J2EE
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  • 1. J2EE Kenneth M. Anderson CSCI 7818 - Web Technologies October 3, 2001
  • 2. Credit where Credit is Due
    • Most (if not all) of the information in this presentation comes from
      • Enterprise JavaBeans
      • by Richard Monson-Haefel
      • O’Reilly & Associates, Inc.
      • © 1999 and 2000
  • 3. J2EE
    • Stands for “Java 2, Enterprise Edition”
    • It is a collection of standards
      • JDBC, JNDI, JMX, JMS
    • It is a component technology
      • Enterprise JavaBeans
    • It is an “application server”
      • Following in the footsteps of Component Transaction Monitors
  • 4. History
    • Distributed Objects
      • CORBA, DCOM, etc.
      • Three-tier scenario: presentation, business logic, and backend databases
        • Hard to “get right” without the proper infrastructure
    • Server-Side Components
      • Focuses on encapsulating “business rules” into objects in the middle tier
    • Component Transaction Monitors
      • Descendant of CORBA’s Object Request Broker
        • provides discovery, persistence, event notification, transactions, etc. for three-tier or n-tier applications
  • 5. Enterprise JavaBeans, Defined
    • From Sun:
      • The Enterprise JavaBeans architecture is a component architecture for the development and deployment of component-based distributed business applications. Applications written using the Enterprise JavaBeans architecture are scalable, transactional, and multi-user secure. These applications may be written once, and then deployed on any server platform that supports the Enterprise JavaBeans specification
  • 6. Enterprise JavaBeans (shorter definition)
    • From O’Reilly’s Enterprise JavaBeans book
      • Enterprise JavaBeans is a standard server-side component model for component transaction monitors
    • Aha! It’s a standard for building server-side components and deploying them in component transaction monitors
  • 7. Distributed Objects Scenario Client Stub Skeleton Server-Side Component Client-Side Network Middle Tier invokes return results connect to remote object invoke return results return results Stub and Skeleton are auto-generated; client “thinks” its making a local call, most networking details are hidden from client; the main detail is obtaining a reference to the remote object  naming service
  • 8. Architectural Overview
    • Two Types of Enterprise JavaBeans
      • Entity Beans
        • Used to model business concepts such as customer, cruise ship, inventory item, etc.
      • Session Beans
        • A server-side “representative” of the client; session beans are responsible for managing processes or tasks; for instance in an airline reservation scenario, the session bean would be responsible for reserving a seat on a particular flight and verifying payment
  • 9. Insight into Session Beans EJB Server Client Using Session Beans Client Using only Entity Beans
  • 10. To Implement an Enterprise Bean
    • Any enterprise bean must define two interfaces and one or two classes
      • Remote interface
        • defines a bean’s external interface
        • must extend javax.ejb.EJBObject (which in turn extends java.rmi.Remote)
      • Home interface
        • The home interface defines a bean’s “life cycle” methods, eg. create bean, remove bean, find bean, etc.
        • must extend javax.ejb.EJBHome which also extends java.rmi.Remote
  • 11. To Implement, continued…
    • Bean Class
      • The java class that actually implements the bean’s external interface, e.g. the bean class provides implementations for the bean’s “business methods”
      • An entity bean must implement the javax.ejb.EntityBean interface, while a session bean must implement the (you guessed it) javax.ejb.SessionBean. Both of these interfaces extend javax.ejb.EnterpriseBean
    • Primary Key
      • The primary key is a very simple class that provides a pointer into a database; Only entity beans need a primary key. This class must implment java.io.Serializable (so the entity bean can automatically be sent to persistent storage)
  • 12. Additional Info
    • Clients never interact directly with a bean class, they use stubs (which connect to skeletons, which connect to “containers” which call the bean class…whew!)
    • Why? This allows the application server to replicate bean instances (for performance reasons), manage transactions, etc.
    • A bean also interacts with its server via a container interface: the container calls the bean’s life cycle methods, manages the bean’s persistence, etc.
  • 13. Architecture Diagram Client EJB Server EJB Container Home Interface Home Stub Remote Interface EJB Stub Home Interface EJB Home Remote Interface EJB Object Bean Class
  • 14. Benefits of an Application Server
    • Resource Management
      • Instance Pooling and Swapping
        • Server can invoke multiple instances of a bean to handle multiple incoming requests (pooling)
        • An instance of a bean class can handle requests from multiple skeletons (swapping)
      • Activation
        • if a bean is “stateful,” application servers automate the process of saving and restoring bean state when a bean is deactived and later activated
  • 15. Benefits of an Application Server
    • Primary Services
      • Concurrency
        • Beans are automatically thread safe; application servers handle concurrent access to a bean; They also handle reentrance
      • Transactions
        • Bean operations can belong to a transaction and the application server handles “rolling back” an application’s state if a partially completed transaction fails
      • Persistence
        • Application servers can map beans into database entries (and back again)
      • Naming (bean discovery) and Security (encrypted communication and access control)
  • 16. References
    • Enterprise JavaBeans
      • by O’Reilly
      • ISBN: 1-56592-869-5
    • Sun’s J2EE website
      • <http://java.sun.com/j2ee/>