How is a JSP page detected and converted to a Servlet by Tomcat?

Question

Is a JSP page detected by the page extension of .jsp only? Is there any other way it can be detected?

Answer 1

JSP pages in Tomcat are handled by a specific servlet that is meant to handle all requests that terminate with .jsp or .jspx in the HTTP request. This configuration exists in the global $CATALINA\conf\web.xml file where one can find the following significant lines. Note that is for Tomcat 6.

JSP Servlet registration

<servlet>
    <servlet-name>jsp</servlet-name>
    <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
    <init-param>
        <param-name>fork</param-name>
        <param-value>false</param-value>
    </init-param>
    <init-param>
        <param-name>xpoweredBy</param-name>
        <param-value>false</param-value>
    </init-param>
    <load-on-startup>3</load-on-startup>
</servlet>

JSP Servlet URL mapping

<!-- The mapping for the JSP servlet -->
<servlet-mapping>
    <servlet-name>jsp</servlet-name>
    <url-pattern>*.jsp</url-pattern>
</servlet-mapping>

<servlet-mapping>
    <servlet-name>jsp</servlet-name>
    <url-pattern>*.jspx</url-pattern>
</servlet-mapping>

You could possibly add more URL mappings for other file extensions that are not already mapped to other servlets, in order to trigger the Jasper compiler, that is eventually responsible for translation of the JSP files into corresponding Java servlets, which are then compiled (using the Eclipse JDT compiler, by default). More information on configuring some of the options in the process can be obtained from the Tomcat documentation on configuring Jasper.

Answer 2

Here's a brief introduction from Built In Servlet Definitions section in $TOMCAT_HOME/conf/web.xml

The JSP page compiler and execution servlet, which is the mechanism
used by Tomcat to support JSP pages.  Traditionally, this servlet
is mapped to the URL pattern "*.jsp".

And JSP page detection is done via servlet mapping (Built In Servlet Mappings section in $TOMCAT_HOME/conf/web.xml):

<!-- The mapping for the JSP servlet -->
<servlet-mapping>
    <servlet-name>jsp</servlet-name>
    <url-pattern>*.jsp</url-pattern>
</servlet-mapping>

<servlet-mapping>
    <servlet-name>jsp</servlet-name>
    <url-pattern>*.jspx</url-pattern>
</servlet-mapping>