The Enterprise Service Bus(ESB) is a hot topic among many SOA vendors, and in the case of Sun the route it opted to take was through a specification by the name of Java Business Integration(JBI).
Get a hands on look at how Java EE and JBI relate to each other, and how you can bridge Java EE Web Services and JBI Components.
Without the Sun Java EE Engine, a Java EE component that is deployed as a servlet or EJB web service cannot be accessed from another JBI component without incurring the overhead of network layer communication. When the application server and JBI runtime environment work together in the same JVM instance, Java EE components can more readily access binding components and service engines.
Read the complete post/article : http://java.sun.com/developer/technicalArticles/J2EE/sunjavaee_engine