Run-time lookup for EJBs in MDB consuming immediately after deploy

Question

I have Java EE 5 project using JBoss 5.1 and problem like this. I have to do kind of run-time lookup for some EJBs in MDBs using a string that results from message content. It's just kind of service locator pattern used in MDBs. Now, since MDBs start consuming just after deploy, I have a lot NameNotFoundException since implicit deployment order doesn't work well here (run-time lookup). What do you think about it? Is it possible to do it really well using EJB 3.0? It's also acceptable for me to use any vendor-specific stuff (JBoss 5.1) if it resolves the problem.

Some code snippet to visualize the situation:

@MessageDriven(mappedName="jms/Queue")
public class MessageBean implements MessageListener {

    @Resource
    private MessageDrivenContext mdc;

    public void onMessage(Message msg) {

        final String beanName = // extract somehow the bean's name from 'msg'
        final Context ctx = new InitialContext();
        final Object obj = ctx.lookup(beanName); // NameNotFoundException
        // do something with 'obj'
    }
}

Answer 1

Use one of these four different approaches.

1. Declare EJB dependencies (EJB references) using "@EJB" annotation (don't use JNDI lookup). For entity bean references, must refer to the entity bean home interface. Container must ensure all dependencies are injected before methods/message-listeners are processed:

   MessageDriven(mappedName="jms/Queue")
   public class MessageBean implements MessageListener {

   @EJB private EntityBeanHomeA entityBeanHomeA;    
   
   @EJB private EntityBeanHomeB entityBeanHomeB;    
   
   @EJB private EntityBeanHomeC entityBeanHomeC;    
   
   @EJB private SessionBeanD sessionBeanD;    
   
   @Resource
   private MessageDrivenContext mdc;
   
   public void onMessage(Message msg) {
   
       final String beanName = // extract somehow the bean's name from 'msg'
       final Object obj = getDependentEJB(beanName);
       // do something with 'obj'
   }
   
   private Object getDependentEJB(String beanName) {
       Object result = null;
       if ("EntityBeanHomeA".equals(beanName)) {
            result = entityBeanHomeA;
       else if ("EntityBeanHomeB".equals(beanName)) {
            result = entityBeanHomeB;
       else ("EntityBeanHomeC".equals(beanName)) {
            result = entityBeanHomeC;
       else ("SessionBeanD".equals(beanName)) {
            result = sessionBeanD;
       }
       return result;
   }
   

   }

2. Use JNDI lookup, but declare EJB dependencies via EJB deployment descriptors. Again, the container must ensure ensure all dependencies are setup before methods/messages are processed:

   @MessageDriven(mappedName="jms/Queue") public class MessageBean implements MessageListener {

   // as given in the original Question...
   

   }

   Deployment descriptor:

   <enterprise-beans>
       <message-driven>
           ... 
           <ejb-name>MessageBean</ejb-name>
           <ejb-class>com.company.pkg.MessageBean</ejb-class> 
           <messaging-type>javax.jms.MessageListener</messaging-type>
           <message-destination-type>javax.jms.Queue</message-destination-type>
           <message-destination-link>ExpenseProcessingQueue</message-destination-link>
           <ejb-ref> 
               <description> This is a reference to an EJB 2.1 entity bean
                 that encapsulates access to employee records. 
               </description>
               <ejb-ref-name>ejb/EmplRecord</ejb-ref-name>
               <ejb-ref-type>Entity</ejb-ref-type>
               <home>com.wombat.empl.EmployeeRecordHome</home>
               <remote>com.wombat.empl.EmployeeRecord</remote> 
               <ejb-link>EmployeeRecord</ejb-link> <-- if in same EJB jar -->
                         <-- ../emp/emp.jar#EmployeeRecord   if in diff EJB jar -->
           </ejb-ref>
           <ejb-local-ref> 
               <description> This is a reference to the local business interface
                  of an EJB 3.0 session bean that provides a payroll service. 
               </description> 
               <ejb-ref-name>ejb/Payroll</ejb-ref-name>
               <local>com.aardvark.payroll.Payroll</local> 
               <ejb-link>Payroll</ejb-link> 
           </ejb-local-ref>
           <ejb-local-ref> 
               <description> This is a reference to the local business interface of an
                 EJB 3.0 session bean that provides a pension plan service. 
               </description>
               <ejb-ref-name>ejb/PensionPlan</ejb-ref-name>
               <local>com.wombat.empl.PensionPlan</local> 
               <ejb-link>PensionPlan</ejb-link> <-- if in same EJB jar -->
           </ejb-local-ref> 
           ...
       </message-driven>
       ... 
   </enterprise-beans>
   

3. Use JNDI lookup but do not declare dependencies using either @EJB annotations or EJB deployment - handle entirely with your own logic, without the container helping. Use delays/error handling.

4. Use JBoss proprietary configuration to control deployment order:

   http://texnoblog.wordpress.com/2010/09/16/depends-in-jboss/

   How to order deployment of EJBs and JMS queue config in JBoss 5?

Answer 2

One way would be to create a dummy-ejb that you inject into your MDB, this way your mdb would not start consuming until that injection can actually take place.

If the dummy-ejb is bundled with the EJBs you intended to do dynamic lookup on, this should work

My answer here solves a similiar use-case.

Answer 3

I think that the best approach would be to delay the deployment of your MDB until all your EJBs are up & running. This is basically approach number 4 in the answer above.

"Use JBoss proprietary configuration to control deployment order:

http://texnoblog.wordpress.com/2010/09/16/depends-in-jboss/

How to order deployment of EJBs and JMS queue config in JBoss 5?"