部署后立即消耗MDB中的EJB的运行时查找

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? "

Answer 4

你可以在查找调用周围实现一个带退避的循环。