©1996-2000 jGuru.com Enterprise JavaBeans Fundamentals

©1996-2000 jGuru.com

Enterprise JavaBeans Fundamentals

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Agenda

• Enterprise JavaBeans defined

• EJB and Distributed Computing

• EJB Architecture

• Entity beans

• Session bean

• Deployment

• EJB Clients


Enterprise JavaBeans: Defined

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What is Enterprise JavaBeans

The Enterprise JavaBeans architecture is a component architecture for the development and deployment of object-oriented distributed enterprise-level applications. Applications written using the Enterprise JavaBeans architecture is scalable, transactional and multi-user secure. These applications may be written once, and deployed on any server platform that supports the Enterprise JavaBeans specification

Sun Microsystems Enterprise JavaBeans™ Specification,

v1.1, Copyright 1999 by Sun Microsystems, Inc.

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In English Please!

• Enterprise JavaBeans is a specification – (a piece of paper) not a product– As is XML, CORBA, TCP/IP

• Enterprise JavaBeans is Java based– Defined by Sun Microsystems– Applications are written in Java

• Enterprise JavaBeans is distributed objects– Similar to CORBA | Java RMI (JRMP) | Microsoft MTS

• Enterprise JavaBeans is components– Similar to regular JavaBeans (similar but not the same)– Similar to COM/DCOM/COM+

• Enterprise JavaBeans products are Transactional Monitors– Similar to CICS | TUXEDO– A complete environment for a distributed object

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Enterprise JavaBeans

• Simple Programming Model– Attribute based programming– Focus on Business Logic– Simple API– Well defined Lifecycle

• Portable– Specification ensures basic services– Component Packaging (JARs and XML DD)– Java

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WORA

• Write-Once, Run Anywhere within Middleware

• Middleware provides all services– Instance management, transactions,

concurrency, persistence

• Beans stay simple!

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EJB Spec

• Spec developed by Sun and dozens of competing vendors– IBM, Oracle, BEA, WebLogic, ...– Interoperable -- a miracle!

• Why did they cooperate?– See next slide...

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The Common Enemy Phenomenon

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EJB Advantages

• With EJB, you can write a business object and easily make it– Persistent– Distributed– Transactional– Secure– Multithreaded

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Persistent

• Beans need to load and store data

• You can let the server do it – (Container-Managed Persistence)

• You can do it yourself – (Bean-Managed Persistence)

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Transactional

• Support for distributed transactions

• You can let the Server manage all transactions– You will if you know what’s good for you– You can give it hints in deployment descriptor

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Secure

• SSL/RMI protects transmitted data

• Client-Server architecture protects– proprietary code– backend database

• SSL authenicates client, server

• ACLs provide fine-grained control of access to objects, methods, services

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Multithreaded

• Programmer delegates all responsibility for multithreading to server– Programmer literally can’t spawn a thread

• Program designer and/or sysadmin establishes multithreading policies

• Server enforces them invisibly

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Naming Services

• JNDI

• Wrapper for many naming services– CORBA, RMI, etc.

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CORBA: Sorta Porta(ble)

• CORBA: Complex Programming Model– Different API for each Service– Explicit use of service APIs– Lifecycle depends on services used

• Fairly Portable– Many Languages, many platforms (pro and con)– Non-Uniform support of Services

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CORBA and EJB

• Transport– EJB uses RMI interface, RMI uses IIOP

• CORBA 3.0 promises object compatibility with EJB– Not quite sure what that means

• Some EJB Servers contain an ORB– All EJB Objects are also CORBA objects

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CORBA and EJB (Cont.)

• All EJB Servers use CORBA Transactions (via JTS)– That means that any client can make a distributed

transaction that includes both CORBA and EJB Objects

• Not an either-or decision– You can have both EJB and CORBA working

together in a single system

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What’s in a name?

• Enterprise Java Beans has absolutely nothing to do with JavaBeans– Except that both are Java-based component architectures

• EJB is server-side, JB is client-side– EJB has no GUI, JB usually has GUI

• JB is basically naming conventions for fully powered Java classes

• EJB rules are much more rigid• EJB classes are less powerful on their own, but

more powerful in a container

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’Tis but thy name that is my enemy

• EJB is all about really poor names

• Architecture makes sense, object names don’t– “Home” is a factory– “Container” is a helper or wrapper– “EJB Object” is an object but is not an EJB– "Enterprise JavaBean" is not a JavaBean

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Disadvantages of N-Tier Architecture

• Learning curve• $$ cost of products• $$ cost of extra servers to achieve performance• time + $$ cost of maintenance• Object-relational mapping

• Basically, app servers are good for large, general solutions (e.g. multiple data sources, multiple applications)


EJB Architecture

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EJB Roles

• Bean developer: creates JAR with:– Remote interface with business functions– Home for accessing instances of bean– Bean itself– Properties and descriptor

• Assembler: integrates beans into clients

• Deployer: modifies and deploys beans– Organization-specifics, ie security

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EJB Roles (cont.)

• Server provider provides Server run-time

• Container provider: provides tools and containers– Creates container classes that wrap bean– Manage installed beans

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The EJB Container

• Think of the container as the server, holding your objects

• Containers host enterprise beans• Containers isolate beans from clients

– Intercept client requests– Set up transactions, security, persistence, etc. before

and after calling EJB methods– Beans can also call methods of the container interface

to get services

• Containers provide JNDI services too

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Component API: Interfaces and Classes

• Home Interface– Extends javax.ejb.EJBHome– Provides remote access to create, find, remove beans

• Remote Interface– Extends javax.ejb.EJBObject– Provides remote access to business methods

• Bean Class– Extends a javax.ejb.EnterpriseBean type– Implements business logic and other functionality

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Implements

Invokes

Creates / uses

EJB Architecture

Client

Server

Home Interface(Factory)

EJB Object(Wrapper)

EnterpriseJava Bean(Biz Logic)

RemoteInterface

Container

RMI

RMI

Naming Service

You write this

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Bean provider: What to write?

• Remote Interface– extend EJBObject interface– Define business method signatures

• Home Interface– extend EJBHome interface– Define create signatures– May define findBy signatures for entities

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What to write (cont.)?

• Enterprise Bean Class– implement EntityBean or SessionBean– Implement business method signatures– Does not need to implement Remote Interface– not abstract– Implement ejbCreate methods matching Home

create• 1 ejbCreate for every Home.create• N.B.: “create” in home interface, “ejbCreate” in Bean

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Remote Interface

import javax.ejb.EJBObject;

import java.rmi.RemoteException;

public interface Customer extends EJBObject {

public Name getName() throws RemoteException;

public void setName(Name name) throws RemoteException;

public Address getAddress() throws RemoteException;

public void setAddress(Address address) throws RemoteException;

}

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Home Interface

import javax.ejb.EJBHome;

import javax.ejb.CreateException;

import javax.ejb.FinderException;

import java.rmi.RemoteException;

public interface CustomerHome extends EJBHome {

public Customer create(Integer customerNumber)

throws RemoteException, CreateException;

public Customer findByPrimaryKey(Integer customerNumber)

throws RemoteException, FinderException;

public Enumeration findByZipCode(int zipCode)

throws RemoteException, FinderException;

}

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Bean Class

import javax.ejb.EntityBean;

public class CustomerBean implements EntityBean { Address myAddress; Name myName; CreditCard myCreditCard;

public Name getName() { return myName; } public void setName(Name name) { myName = name; } public Address getAddress() { return myAddress; } public void setAddress(Address address) { myAddress = address; }

...}

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Types of Beans

• Entity Beans– Represent data in the database– Include behavior (business methods)

• Session Beans– Represent tasks or processes– Do not represent data, but can access data– Manage the interactions of other beans

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Home Interface

• Defined by Bean developer

• Implemented by server tools (autogenerated)

• Must extend interface EJBHome– EJBMetaData getEJBMetaData()– void remove(Handle ejbHandle)– void remove(Object primaryKey)

• Must provide your own create() methods– Foo create()– Foo create(Bar b, Baz z)…

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Home Interface: Entity Beans

• Entity Beans are persistent, therefore they need more than a “create” method

• Need findXXX methods– public Foo findByPrimaryKey(Object key);– public Foo findByBar(Bar bar);– public Enumeration findOverdrawnAccounts();

• Implement ejbFindXXX methods in bean– N.B.: “find” in home interface, “ejbFind” in Bean

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Remote Interface

• Written by developer

• Defines methods accessible by client– Your business methods go here

• extends javax.ejb.EJBObject– standard methods provided by all EJBs– getEJBHome(), getPrimaryKey(), getHandle(),

remove(), isIdentical(EJBObject obj)

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Remote Interface vs. EJBObject vs. EJB

• Developer writes Remote Interface • Tool uses RI to automatically generate the

EJBObject class• Developer writes the EJB source file• N.B.: The EJB does not implement the Remote

Interface– However, you still must implement all the business methods– Lame-o-rama: no type safety provided by compiler– Better: if EJB tool auto-created a new interface for the

EJBObject (oh well)

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Implementing the EJB

• Implements all your business methods• Must also implement

– ejbCreate() methods– ejbFind() methods (for Entity Beans)

• Also callback methods– ejbRemove()– ejbActivate()– ejbPassivate()

• Implement which interface?– javax.ejb.SessionBean– javax.ejb.EntityBean

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Interfaces and Implementations

EJBObjectgetEJBHome()

getPrimaryKey()getHandle()isIdentical()

remove()deposit()

getBalance()

Remote Interfacedeposit()

getBalance()

EJBejbCreate()

ejbRemove()ejbActivate()ejbPassivate()

deposit()getBalance()

Home Interfacecreate()remove()


Beans and Persistence

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Persistence

• Container-Managed Persistence– Server reads values from your bean– Server stores/loads data for you– Very easy to write - just define data and server

magically takes care of persistence

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Container-Managed Persistence (CMP)

• Persistence is automatic– inserts, updates, and deletes are automatic– transactions managed automatically– server reads/writes your bean instance variables– Server also informs you before/after it's done

something • you implement callback methods

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Container-Managed Persistence (CMP) cont.

• Easier to write beans– Focus on business logic, not persistence– But, requires sophisticated, vendor-specific data

mapping tools

• Components more portable– not backend-specific– shrink-wrapped components

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Bean-Managed Persistence

• Bean-Managed Persistence– You code access database directly– Callback methods dictate when to insert, update,

delete

• More control over persistence, performance• Much harder to write• Don’t need sophisticated data mapping tools

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Entity Beans

• Model entities in a system– represent their data and associated behavior

• one or many relational database tables• an object in an object database• an entity in a legacy system

– Nouns: People, Places or Things • Customer, Employee, Student• City, Building, Hotel Room• Order, Organization, Health Benefit

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Session Beans

• Not persistent, but can access database• Model tasks, process or agents

– Charge a credit card– Process a stock purchase– Perform hotel reservations

• Manage interactions of other beans

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Stateless Session Beans

• Perform transitive tasks

• Independent business methods– Act on method arguments, not bean state– Similar to procedures in traditional TPM systems

• Most performant bean type

• Not dedicated to one client

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Stateful Session Beans

• Act on behalf of one client– Extension of the client– Agent that performs work for the client

• Interdependent business methods– Bean has conversational state– Method depend on conversational state

• Dedicated to exactly one client


EJB Deployment

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Function of a Deployment Descriptor

• Describe bean(s) to the container– interfaces and class– type of bean (entity or session)– identify primary key & container-managed fields

• Declare runtime behavior– transaction attributes of methods– authorization access to method– persistence type (BMP vs. CMP)

• Written in XML


EJB Clients

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Types of Clients

• Enterprise beans

• J2EE Application Components

• Java Applications

• Java Applets

• Servlets

• JSP

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Example: J2EE Application Component

CustomerHome home;

Object ref;

// Obtain a reference to the CustomerHome

ref = jndiContext.lookup("java:comp/env/ejb/Customer");

// Cast object returned by the JNDI lookup to the

// appropriate datatype

home = PortableRemoteObject.narrow(ref, CustomerHome.class);

// Use the home interface to create a

// new instance of the Customer bean.

Customer customer = home.create(customerID);

// Use a business method on the Customer.

customer.setName(someName);

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Messaging Beans

• New with EJB 2.0• Use JMS for asynchronous message queue

services• Allows asynchronous response

– I will gladly pay you Tuesday for a hamburger today

– Get some of the missing Thread functionality back

• Not completely supported yet– Must sit on a message queue product

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Value Objects

• Fact: – EJBs are RMI remote objects

• Problem:– Calling each setter or getter goes across the network– Slow if you're accessing more than one value at once

• Solution:– Value object which mimics the EJB, only it isn't remote– EJB has methods to get and set value object– Client sets/gets values locally, sends/receives en masse

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Online Resources

• jGuru EJB FAQ– http://www.jguru.com/faq/EJB

• Sun EJB Page– http://java.sun.com/products/EJB

• EJBNow– http://www.ejbnow.com/

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Thanks To

• Richard Monson-Haefel

• Dave Orchard

• Random Walk Computing

Documents

©1996-2000 jGuru.com Enterprise JavaBeans Fundamentals