The Deployment Descriptor: web.xml

Before I explain what a deployment descriptor is, we should first understand the web application directory structure

WEB APPLICATION DIRECTORY STRUCTURE:

A Web application contains a structure hierarchy of directories and files in a standard layout. Such a hierarchy can be accessed in its "unpacked" form, where each directory and file exists in the filesystem separately(This format is more useful during development), or in its "packed" WAR file(This format is used when you distribute your application to be installed). Any Web Application that is to be deployed in Tomcat has to conform to this directory structure. The format of the structure can be seen in the figure below:

Web Application Directory Structure

• Application Root Directory - This is the root directory of the Web Application. The name of this directory is named by the developer - for example the default context we created for this module was called "student". All JSP, HTML and image files are stored here. Typically, there would be an index.html or index.jsp file in this directory. The "other directories" could be, for example, "images" (containing gifs and jpgs for the html pages and servlets used in the Application), "style" (containing the .css or other style files used) or simply subdirectories of HTML or JSP pages.
• /WEB-INF - This directory contains all resources related to the application that are not in the document root of the application. It should be noted that the WEB-INF directory is not part of the public document (ie. paths beginning in http://localhost:8080/student/WEB-INF cannot be viewed in a browser) and no files contained in this directory can be served directly to the client. The WEB-INF directory also contains the Web Application Deployment Descriptor.
• /WEB-INF/classes - contains the application Java Servlets, class files, JavaBeans and utility classes.
• /WEB-INF/lib - The lib directory contains the Java ARchive (JAR) files (or ZIP) that the web application depends upon. For example, this directory might contain a JDBC Database Connectivity driver in JAR or Zip format.
• /WEB-INF/web.xml - The deployment descriptor is an XML file named web.xml and forms the heart of the Web Application. This file is discussed below.

  Web Application Deployment Descriptor:

A web application's deployment descriptor describes the classes, resources and configuration of the application and how the web server uses them to serve web requests. When the web server receives a request for the application, it uses the deployment descriptor to map the URL of the request to the code that ought to handle the request. This file is named web.xml, and resides in the app's WAR under the WEB-INF/ directory. web.xml is part of the servlet standard for web applications.

1.     Configuring and Mapping a Servlet

2.     Servlet Init Parameters

3.     Servlet Load-on-Startup

4.     Context Parameters

Configuring and Mapping a Servlet

To configure a servlet in the web.xml file, you write this:

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app

    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"

    "http://java.sun.com/dtd/web-app_2_3.dtd"> 

<web-app>

  <servlet>

    <servlet-name>HelloWorld</servlet-name>

    <servlet-class>servletexample.createHelloWorldExample

      </servlet-class>

  </servlet>

  <servlet-mapping>

    <servlet-name>HelloWorld</servlet-name>

    <url-pattern>/HelloWorldExample/*</url-pattern>

  </servlet-mapping>

</web-app>   

Servlet-mapping has two child tags, url-pattern and servlet-name. url-pattern specifies the type of urls for which, the servlet given in servlet-name should be called. Be aware that, the container will use case-sensitive for string comparisons for servlet matching.

Servlet Init Parameters

You can pass parameters to a servlet from the web.xml file. The init parameters of a servlet can only be accessed by that servlet. Here is how you configure them in the web.xml file:

  <servlet>

    <servlet-name>HelloWorld</servlet-name>

    <servlet-class>servletexample.createHelloWorldExample

      </servlet-class>  

    <init-param>

        <param-name>myParam</param-name>

        <param-value>paramValue</param-value>

    </init-param>

</servlet>

Here is how you read the init parameters from inside your servlet - in the servlets init() method:

public class createHelloWorldExample  extends GenericServlet {

  protected String myParam = null;

  public void init(ServletConfig servletConfig) throws ServletException{

    this.myParam = servletConfig.getInitParameter("myParam");

  }

  public void service(ServletRequest request, ServletResponse response)

        throws ServletException, IOException {

    response.getWriter().write("<html><body>myParam = " +

            this.myParam + "</body></html>");

  }

}

A servlets init() method is called when the servlet container loads the servlet for the first time. No one can access the servlet until the servlet has been loaded, and the init() method has been called successfully.

Servlet Load-on-Startup

The <servlet> element has a subelement called <load-on-startup> which you can use to control when the servlet container should load the servlet. If you do not specify a <load-on-startup> element, the servlet container will typically load your servlet when the first request arrives for it.

By setting a <load-on-startup> element, you can tell the servlet container to load the servlet as soon as the servlet container starts. Remember, the servlets init() method is called when the servlet is loaded.

Here is an example <load-on-startup> configuration:

<servlet>

    <servlet-name>HelloWorld</servlet-name>

    <servlet-class>servletexample.createHelloWorldExample

      </servlet-class>

    <init-param><param-name>container.script.static</param-name>

                <param-value>/WEB-INF/container.script</param-value>

    </init-param>

    <load-on-startup>1</load-on-startup>

</servlet>

The number inside the <load-on-startup>1</load-on-startup> element tells the servlet container in what sequence the servlets should be loaded. The lower numbers are loaded first. If the value is negative, or unspecified, the servlet container can load the servlet at any time.

Context Parameters

You can also set some context parameters which can be read from all servlets in your application. Here is how you configure a context parameter:

<context-param>

    <param-name>myParam</param-name>

    <param-value>the value</param-value>

</context-param>

Here is how you access the parameter from inside an HttpServlet subclass:

String myContextParam   

                                       =request.getSession().getServletContext().getInitParameter("myParam");