Providing sessionbeans through init()

[Mark Mescher]

Hi,
I have a Servlet that calls many methods of stateless sessionbeans. This Servlet is called very often. So it is really expensive for me to get a link to the sessionbean. I think a solution for me is to provide the sessionbeans by the init()-Method. So the sessionbeans are only generated once.
I think of something like this:

  
public class MyServlet extends HttpServlet{
   private SessionBeanInterface mySessionBean;
 
   public void doGet(...){
      ...
      mySessionBean.doSomething();
      ...
   }
 
   public void init(){
       //generate sessionbean here...so look for homeinterface and than do: 
       ...
       mySessionBean=home.create();
       ...
   }
 
}

Is this a good solution? Are there any problems in a concurrent situation?

Thx for your advice!!
Bye
Mark

[Leandro Melo]

Originally posted by Mark Mescher:

Is this a good solution?

Not really. This is typical situation where you should go after Design Patterns. What you're looking for is described by the Service Locator pattern.
Take a look

here

And if you don't know about j2ee patterns (or maybe just GOF patterns). Don't think it's a waste of time studying them. It's actually the opposite (I mean, don't studying them is a waste of time

[Mark Mescher]

Hi,
the Service Locator Pattern seems to be a good solution for me. One other question: I would initialize the Singleton in the init()-Method of the Servlet. If I deploy my different Servlets in Tomcat, it is possible that each Servlet has its own Instance of the singleton. Do you think this is a problem?
I have read something of workarounds by using a defined classloader. What do you think of this?

Bye

Mark

[Leandro Melo]

Originally posted by Mark Mescher:
If I deploy my different Servlets in Tomcat, it is possible that each Servlet has its own Instance of the singleton. Do you think this is a problem?

By definition, the Singleton has only one instance. So, if you create a Singleton in a correct manner it will never have more than one instance. Therefore, it's impossible for servlets to have their own instance of Singleton.

Also, you don't need to worry about the initialization of a Singleton. Because the Singleton itself is responsible for it's initialization. So, just call the methods you need and if the Singleton has not been initialized yet, it'll do so.

Example. When you call the getInstance() method of a Singleton, you'll have code like this:

public static SingletonServiceLocator getInstance(){
  if (instance == null)
    instance = new SingletonServiceLocator(); //Remember the constructor is private
  return instance;
}

[Mark Mescher]

Thx for your advice.
I have googled for singletions in combination with Servlets. I found that some Servlet-Engines use another Classloader for each servlet. Therefore every Servlet would have a own instance of the Singleton. A solution for this would be to use an own classloader (I have found some advices to this problem). I will test this today and tell you how Tomcat is doing this things...
Bye

Mark

[Leandro Melo]

Mark,
maybe you haven't gotten the point yet. The Singleton should NOT be a servlet. The Singleton is POJO (plain old java class). You should just call it's methods from the Servlets. As I said, just build a real Singleton and that's guaranteed it'll not have more than one instance.
So, I don't get why are you googling for this "singleton in combination with servlets". There's no need for this.
Also, don't forget that you will always have only one instance of each servlet (if this helps you in anyway).
What are you trying to do exactly?
[ August 11, 2005: Message edited by: Leandro Melo ]

[Mark Mescher]

Hi,
well I know that there is normally only one Instance of a Singleton. But I want to call methods of the Singleton through different Servlets! And if different Servlets (which are not initialized by me but by Tomcat) want to call the same Singleton, the Servlets must be instantiated by the same classloader (as I understand). And that is not so in every Servlet-Engine, I dont know how Tomcat does this.
Hopefully I'm wrong:-) That would make things a little bit easier
At the end I want to call methods of stateless session Beans (which hide my Business Logic). Till now I generate the home-Interface through JNDI everytime I enter the Servlet. I want to optimize this by the Singleton. It would be nice if a Singleton in a many-Servlets-Tomcat / Jboss - Webapp would be really single:-)
Let me do some tests and than we will see:-) I will post my results here!
[ August 11, 2005: Message edited by: Mark Mescher ]

[Leandro Melo]

Mark, you should re-read the ServiceLocator and Singleton Design Patterns.
You're making a huge confusion with a very very simple thing.

Originally posted by Mark Mescher:

well I know that there is normally only one Instance of a Singleton. But I want to call methods of the Singleton through different Servlets!

That's the point!!! You can call the methods of the Singleton from different places (from servletA, servletB, servletC, a jsp, a enterprise java bean or a pojo). As long as they're in the same java virtual machine you'll ALWAYS get the same instance of the Singleton.

Suppose you're in ServletA (you'd have code like this).

ServiceLocatorSingleton locator = ServiceLocatorSingleton().getInstance() // note this is a static method!!!
MyObject o = locator.findMeSomething("name");

Suppose you're now in ServletB:

ServiceLocatorSingleton locator = ServiceLocatorSingleton().getInstance() // note this is a static method!!!
MyObject o = locator.findMeSomething("name");

Suppose you're now in ServletC:

ServiceLocatorSingleton locator = ServiceLocatorSingleton().getInstance() // note this is a static method!!!
MyObject o = locator.findMeSomething("name");

Can you see now? The code is the same. When you call the getInstance() method of the singleton you'll be always returned the same instance!
If you call the getInstance() method on the singleton from any part of your application (a servlet, a jsp, a enterprise bean, a java bean or a pojo) you'll will ALWAYS be returned the only instance of the Singleton (as long as they're running on the same java virutal machine, which is propably your case).

Originally posted by Mark Mescher:
And if It would be nice if a Singleton in a many-Servlets-Tomcat / Jboss - Webapp would be really single:-)

It is really single!!! Well, as I said, I think you should re-read these Design Patterns, because from your comments one can tell you that you did not fully understand them.

[ August 11, 2005: Message edited by: Leandro Melo ]
[ August 11, 2005: Message edited by: Leandro Melo ]

[Leandro Melo]

Ok, I'll give you an extra help here.
I'm sending you the code of a fully working ServiceLocatorSingleton. It works perfectly on jboss 3.2.3 (for the default structure).
Check out the static method getInstance(), this is them method you should call to get the instance from any servlet.
Just insert this class in your package structure and it's ready to use (in the way I described on the previous post).

 
package your.package;
 
import java.util.Collections;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Map;
 
import javax.ejb.EJBHome;
import javax.ejb.EJBLocalHome;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.sql.DataSource;
 
/**
 * @author Leandro Terra C. Melo
 * 
 * This class is also a ServiceLocator, a Singleton and a CachingFactory, it provides functionality
 * to find resources and caches them in a Map.
 *
 */
public class ServiceLocator {
 
/*=================*/
/*   Class data    */
/*=================*/
private static ServiceLocator instance = null;
private Context initialContext;
private Map ejbHomes;
private DataSource jdbcDataSource = null;

/*========================*/
/*    Constructor(s)      */
/*========================*/
/**
 * 
 * Private Constructor for the class.
 * 
 */
private ServiceLocator() throws NamingException {
  
  initialContext = this.getInitialContext();
  ejbHomes = Collections.synchronizedMap(new HashMap());
  
}
 

/*=====================================================*/
/*   Accessor for the Class (as it's a Singleton)      */
/*=====================================================*/
/**
 * 
 * All instances of this class must be obtained trough this method.
 * 
 * @return ServiceLocator - A Factory for Home interfaces.
 * 
 */
public static ServiceLocator getInstance() throws NamingException {

  if(instance == null) {  
instance = new ServiceLocator();
  }
  
  return instance;
}
 

/*========================*/
/*    Lookup Methods      */
/*========================*/
/**
 * 
 * Gets the initial context
 * 
 * @return InitialContext
 * 
 * @throws NamingException
 * 
 */
private InitialContext getInitialContext() throws NamingException{

Hashtable props = new Hashtable();
 
props.put(InitialContext.INITIAL_CONTEXT_FACTORY, "org.jnp.interfaces.NamingContextFactory");
props.put(InitialContext.PROVIDER_URL, "jnp://127.0.0.1:1099");
 
InitialContext initialContext = new InitialContext(props);

return initialContext;
}


/**
 * Looks up for a local interface in the COMP enviroment.
 * 
 * @param ejbReferenceComponent - Name of the EJB. 
 * 
 * @return EJBLocalHome - LocalHome interface for the EJB.
 * 
 */
public EJBLocalHome lookupByCOMPLocalEJBReference(String ejbReferenceComponent) throws NamingException {
 
  java.lang.Object home = ejbHomes.get(ejbReferenceComponent);
  
  if(home == null) {  
home = initialContext.lookup("java:comp/env/ejb/" + ejbReferenceComponent);
ejbHomes.put(ejbReferenceComponent, home);
  }
  
  return (EJBLocalHome) home;
  
}


/**
 * Looks up for a local interface in the JNDI service.
 * 
 * @param ejbReferenceComponent - Name of the EJB. 
 * 
 * @return EJBLocalHome - LocalHome interface for the EJB.
 * 
 */
public EJBLocalHome lookupByJNDILocalEJBReference(String ejbReferenceComponent) throws NamingException {
 
  java.lang.Object home = ejbHomes.get(ejbReferenceComponent);
  
  if(home == null) {  
home = initialContext.lookup(ejbReferenceComponent);
ejbHomes.put(ejbReferenceComponent, home);
  }
  
  return (EJBLocalHome) home;
  
}

 
/**
 * 
 * Looks up for a remote reference in the JNDI service.
 * 
 * @param ejbReferenceComponent - Name of the EJB. 
 * @param homeClass - EJB's home class.
 * 
 * @return EJBLocalHome - LocalHome interface for the EJB.
 * 
 */
public EJBHome lookupByJNDIRemoteEJBReference(String ejbReferenceComponent, 
Class homeClass) throws NamingException {
 
java.lang.Object home = ejbHomes.get(ejbReferenceComponent);

  if(home == null) {  
  //Lookup by COMP_NAME.
//java.lang.Object obj = initialContext.lookup("java:comp/env/ejb/" + ejbReferenceComponent);

//Lookup by JNDI_NAME.
Object obj = initialContext.lookup(ejbReferenceComponent);
home = javax.rmi.PortableRemoteObject.narrow(obj, homeClass);
ejbHomes.put(ejbReferenceComponent, home);
}

  return (EJBHome) home;
}


/**
 * Looks up for a specific DataSource.
 * 
 * @param String dbURL - The data source url.
 * 
 * @return DataSource - The data source.
 * 
 * @throws NamingException
 */
public DataSource lookupDataSourceByURL(String dbURL) throws NamingException{

//Retrieves the data source.
if (jdbcDataSource == null){
jdbcDataSource = (DataSource)initialContext.lookup(dbURL);
}
return jdbcDataSource; 
} 
 
}

[Mark Mescher]

Hi Leandro,
now I had the time to test the Singleton Pattern. It works really fine. Thx for your detailed help and the example code.
I have worked before with Singletons in "normal" Apps so i knew the concept of Singletons.
The only problem I had was that I have read of some issues when using Singletons in Servlets in different Servlet-Engines. Look here for detailed informations:
http://www.churchillobjects.com/c/11043f.html
But not important yet...it works fine with Tomcat so I will change my application by using the Singleton-Pattern.
Thx a lot Leandro!!!

Bye

Mark
[ August 12, 2005: Message edited by: Mark Mescher ]