How is JSP configuration performed in the deployment descriptor?

The JSP configuration in the deployment descriptor web.xml is done using <servlet> and <servlet-mapping> elements. The JSP file is processed as a special type of servlet.

1. Declaring JSP as a servlet:

   The <servlet> element is used to declare the resource (<servlet-class> points to the servlet implementation, but for JSP it is provided by the container) and assign it a logical name (<servlet-name>). In the case of JSP, the class name is not specified, and the <jsp-file> element is used.

   <!--
    * Declaring a JSP file as a servlet
   -->
   <servlet>
       <servlet-name>myJspPage</servlet-name>
       <jsp-file>/myJsp.jsp</jsp-file> <!-- Path to the JSP file relative to the web application root -->
       <!--
        * Optional initialization parameters for the JSP page
       -->
       <init-param>
           <param-name>paramName</param-name>
           <param-value>paramValue</param-value>
       </init-param>
   </servlet>
   

2. Mapping URL pattern to the JSP servlet:

   The <servlet-mapping> element associates the servlet's logical name (<servlet-name>) with a URL pattern (<url-pattern>), through which this servlet (and consequently, the JSP) will be accessible.

   <!--
    * Mapping a URL pattern to the declared JSP servlet
   -->
   <servlet-mapping>
       <servlet-name>myJspPage</servlet-name>
       <url-pattern>/mypage</url-pattern> <!-- URL where the JSP will be accessible -->
   </servlet-mapping>
   

3. Other configurations:

   You can also configure other aspects, such as:

   • <load-on-startup>: Defines the order of JSP loading (lower values load earlier).
   • <security-constraint>: Applies security restrictions.
   • <error-page>: Defines error pages.

Full configuration example:

<!--
 * Deployment descriptor for the web application
-->
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <!--
     * Defining a JSP file as a servlet "myJspPage"
    -->
    <servlet>
        <servlet-name>myJspPage</servlet-name> <!-- Logical name of the servlet -->
        <jsp-file>/myJsp.jsp</jsp-file> <!-- Path to the JSP file -->
        <load-on-startup>1</load-on-startup> <!-- Load on startup -->
    </servlet>

    <!--
     * Mapping URL pattern "/mypage" to the servlet "myJspPage"
    -->
    <servlet-mapping>
        <servlet-name>myJspPage</servlet-name>
        <url-pattern>/mypage</url-pattern> <!-- Accessible URL -->
    </servlet-mapping>

    <!--
     * Example of another mapping for a different JSP
    -->
    <servlet-mapping>
        <servlet-name>anotherJsp</servlet-name>
        <url-pattern>/another</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>anotherJsp</servlet-name>
        <jsp-file>/anotherPage.jsp</jsp-file>
    </servlet>

</web-app>

Thus, web.xml manages how JSP files are transformed into servlets and which URLs invoke their execution, as well as configuring their initialization and loading behavior.