Java™ Application Development on Linux - Dator

482Chapter 21Introduction to Enterprise JavaBeans21.2.3 Under the HoodWe’re not going to take apart the source code for JBoss here, but we do wantto spend some time talking about what is going on in an EJB container. It isn’tmagic. The whole system is built on some familiar technologies that you canlearn all about if you want to. First of all, the default protocol for EJB sessionand entity beans is RMI. That’s right. A major part of an EJB container is thecode to set up RMI connections between beans and clients. RMI is an applicationprotocol that defines how to encode the state of a Java class, transfer it overa network (using TCP/IP sockets), and restore the coded data to a local implementationof the class (the encoding/decoding process is called marshaling/unmarshaling). That’s part one. Knowing where to find the appropriate beanis the next part.21.3WHAT’S INA NAME? AN INTRODUCTION TO JNDIJNDI abstracts a type of service known generically as a directory service. Weneed to introduce that concept and then describe a few common examples ofsuch systems. Then we can explain how JNDI abstracts these various services.21.3.1 Naming and Directory System ConceptsDirectory services are one of the dark mysteries of modern computing. Why?Because if the people who developed these systems ever let on how simple theyactually are, everyone would understand and be able to use them well. Thenwhere would we be?In practice, a naming system is what we programmers call an associativearray, or, when we are feeling less verbose, a simple hash of name/value pairs.That’s it. The core concept isn’t any more complicated than that. The mostfamiliar naming service out there (one that we are sure you use every day)is the Internet Domain Name Service, or DNS. This is a system that mapsdomain names (like www.somedumbnetwork.net) to IP addresses (like205.117.29.1). In the world of directory services, such a name/value pair iscalled a binding.Of course, the devil is in the details. DNS is implemented by a complexnetwork of name servers that pass requests up and down a distributed hierarchyof name servers. That part can get quite complex, but the core idea is that youhave a name (the domain name) and a value (an IP address) that you join