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Servlets in Apache Tomcat and BEA Systems' WebLogic Server

Deploy servlets and Web applications in two popular application servers

By Steven Gould, JavaWorld.com, 02/23/01

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In "Develop N-Tier Applications Using J2EE" (JavaWorld, December 1, 2000), I gave an introduction to Java 2 Enterprise Edition (J2EE) and an overview of the technologies it includes. In this article, we delve into a little more detail with one of the more widely used J2EE technologies: servlets. We begin with a brief recap of servlet development fundamentals, then show how to build a Web application to house them. We discuss the use of Web Application Archives (WARs), then illustrate deployment of the servlet and Web application in Apache Tomcat. In addition, we'll look at deploying the same servlet and Web application in one of the most widely-used servlet containers -- BEA's WebLogic Server.

Develop servlets

Servlets were designed to allow for extension of a server providing any service. Currently, however, only HTTP and JSP page servlets are supported. In the future, a developer may be able to extend an FTP server or an SMTP server using servlets.

Generic servlets

A servlet extends a server's functionality by offering a specific service within a well-defined framework. It is a small piece of Java code -- often just a single class -- that provides a specific service. For example, an HTTP servlet may provide a bank customer with details of her recent deposits and withdrawals. Another HTTP servlet could allow a customer to view, and even edit, his mailing address.

To deploy a servlet usually requires configuration of the hosting server application. When the server encounters a particular type of request, it invokes the servlet, passing to it details about the request and a response object for returning the result.

All servlets implement the javax.servlet.Servlet interface either directly -- in the case of generic servlets -- or indirectly, in the case of HTTP or JSP servlets. The javax.servlet.Servlet interface's important methods include:

  • init(): defines any initialization code that should be executed when the servlet loads into memory.
  • service(): the main method called when the servlet receives a service request. It defines the bulk of the processing logic provided by the servlet.
  • destroy(): defines any clean-up code required before removing the servlet from memory.


When the servlet container first loads a servlet it invokes the servlet's init() method to initialize the servlet. Then, as requests are made to execute the servlet, the servlet container repeatedly invokes the servlet's service() method to provide the required service. Finally, when the servlet container no longer needs the servlet, it invokes the servlet's destroy() method and unloads it from memory. Note that during the lifetime of a single servlet instance, the init() and destroy() methods will be invoked only once, whereas the service() method will be invoked many times -- once each time a request is made to execute the servlet.

JSP Page servlets are mostly of interest to implementers of JSP containers and are beyond the scope of this article. Rather, we now go on to look specifically at HTTP servlets.

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