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Component-Based Software Engineering

ECE493-Topic 4Winter 2006

Lecture 17– Java Enterprise(Part B)

Ladan TahvildariLadan TahvildariAssistant ProfessorAssistant Professor

Dept. of Elect. & Comp. Eng.Dept. of Elect. & Comp. Eng.University of WaterlooUniversity of Waterloo

March 2, 2006March 2, 2006 ECE493ECE493--T4T4 22

J2EE Application Server

Java 2 Enterprise Edition standardizes interfaces for Application Server components


EJB Overview


EJB – Enterprise Java Beans

Enterprise Java Beans are components that are deployed into containers

The container provides services– Loading / Initialization

– Transactions

– Persistence

– Communication with EJB clients

– Enterprise Naming Context (JNDI name space)


Deployment Phase


Abstract Schema

Part of an entity bean's deployment descriptor

Defines the bean's persistent fields and relationship fields.

The term abstract distinguishes this schema from the physical schema of the underlying data store

Specify the name of an abstract schema in the deployment descriptor


Abstract Schema

Persistent Fields– Are stored in the underlying data store

– Constitute the state of the bean. At runtime, the EJB container automatically synchronizes this state with the database

– During deployment, the container • Maps the entity bean to a database table

• Maps the persistent fields to the table's columns

Relationship Fields– It’s like a foreign key in a database table.It identifies a related bean

– Like a persistent field, a relationship field is virtual and is defined in the enterprise bean class with access methods

– Unlike a persistent field, a relationship field does not represent the bean's state


Abstract Schema

Multiplicity in Container-Managed Relationships– One-to-one: Each entity bean instance is related to a single

instance of another entity bean

– One-to-many: An entity bean instance may be related to multiple instances of the other entity bean

– Many-to-one: Multiple instances of an entity bean may be related to a single instance of the other entity bean

– Many-to-many: The entity bean instances may be related to multiple instances of each other

Direction in Container-Managed Relationships– Bidirectional relationship: each entity bean has a relationship field

that refers to the other bean. Through the relationship field, an entity bean's code can access its related object

– Unidirectional relationship: only one entity bean has a relationship field that refers to the other


Example of Abstract Schema


Deployment Phase


Anatomy of an EJB

Remote Interface– Methods that can be accessed by the outside world.– Extends javax.ejb.EJBObject

Remote Home Interface– Life-cycle methods (create, findByPrimaryKey)– Extends javax.ejb.EJBHome which extends

java.rmi.Remote

Bean class– The class performing the actual business process– Implements an interface based on type of bean


Client / EJB RelationshipHow does a client application (Java class) utilize EJBs?

– Lookup • JNDI (Java Naming and Directory Interface)

• ENC (Environment Naming Context)

– Network protocol - RMI

– EJB container creates object with RemoteHome and Home interfaces –this object passes calls to the bean class


Anatomy of an EJB

EJB 2.0 New Interfaces– New Interfaces allow bean to bean method calls within

the same container

– Local Interface• Similar to the remote interface, but without RMI

• Extends javax.ejb.EJBLocalObject

– Local Home Interface• Similar to the remote home interface, but without RMI

• Extends javax.ejb.EJBLocalHome


Java versus .NET for the Enterprise

Activation/Passivate (on stateful beans)JIT Activation

Java Server Page (JSP) ASP.NET

Instance PoolingObject Pooling

Can be implemented through JMSQueued Components

Java Message Service (JMS)Loosely Coupled Events

Java Transaction Service (JTS)Transactions

EJB

Session Beans (Stateless, Stateful)

Entity Beans

Message-Driven Beans

COM+ Components

Java.NET


JTA / JTS – TransactionsThe Java Transaction API (JTA) and the Java Transaction Service (JTS) allow J2EE application servers to take the burden of transaction management from the component developer

Developers can define the transactional properties of Enterprise JavaBeans technology based components during design or deployment using declarative statements in the deployment descriptor

The application server takes over the transaction management responsibilities


Transactions

In EJB only addresses persistent data– Application code required to rollback changes in application

variables

Either Bean or Container Managed Transaction– Bean Managed

• Explicit use of the Java Transaction API by the Bean

– Container Managed• Completely declarative in deployment descriptor

• Container invokes business method in specified scope

– Entity Beans must use Container Managed transactions

– Session and Message Beans may use Bean Managed

Clients can also establish transactional scope


Transaction Attributes

NotSupported– Method never called within a transaction

– Container suspends client context if it exists

Required– Runs in client’s context if it exists otherwise Container create a

new context

– Used for a method that requires transaction, but can participate in a broader unit of work

• Example: depositing money in an account

• Can be atomic by itself or part of a greater transaction involving other operations


Transaction Attributes

Supports– Uses client’s context if it exists otherwise runs without a context– Needs to be used with caution

RequiresNew– Container always runs the method in a new transaction– Useful for work that commits regardless of results of outer unit of

work

Mandatory– Client must invoke the method from within a transaction– Container uses this context

Never– Client must not invoke the method from within a transaction– Container does not provide transaction context









EJB


Entity Beans


COM+ Components

Java.NET


JMS – Java Message ServiceEnterprise messaging provides a reliable, flexible service for the asynchronous exchange of critical business data and events throughout an enterprise

The JMS API adds to this a common API and provider framework that enables the development of portable, message based applications in the Java programming language

Why should we use JMS?– Loosely-Coupled systems

• Connectionless• Removes dependence on client and server platform / programming language /

version – Publish / Subscribe metaphor

• Send / receive information with many, unknown clients– Integration with other messaging systems

• IBM MQ-Series• Microsoft Message Queue


JMS – Java Message Service

JMS Queue

JMS Topic









EJB


Entity Beans


COM+ Components

Java.NET


J2EE Application Server

Java 2 Enterprise Edition standardizes interfaces for Application Server components


Java ServletsServlets are the Java platform technology of choice for extending and enhancing web servers

Servlets provide a component-based, platform-independent method for building web-based applications

Servlets have access to the entire family of Java APIs, including the JDBC API to access enterprise databases

Servlets can also access a library of HTTP-specific calls and receive all the benefits of Java language, including– Portability– Performance– reusability


Anatomy of a Servletinit() – the init() function is called when the servlet is initialized by the server. This often happens on the first doGet() or doPost() call of the servlet

destroy() – this function is called when the servlet is being destroyed by the server, typically when the server process is being stopped


Anatomy of a ServletdoGet() – the doGet() function is called when the servlet is called via an HTTP GET

doPost() – the doPost() function is called when the servlet is called via an HTTP POST

– POSTs are a good way to get input from HTML forms


JSP – Java Server Pages

Java Server Pages technology uses XML-like tags and scripts written in the Java programming language to encapsulate the logic that generates the content for the page

Any and all formatting (HTML or XML) tags are passed directly back to the response page

By separating the page logic from its design and display and supporting a reusable component-based design, JSP technology makes it faster and easier than ever to build web-based applications


EJB ExampleThe OnLine Bank

– Services: what the client can do in the system such as see the foreign currency , listed shares or make operations on his hown account.

– Accounts: a database containing the accounts of all the clients of the bank with information about credit,debit,access etc.

– Security: is a subsystem that receives all the alarm caused from wrong access and performs action about the situation (calls police and stops operation of that client keeping information about him/her)

Virtual Bank

Client

Security Accounts

Services


EJB Example: The OnLine Bank

Easy to create an EJB structure:

– Client will have a web page at client side to insert values and connect the system

• This will be done using JSP (Java Servlet Pages )

– Services will be a Stateful Session Bean• it will be different for each client connecting the system mantaining data about

the client connected.

– Accounts will be formed by an Entity Bean for each account in the system with a code-account as primary key.

– Security will be a Message Driven Bean and will be called only from container if some operation are abnormal for result or the autentification for the same client fails too much times.

j2ee application serverltahvild/courses/ece... · march 2, 2006 ece493-t4 13 anatomy of an ejb zejb...

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