The "web.xml" is called web application deployment descriptor

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3.3 Configure the Application Deployment Descriptor - "web.xml"A web user invokes a servlet, which is kept in the web server, by issuing a specific URL from the browser. In this example, we shall configure the following request URL to trigger the "HelloServlet":

http://hostname:port/helloservlet/sayhello

Create a configuration file called "web.xml", and save it under "webappshelloservletWEB-INF", as follows:

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< ?xml version="1.0" encoding="ISO-8859-1"?> < web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"> < !-- To save as < CATALINA_HOME> webappshelloservletWEB-INFweb.xml --> < servlet> < servlet-name> HelloWorldServlet< /servlet-name> < servlet-class> mypkg.HelloServlet< /servlet-class> < /servlet> < !-- Note: All < servlet> elements MUST be grouped together and placed IN FRONT of the < servlet-mapping> elements --> < servlet-mapping> < servlet-name> HelloWorldServlet< /servlet-name> < url-pattern> /sayhello< /url-pattern> < /servlet-mapping> < /web-app>

The "web.xml" is called web application deployment descriptor. It provides the configuration options for that particular web application, such as defining the the mapping between URL and servlet class.
The above configuration defines a servlet named "HelloWroldServlet", implemented in "mypkg.HelloServlet.class" (written earlier), and maps to URL "/sayhello", where "/" denotes the context root of this webapp "helloservlet". In other words, the absolute URL for this servlet is http://hostname:port/helloservlet/sayhello.
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Take note that EACH servlet requires a pair of < servlet> and < servlet-mapping> elements to do the mapping, via an arbitrary but unique < servlet-name>. Furthermore, all the < servlet> elements must be grouped together and placed before the < servlet-mapping> elements (as specified in the XML schema).