is whether you will have a Singleton in a cluster, if you want that have a look to this quickstart

[madchedar0]

Ah good point.  In the definition I missed this part:

(From http://docs.oracle.com/javaee/6/tutorial/doc/gipjg.html#gipmt)

"Singleton session beans offer similar functionality to stateless session beans but differ from them in that there is only one singleton session bean per application, as opposed to a pool of stateless session beans, any of which may respond to a client request. Like stateless session beans, singleton session beans can implement web service endpoints.

Singleton session beans maintain their state between client invocations but are not required to maintain their state across server crashes or shutdowns."

So I guess I can use a @Singleton after all...  Thanks for the quickstart link too - I'm sure that will come in handy.

[wdfink]

No problem your are welcome

[sgilda]

There is a simpler singleton quickstart example here: https://github.com/jbossas/quickstart/tree/master/helloworld-singleton

Wolf's deals with clustering, so you might want to start wit hthe simpler version and then move on to his. :-)