Spring MVC to EJB 3.
0
Introduction
Yet Another
Easy Enterprise
WebDevelopment
Framework
copyright 2009 Trainologic LTD
Spring MVC –to
Introduction part
EJB1 3.0
• Introduction
Overview
• The EJB
MVCModel
Pattern
• Key
Spring
Services
MVC Implementation
of the Application Server
• Controllers
• Mapping Request Handlers
• Views
• I18N and Locale Resolvers
• Customizing Look & Feel using Themes
• File Uploading
• Error Handling
• Using Annotations copyright 2009 Trainologic LTD
EJB 3.0
Introduction
• EJB (Enterprise Java Beans) is one of the main
standards under the JavaEE umbrella.
• Version 3.0 is part of JavaEE 5.
• As we are going to see, EJB3 provides a great solution
for building enterprise applications.
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EJB 3.0
Relation to Other Standards
• Many standards exist in Java which have the ‘beans’
word in it:
• JavaSE – Java Beans.
• JMX – Managed Beans.
• JSF – Managed Beans.
• JavaEE – Enterprise Java Beans.
• There is no relationships between those standards!
(except for the word ‘bean’).
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EJB 3.0
The Motivation for EJB
• Question: why do we need EJB?
• How can it help us in our enterprise applications?
• Answer: when working with EJB we are provided with
out-of-the-box middleware services that ease our
development and allow us to focus on the business logic.
• E.g., transactions, messaging, resource management,
clustering, security, etc…
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EJB 3.0
The Application Server
• EJBs must be deployed into an Application Server.
• The application server provides the middleware services
that the EJBs require.
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EJB 3.0
Versions
• EJB 3.0 is a part of JavaEE 5.
• The previous version (2.1) was part of J2EE 1.4.
• EJB3 greatly improved the usability of the EJBs.
• By making use of annotations, it is now very easy to
write EJBs and to configure them.
• The next version (3.1) will be a part of JavaEE 6.
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Introduction to EJB 3.0
• Overview
• The EJB Model
• Key Services of the Application Server
copyright 2009 Trainologic LTD
EJB 3.0
The EJB Model
• It is important to understand how EJBs work and why
the creators of the standard did it this way.
• Remember, the motivation behind EJBs is to have
middleware services that will be provided by the
Application Server!
• Let’s see how this can be done…
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EJB 3.0
Example Scenario
• Let’s say we have the following business logic class:
public class Store {
public void submitOrder(Order o) {
// do the business logic
}
}
• And now, let’s say that we want the Application Server
to wrap the submitOrder method with a transaction.
• Of course, if we invoke the method directly, the
Application Server will not be able to intercept and inject
the middleware services!
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EJB 3.0
The EJB Object
• The only way the Application Server can perform its
middleware services, is by having the invocation pass
through a mediator object.
• Known as the EJB Object.
Add
Transaction
Logic
submitOrder EJB submitOrder
Client Store
Object
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EJB 3.0
The EJB Object
• It is important to mention that the EJB Object is
automatically generated by the Application Server.
• Another important conclusion: you must never ever
make a direct invocation of the EJB class!
• All invocations must be passed through the EJB Object.
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EJB 3.0
Creating Instances
• Ok, if in order to use an EJB we need to have a
reference to the EJB Object, how do we get it?
• Do we use the new keyword?
• The answer is: No.
• We need to ask the Application Server to generate an
EJBObject for us (and we need to tell it for which bean).
• So, we use the JNDI…
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EJB 3.0
The Full Picture
Application Server
Client JNDI EJB Container
Code EJB
EJB Bean
Object
Object Class
Proxy
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EJB 3.0
Restrictions
• When developing EJBs, the specification forbids the use
of a number of Java APIs:
• You must not create threads, manage threads or
synchronize with other beans.
• You must not load a native library.
• You must not use System.exit().
• You must make changes to the filesystem.
• You must not use GUI (AWT & Swing).
• You must not read/write nonfinal static fields.
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EJB 3.0
Restrictions
• You must not listen to connections (ServerSocket).
• You must not pass a direct reference from one EJB to
another (can’t pass this to another EJB).
• Breaking these restrictions will result in vendor
dependant, unexpected behavior and security
vulnerabilities.
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EJB 3.0
EJB Types
• There are 2 types of EJBs:
• Session Beans – handle client invocations.
• Message Driven Beans (MDB) – handle messaging.
• We will also learn to use Entities which are part of JPA
(Java Persistence API).
• Note, Entities are not EJBs.
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Introduction to EJB 3.0
• Overview
• The EJB Model
• Key Services of the Application Server
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EJB 3.0
Supplied Services
• The Application Server supplies the following services for
EJBs:
• Concurrency – all multithreading/ concurrency
issues. You are not allowed to use multi-threaded
code in your EJBs.
• Resource Management – the life-cycle of the EJBs,
namely creating, destroying and reusing EJBs.
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EJB 3.0
Supplied Services
• Transactions – distributed/global transactions for the
EJBs.
• Security – role-based security for the EJBs.
• Messaging – there is a special type of EJB (Message
Driven Bean) that handles messaging.
• Scheduling – scheduling services for EJBs.
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EJB 3.0
Supplied Services
• Persistence – through the JPA specification, EJBs can
use persistence services.
• Clustering – Although not part of the JavaEE
specification, most Application Servers provide
clustering services (load balancing & failover).
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EJB 3.0
More Details
• In this course we’ll discuss most of the aforementioned
services in details.
• But not clustering.
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