J2EE Features

Every J2EE application server needs to provide "J2EE containers" that allow developers to build and deploy web applications, EJBs, J2EE Connectors, and application clients . It must also implement all of the supporting J2EE technologies such as Java Message Service (JMS) and Java Database Connectivity (JDBC). Oracle 10g AS 9.0.4 supports J2EE version 1.3, which requires support for the following specifications listed in Table 1-3.

Table 1-3: J2EE 1.3 Technologies

Specification	Version
Servlet	2.3
JavaServer Pages	1.2
Enterprise JavaBeans	2.0
J2EE Connector	1.0
Java Transaction API	1.0.1b
JDBC	2.0
Java Message Service	1.0.2b
Java Naming and Directory Interface	1.2.1
JavaMail	1.2

The following sections briefly describe the major J2EE technologies. Later chapters will discuss the configuration and deployment of these components in detail.

Servlets and JavaServer Pages

Java Servlets are server-side Java classes that respond to HTTP requests. JSPs use a combination of Java scriptlets, custom tags, and markup (HTML, WML, XML, and so on) to render an HTTP response. The two technologies are often used together to build web applications based on a Model-View-Controller (MVC) design. In this common setup (also called a "Model 2 architecture"), the Servlets parse incoming requests , execute business logic, update data objects and pass control to JSPs, which display the results. Chapter 10 covers the configuration and deployment of web applications.

Enterprise JavaBeans

EJBs are business components that can take advantage of container-managed security, transactions, persistence, remoting, and other J2EE services. There are three types of EJBs: entity beans, session beans, and message-driven beans. Entity beans correspond to rows of data in a database and are often used strictly for data access. Session beans contain complex business logic and often manage one or more entity beans or database tables. Message-driven beans respond to messages posted asynchronously to JMS topics or queues. See Chapter 11 for more information about the configuration and deployment of EJBs.

Java Message Service

JMS allows J2EE components to communicate asynchronously through topics (publish/subscribe) and queues (point-to-point). This is very useful for throttling requests, kicking off long-running processes, or responding to asynchronous events. Chapter 6 covers the configuration and use of JMS.

Java Database Connectivity and J2EE Connectors

JDBC data sources allow for easy interaction with relational databases. J2EE Connectors allow interaction with other types of external systems. Both support transactions and allow for changes across several systems that will be committed or rolled back together. Chapter 5 discusses the configuration and use of JDBC data sources. Chapter 12 covers the configuration, deployment, and use of J2EE Connectors.

Transactions

Transactions allow developers to prevent data corruption by ensuring that related changes to databases and other systems are committed or rolled back together. J2EE provides the Java Transaction API (JTA) for programmatic initiation of transactions, and it also allows EJB transaction settings to be configured declaratively inside the EJB deployment descriptor. See Chapter 11 for information on configuring declarative transactions.

Security

J2EE supports both programmatic security (manually checking permissions from within code) and declarative security (configuring which roles should be able to access which components and methods ). Most applications will need both, and both rely on the same underlying security architecture, described in Chapter 7. Security for the Application Server is covered in detail in Chapter 20.

Other J2EE Services

J2EE includes a number of other services and APIs that simplify programming but don't require significant server configuration. In particular, Chapter 5 discusses Java Naming and Directory Interface (JNDI), and Chapter 8 covers JavaMail and the Java APIs for XML (JAX).