Why do we use web.xml?

Generally speaking, this is the configuration file of web applications in java. It instructs the servlet container (tomcat for ex.) which classes to load, what parameters to set in the context, and how to intercept requests coming from browsers.

There you specify:

• what servlets (and filters) you want to use and what URLs you want to map them to
• listeners - classes that are notified when some events happen (context starts, session created, etc)
• configuration parameters (context-params)
• error pages, welcome files
• security constraints

In servlet 3.0 many of the web.xml parts are optional. These configurations can be done via annotations (@WebServlet, @WebListener)

It says all the requests to go through WicketFilter

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Also, if you use wicket WicketApplication for application level settings. Like URL patterns and things that are true at app level

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This is what you need really, http://wicket.apache.org/learn/examples/helloworld.html

The web.xml file is the deployment descriptor for a Servlet-based Java web application (which most Java web apps are). Among other things, it declares which Servlets exist and which URLs they handle.

The part you cite defines a Servlet Filter. Servlet filters can do all kinds of preprocessing on requests. Your specific example is a filter had the Wicket framework uses as its entry point for all requests because filters are in some way more powerful than Servlets.

Servlet to be accessible from a browser, then must tell the servlet container what servlets to deploy, and what URL's to map the servlets to. This is done in the web.xml file of your Java web application.

use web.xml in servlet

<servlet>
    <description></description>
    <display-name>servlet class name</display-name>
    <servlet-name>servlet class name</servlet-name>
    <servlet-class>servlet package name/servlet class name</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>servlet class name</servlet-name>
    <url-pattern>/servlet class name</url-pattern>
</servlet-mapping>

manly use web.xml for servlet mapping.