Using Java2 Enterprise Edition in the Application Server 10g Oracle Application Server Tips by Burleson Consulting

This chapter and the next revolve around supporting Java and J2EE applications with Oracle Application Server 10g. Providing access to J2EE Enterprise JavaBeans along with connectivity to the web and customer databases is the basis of any application server. As such, this chapter introduces the components needed to understand and support J2EE within the Oracle Application Server. Many DBAs are tasked to administer the application server without a basic understanding of J2EE components and their interaction with the application server.

The Java language and the Enterprise Edition extensions are beyond the scope of this book. Indeed, a trip to the local bookstore will reveal shelves of books covering this subject. If you wish to dig deeper into developing J2EE applications, it is covered in detail in the excellent Oracle Press books ORACLE9iAS Building J2EE Applications, by Nirva Morisseau-Leroy (200x), or ORACLE9i Web Development, by Brad Brown (200x). Because Oracle?s Application Server and Oracle?s JDeveloper are very tightly integrated, another important reference is the Oracle9i JDeveloper Handbook, by Peter Koletzke (200x). Here we will only discuss J2EE application components and how to administer them within the Oracle Application Server and leave the application development to others.

J2EE Introduction

Java is both a compiled and interpreted language. Java source code is compiled into a file containing byte-code. The Java Virtual Machine (JVM) interprets the byte-code into machine code and executes it. It is because of this intermediate step that the Java code is portable to many different operating systems. Any operating system that implements a JVM can run the Java class file. Java also implements a number of technologies that allow it to communicate easily across a network or the Internet. As Java matured, more and more developers were using it to produce large, complex, distributed transaction programs. To assist this development, Sun produced the Java2 Enterprise Edition (J2EE), which consists of a component-based approach to designing, developing, assembling, and deploying an enterprise application. J2EE uses a multitiered set of technologies that allow developers to focus attention on each tier during development. Developers implementing Enterprise JavaBeans may not know how to program Java Server Pages, but because the J2EE specification defines methods for EJBs to communicate, they can be developed separately and still seamlessly mesh within the application. The J2EE specification divides its technologies into three tiers: the client tier, web tier, and business tier.

The client tier consists of either an application (possibly using JavaBeans) or the user?s browser with static or dynamic pages and Java applets. The client tier runs on the user?s desktop. Utilizing the user?s browser to serve the application interface allows the flexibility of having the entire application located on the server. Clients are always working on the latest version because their application is served from the application server each time it starts. Having a client application running on the user?s desktop that communicates with the server portion of the application can allow for extensive processing locally and reduced server loads; however, updating the client application involves reinstalling on each client.

The web tier consists of an HTTP server providing static and dynamic HTML pages, Java Server Pages, and servlets.

Behind the web tier is the business tier. Here is where the business logic is executed within Enterprise JavaBeans (EJBs). This tier may be located on one server or spread across a number of possibly geographically dispersed servers. Co-locating the business logic in one tier allows for easier maintenance. The business tier may communicate with a back-end database or a legacy information system.

Figure 1: J2EE component tiers

Now that you have an idea of how J2EE divides an enterprise application into tiers, we need to define some J2EE terms.

Applets

Applets are usually small Java programs that are downloaded to the browser and run locally inside the browser?s Java Virtual Machine. Applets can be small programs like banner ads or large applications that run a GUI. A request for an applet is embedded into an HTML page, and when the browser renders the page, it will find the tag and request the applet. Once the applet is downloaded to the client machine, the browser executes the applet. All applets require that the browser have a JVM installed, and more complicated applets may require a security policy file in order to execute successfully. Applets are a powerful component that can provide a rich user interface, but they must be downloaded from the server, which can take a considerable amount of time.

Client Applications

Client applications are Java programs that run on the client system and communicate with an application server. They are usually rich clients, meaning that they can perform extensive user interaction through a GUI. Client applications are normally implemented using JavaBeans, which will be discussed later in this section. They can also access EJBs on the application server or access a servlet through an HTTP connection. In multitiered systems, client applications normally execute the user interface, leaving all the business logic to the application server.

Servlets

Servlets are Java web components that execute on the HTTP server side (the web tier). Called in an HTTP or Java Server Page (JSP) document, or directly from an application, servlets are used to produce dynamic data. Since they operate on the server side of the communication, they have full access to the server?s Java APIs. They also remain active on the server and can be used to maintain persistent data. Servlets are called as the document is being served and normally produce HTML that is served to the client browser for displaying. Servlets can access JavaBeans and/or Enterprise JavaBeans during execution.

Java Server Pages

Java Server Pages are Java web components used to embed Java into an HTML coded page. JSPs can place Java code directly into the HTML code so that it is compiled and executed when served.. Java Server Pages allow for easy interaction between the user and the server application. JSPs are not served directly by the HTTP server, but are instead served by the JSP container through the HTTP server. You?ll learn more about this later in the chapter.

JavaBeans

A JavaBean is a reusable software component that implements Sun?s JavaBean specification. Beans are normally implemented in some type of integrated development environment (IDE) used to create applications and build user interfaces, such as Oracle?s JDeveloper. Think of a bean as a black box that implements some needed functionality and can be dropped into a Java program and used. JavaBeans are part of the Java programming language and are not really part of the J2EE specification. However, since JavaBeans are just reusable ?chunks? of Java code, they can be found on all three tiers.

Enterprise JavaBeans

Enterprise JavaBeans (EJB) are server-side, reusable components that are used to implement business logic. EJBs run inside containers that implement all of the required support infrastructure, such as system resource security, locating, and connecting to other EJBs and clients. EJBs will be discussed in greater detail later in this chapter. There is a significant difference between a JavaBean and an Enterprise JavaBean. A bean is a chunk of reusable code that is integrated into an application. An enterprise bean must run in a container and can be thought of as an independent subapplication that waits for requests and then answers them. Thus a J2EE application can be thought of as a series of subapplications, all running to support the main application.

J2EE Containers

A container provides the infrastructure for running EJBs. An EJB must be deployed into a container before it can be used. The container starts the EJB when it is accessed and provides all required services the EJB needs. If EJBs are the heart of a J2EE application, the container is the heart of the application server. Oracle?s container, OC4J, is covered in detail in Chapter 7.

 

This is an excerpt from "Oracle 10g Application Server Administration Handbook" by Don Burleson and John Garmany.