# Your First Cup: An Introduction to the Java EE Platform

# 2.1 Overview of Enterprise Applications

This section describes enterprise applications and how they are designed and developed.

As stated above, the Java EE platform is designed to help developers create large-scale, multi-tiered, scalable, reliable, and secure network applications. A shorthand name for such applications is "enterprise applications," so called because these applications are designed to solve the problems encountered by large enterprises. Enterprise applications are not only useful for large corporations, agencies, and governments, however. The benefits of an enterprise application are helpful, even essential, for individual developers and small organizations in an increasingly networked world.

The features that make enterprise applications powerful, like security and reliability, often make these applications complex. The Java EE platform reduces the complexity of enterprise application development by providing a development model, API, and runtime environment that allow developers to concentrate on functionality.

## 2.1.1 Tiered Applications

In a multi-tiered application, the functionality of the application is separated into isolated functional areas, called tiers. Typically, multi-tiered applications have a client tier, a middle tier, and a data tier (often called the enterprise information systems tier). The client tier consists of a client program that makes requests to the middle tier. The middle tier is divided into a web tier and a business tier, which handle client requests and process application data, storing it in a permanent datastore in the data tier.

Java EE application development concentrates on the middle tier to make enterprise application management easier, more robust, and more secure.

### 2.1.1.1 The Client Tier

The client tier consists of application clients that access a Java EE server and that are usually located on a different machine from the server. The clients make requests to the server. The server processes the requests and returns a response back to the client. Many different types of applications can be Java EE clients, and they are not always, or even often Java applications. Clients can be a web browser, a standalone application, or other servers, and they run on a different machine from the Java EE server.

### 2.1.1.2 The Web Tier

The web tier consists of components that handle the interaction between clients and the business tier. Its primary tasks are the following:

- Dynamically generate content in various formats for the client

- Collect input from users of the client interface and return appropriate results from the components in the business tier

- Control the flow of screens or pages on the client

- Maintain the state of data for a user's session

- Perform some basic logic and hold some data temporarily in managed beans

Table 2-1 lists some of the main Java EE technologies that are used in the web tier in Java EE applications.

Table 2-1 Web-Tier Java EE Technologies

Technology | Purpose
--- | ---
JavaServer Faces technology | A user-interface component framework for web applications that allows you to include UI components (such as fields and buttons) on a XHTML page, called a Facelets page; convert and validate UI component data; save UI component data to server-side data stores; and maintain component state
Expression Language | A set of standard tags used in Facelets pages to refer to Java EE components
Servlets | Java programming language classes that dynamically process requests and construct responses, usually for HTML pages
Contexts and Dependency Injection for Java EE | A set of contextual services that make it easy for developers to use enterprise beans along with JavaServer Faces technology in web applications

### 2.1.1.3 The Business Tier

The business tier consists of components that provide the business logic for an application. Business logic is code that provides functionality to a particular business domain, like the financial industry, or an e-commerce site. In a properly designed enterprise application, the core functionality exists in the business tier components.

The following Java EE technologies are among those that are used in the business tier in Java EE applications:

- Enterprise JavaBeans (enterprise bean) components

- JAX-RS RESTful web services

- Java Persistence API entities

### 2.1.1.4 The Enterprise Information Systems Tier

The enterprise information systems (EIS) tier consists of database servers, enterprise resource planning systems, and other legacy data sources, like mainframes. These resources typically are located on a separate machine from the Java EE server, and are accessed by components on the business tier.

The following Java EE technologies are used to access the EIS tier in Java EE applications:

- The Java Database Connectivity API (JDBC)

- The Java Persistence API

- The Java EE Connector Architecture

- The Java Transaction API (JTA)