Tech Erudition

Tomcat maps the static file request to DefaultServlet class (org.apache.catalina.servlets.DefaultServlet in Tomcat/server/lib/servlets-default.jar).

If you want to customize file serving the following steps need to be followed.
1. Mapping in web.xml for you application

<servlet>
  <servlet-name>CustomFileServlet</servlet-name>
  <servlet-class>com.example.servlet.CustomFileServlet</servlet-class>
    <init-param>
      <param-name>listing</param-name>
      <param-value>true</param-value>
    <!-- allow browsing directory inside web application -->
</servlet>
<servlet-mapping>
    <servlet-name>CustomeFileServlet</servlet-name>
    <url-pattern>/</url-pattern>  
    <!-- Maps all request except the ones which are mapped separately -->
</servlet-mapping>

2. Enable privilege for the application (since server library class is used).
Here is an example context file I created for such a application (Tomcat/conf/Catalina/localhost/customapp.xml)
<?xml version="1.0"?>
<context path="/customapp" docbase="/home/prasad/CustomApp/web" privileged="true" reloadable="true"></context>

3. Write a CustomFileServlet class.

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.catalina.servlets.DefaultServlet;
public class CustomFileServlet extends DefaultServlet {
    private static final long serialVersionUID = 1L;
    public void doPost(HttpServletRequest request, 
       HttpServletResponse response)
    throws IOException, ServletException {
        if (processRequest(request, response)) {
         super.doPost(request, response);
        }
    }

    public void doGet(HttpServletRequest request, 
      HttpServletResponse response)
    throws IOException, ServletException {
        if (processRequest(request, response)) {
 super.doGet(request, response);
        }
    }
    
    public boolean processRequest(HttpServletRequest request, 
               HttpServletResponse response) 
    throws IOException, ServletException {
     String requestURI = request.getRequestURI();
     String contextPath = request.getContextPath();
     
     String fileURI = "";
            if (requestURI.indexOf(contextPath) != -1) {
                // Request URI is CONTEXTPATH + RESOURCE URI
                fileURI = requestURI.substring(
                         requestURI.indexOf(contextPath) + 
                         contextPath.length());
            }
 
 boolean checkPassed = false;
 // DO SOME CHECKS BEFORE SERVING FILE...

        return checkPassed; 
    }