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

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.
• 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).

https://www.ntu.edu.sg/home/ehchua/programming/java/JavaServlets.html