Where is it declared?

It is declared globally in $CATALINA_BASE/conf/web.xml. By default here is it’s declaration:

 

<servlet>

<servlet-name>default</servlet-name>

<servlet-class>

org.apache.catalina.servlets.DefaultServlet

</servlet-class>

<init-param>

<param-name>debug</param-name>

<param-value>0</param-value>

</init-param>

<init-param>

<param-name>listings</param-name>

<param-value>false</param-value>

</init-param>

<load-on-startup>1</load-on-startup>

</servlet>

 

…

 

<servlet-mapping>

<servlet-name>default</servlet-name>

<url-pattern>/</url-pattern>

</servlet-mapping>

 

So by default, the default servlet is loaded at webapp startup and directory listings are disabled and debugging is turned off.