1. Enterprise Java Beans: an introduction

2. Java E-Commerce © Martin Cooke, 2003
Today’s lecture
Why is enterprise computing so complex?
Component models and containers
Session beans
Entity beans
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Java E-Commerce © Martin Cooke, 2003

3. Why is enterprise computing so complex?
session management
authorisation
persistence
transactions
Core code
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A solution
Benefits
Programming becomes easier
Certain aspects are hard eg transactions …
… or tedious eg persistence
No need to duplicate code or reinvent wheel
Can take advantage of 3rd party solutions
Business logic can be ported between container providers
Scaleability issues taken out of webapp
container
business
logic
enterprise bean
persistence
transactions
authorisation
sessions
object pooling
load-balancing
network services
Drawback
Loss of control can impact upon efficiency
“middleware” services
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5. Distributed object technologies
CORBA (Object Management Group)
Revolutionised distributed computing, but overcomplex programming model, and less portable than expected
Java RMI (Sun)
DCOM/MTS (Microsoft)
EJB (Sun)
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6. Containers and components

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Definitions
Component
reusable software building block
often customisable
Container
shell in which the component executes
provides services
all communication with component goes via container
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Definitions
The relationship between a container and its components is analogous to that between
a webserver and servlets
a browser and applets
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Component model
Defines structure of interfaces and mechanisms by which it interacts, both with container and with client applications
Components are called Enterprise Java Beans
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Containers
Manages many beans simultaneously
Intercepts all method invocations on beans
Pools resources
Can ‘swap’ beans in and out: transparent to client applications
Provides everything needed by bean
eg JDBC access
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11. Comparison with java beans
Enterprise java bean
Typically used for widgets, controls
Manipulated using visual tools
Configured at compile time
Run on one machine, in one address space
Typically used for larger-grained business processes and objects
Can be configured at deploy time
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Using the bean
Client
Anything: webapp, app or another EJB
Container
Home interface
Used to control the lifecycle methods of the bean: create, remove, find; uses JNDI
Home
interface
create
find
remove
Client
app
EJB
Remote interface
Exposes methods that make the bean tick
Remote
interface
Business
methods
Deployment
descriptor
Deployment descriptor
Defines security, transactional, … behaviour
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Two types of EJB
Entity bean
Represents persistent data
Can be viewed as an object representation of a row in a database (but is more general)
Think of it as a noun
Session bean
Represents transient data and processes
Think of it as a verb
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Two types of EJB
entity
session
Object representation of persistent data
Identified by primary key
Transactional
Recoverable after system crash
Persistence can be managed by container or by bean
Egs: a booking, a customer
Represents work being performed by client, and may span several method calls
Created by client
Exists for a single session
Can be stateful or stateless
May be transactional
usually not recoverable
Eg: a reservation handler
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15. Session beans

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Typical uses
Price quoting
Order entry
Compression
Complex calculations
Database operations
Credit card verification
ie business logic
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17. Session beans: 2 flavours
Stateless
don’t maintain state across method calls
Any instance can be used at any time by any client
Eg audio compression
Stateful
Maintain state within and between transactions
Each is associated with a specific client
Containers manage instance pool
Eg shopping cart
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18. Session bean lifecycle
Pooling
Number of instances < number of current clients due to thinking time
Eg threads, socket connections, database connections, …
Concurrency
All methods are thread-safe (one-at-a-time)
Extra beans created to handle bottlenecks
Source: middleware-company.com book
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A first example
Simplest possible bean: stateless session “hello world” bean
Recall that we need
Home interface
Remote interface
Bean implementation
Deployment descriptor
Container
create
find
remove
Home
interface
Client
app
EJB
Remote
interface
Business
methods
Deployment
descriptor
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20. Remote method invocation (RMI)
RMI enables invocation of methods on remote objects, passing in and receiving real Java objects as arguments and return values
Remote objects implement a remote interface which specifies which methods can be invoked remotely
Any object that implements java.rmi.remote is callable across the network
EJB objects implement java.rmi.remote, hence all EJBs are networkable objects
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The home interface
public interface HelloHome extends javax.ejb.EJBHome {
Hello create()
throws java.rmi.RemoteException,
javax.ejb.CreateException; }
All home interfaces extend javax.ejb.EJBHome
All home interfaces must supply a create method which is used for initialisation and can take params, but not here (stateless)
Note exceptions: all remote methods must throw a RemoteException
Container provides implementation
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The remote interface
public interface Hello extends javax.ejb.EJBObject {
public String hello()
throws java.rmi.RemoteException; }
All remote interfaces extend javax.ejb.EJBObject
hello is our single business method
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The bean class
public class HelloBean
implements javax.ejb.SessionBean {
public void ejbCreate() {}
  public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext
(javax.ejb.SessionContext ctx) {}
public String hello() {
return "Hello, World!"; }
Business methods
Container methods
allow container to interact with bean
ejbCreate must match create method of home interface
ejbRemove called when bean is destroyed
ejbActivate and ejbPassivate concepts don’t apply to stateless session beans
setSessionContent allows beans to access certain resources eg info about caller identity
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24. The deployment descriptor
<!DOCTYPE ejb-jar PUBLIC "-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 2.0//EN" "
<ejb-jar>
<enterprise-beans>
<session>
<ejb-name>Hello</ejb-name>
<home>examples.HelloHome</home>
<remote>examples.Hello</remote>
<ejb-class>examples.HelloBean</ejb-class>
<session-type>Stateless</session-type>
<transaction-type>Container</transaction-type>
</session>
</enterprise-beans>
</ejb-jar>
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What’s missing
Simplest possible bean; many features not covered or used
Vendor-specific deployment information
Packaging
Deployment
Client-access code (next)
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Client application
import javax.naming.Context;
import javax.naming.InitialContext;
import java.util.Properties;
public class HelloClient {
public static void main(String[] args)
throws Exception {
Context ctx = new
InitialContext(System.getProperties());
HelloHome home = (HelloHome)
javax.rmi.PortableRemoteObject.narrow(
ctx.lookup("HelloHome"), HelloHome.class);
Hello hello = home.create();
System.out.println(hello.hello());
hello.remove(); }
Steps
Use of JNDI to find bean
RMI-style cast
Use home object to create EJB
Call business method on bean
Remove bean
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27. Stateful session beans
Issue
Not so easy to pool beans because of need to maintain conversational state: could easily run out of resources
Solution
cf operating systems and applications:
Passivate: Swap out state and save
Activate: Swap in when needed to restore state
Bean instance receiving state might not be same as that previously used, but doesn’t matter
Has effect of pooling beans
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28. More on passivation & activation
Which?
Container dependent, but least recently used is typical
When passivated?
Any time bean is not in a method call or in a transaction
When activated?
Container dependent, but often ‘just in time’
What is saved?
All non transient member variables
How used?
ejbPassivate called just before saving, ejbActivate just after restoration. Gives bean chance to relinquish resources or reconstruct transient objects
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Example: counter
public class CountBean implements SessionBean {
public int val;
public int count() {
System.out.println("count()");
return ++val; }
public void ejbCreate(int _val) throws CreateException {
val = _val;
etc
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30. Deployment descriptor
<!DOCTYPE ejb-jar PUBLIC
"-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 2.0//EN"
"
<ejb-jar>
<enterprise-beans>
<session>
<ejb-name>Count</ejb-name>
<home>examples.CountHome</home>
<remote>examples.Count</remote>
<ejb-class>examples.CountBean</ejb-class>
<session-type>Stateful</session-type>
<transaction-type>Container</transaction-type>
</session>
</enterprise-beans>
</ejb-jar>
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31. Client application main
Context ctx = new InitialContext(System.getProperties());
CountHome home = (CountHome)
javax.rmi.PortableRemoteObject.narrow(
ctx.lookup("CountHome"), CountHome.class);
Count count = home.create(0);
System.out.println(count.count());
count.remove();
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32. Entity beans

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Reminder
Entity beans provide an in-memory representation of persistent data
Can read themselves from storage and persistent themselves back to storage
Survive system failures
Used to model the nouns of an application
Customer
Order
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Persistence
Container-managed
Simple to use
Possibly very inefficient given current state-of-the-art in containers
Bean-managed
Customisable
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Transactional issues
Like session beans, entity beans are single-threaded
Multiple threads makes transactions very difficult
Difficult to produce reliable thread-safe code
Like session beans, containers can create multiple instances (pools)
For entity beans, this raises the issue of multiple instances of the same data becoming out of synch.
Dealt with using transactional isolation
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Pooling
Like session beans, entity beans may be pooled
ie implement ejbActivate() and ejbPassivate()
In addition, state is saved just prior to passivation and loaded just prior to activation
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BMP vs CMP
Source: the J2EE tutorial (2002), Sun
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Example entity bean
From WebTomorrow tutorial (see resources)
Models a CD
Uses container-managed persistence
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The remote interface
import javax.ejb.*;
import java.rmi.RemoteException;
public interface CD extends EJBObject {
  public abstract String getTitle() throws RemoteException;
  public abstract void setTitle(String title)
throws RemoteException;
  public abstract String getId() throws RemoteException;
  public abstract void setId(String id)
// … more get/set methods for other fields }
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The home interface
import javax.ejb.*;
import java.rmi.RemoteException;
public interface CDHome extends EJBHome {
public CD create(String id)
throws RemoteException, CreateException;
public CD findByPrimaryKey (String id)
throws RemoteException, FinderException; }
NB: this and other finder methods implemented by container
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41. The bean implementation
import javax.ejb.*;
import java.rmi.RemoteException;
public class CDBean implements EntityBean {
public String id;
public String title;
// other fields
public String getTitle() {
return title; }
public void setTitle(String _title) {
title = _title;
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42. The bean implementation contd
public String ejbCreate(String _id) {
id = _id;
return null; }
// mandatory methods
public void ejbPostCreate(String id) {}
public void setEntityContent(EntityContext ctx) {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void ejbLoad() {}
public void ejbStore() {}
public void ejbRemove() {}
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43. The deployment descriptor
<enterprise-beans>
<entity>
<ejb-name>CD</ejb-name>
<home>examples.CDHome</home>
<remote>examples.CD</remote>
<ejb-class>examples.CDBean</ejb-class>
<persistence-type>Container</persistence-type>
<prim-key-class>java.lang.String</prim-key-class>
<reentrant>False</reentrant>
<cmp-field><field-name>id</field-name></cmp-field>
<cmp-field><field-name>title</field-name></cmp-field>
<primkey-field>id</primkey-field>
</entity>
</enterprise-beans>
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44. The deployment descriptor, contd
<assembly-descriptor>
<container-transaction>
<method>
<ejb-name>CD</ejb-name>
<method-name>*</method-name>
</method>
<trans-attribute>Required</trans-attribute>
</assembly-descriptor>
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What else is in EJB?
Message-driven beans
EJB-QL
Security
Network services
Tools for deployment
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jBoss
jBoss.org
Free
widely-used
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Summary
Distributed component models focus developers’ attention on business logic
Other middleware services provided by container
2 principal bean types: session & entity, representing process and persistent data
Some efficiency concerns at present, but better object-relational mapping tools will help
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Course summary
eCommerce wasn’t a blip
Now in a more mature phase, but new techniques, notations, tools, etc being developed at a rapid pace
Beware investment of time in recent developments -- tendency to disappear just as quickly
Don’t reinvent wheel: use 3rd party services for critical aspects (eg online monetary transactions)
Distributed, multi-client computing is here to stay with its associated issues: concurrency, load-balancing, security, …
Architectural design is key
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Resources
Some of the examples in this lecture are derived from a book on EJBs which is in preparation, and available for public comments, at
An excellent tutorial on the use of jBoss is given in Creating a distributed Web application in Java using jBoss and Tomcat by Kevin Boone (available at
Another useful tutorial is Enterprise Java Beans Fundamentals from the IBM DeveloperWorks site.
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