From http://javaevangelist.blogspot.com/2014/01/sourceservlet-displaying-source-code.html

SourceServlet.java

package com.bluelotussoftware.example.richfaces;
 
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLDecoder;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
/**
 * A servlet that displays XHTML/Java Source code as plain text from the project
 * in which the servlet is included.
 *
 * @author John Yeary
 * @version 1.0
 */
@WebServlet(name = "SourceServlet", urlPatterns = {"/source/*"})
public class SourceServlet extends HttpServlet {
 
    private static final long serialVersionUID = 3859447979763694232L;
 
    /**
     * Processes requests for both HTTP
     * <code>GET</code> and
     * <code>POST</code> methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String basePath = getServletContext().getRealPath("/");
 
 
        if (request.getPathInfo() == null || "/".equals(request.getPathInfo())) {
            //NOI18N
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "An file name is required.");
            return;
        }
 
        String filename = URLDecoder.decode(request.getPathInfo(), "UTF-8");
        File file = new File(basePath, filename);
 
        if (!file.exists()) {
            //NOI18N
            response.sendError(HttpServletResponse.SC_NOT_FOUND, file.getName() + " was not found.");
            return;
        }
        response.setContentType("text/plain");
        response.setContentLength((int) file.length());
        response.setHeader("Content-Disposition", "inline; filename=\"" + file.getName() + "\"");
 
        BufferedInputStream input = null;
        BufferedOutputStream output = null;
 
        try {
            input = new BufferedInputStream(new FileInputStream(file));
            output = new BufferedOutputStream(response.getOutputStream());
 
            byte[] buffer = new byte[8192];
            int length;
            while ((length = input.read(buffer)) > 0) {
                output.write(buffer, 0, length);
            }
        } finally {
            if (output != null) {
                try {
                    output.close();
                } catch (IOException ignore) {
                }
            }
            if (input != null) {
                try {
                    input.close();
                } catch (IOException ignore) {
                }
            }
        }
    }
 
    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }
 
    /**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }
 
    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "This servlet will serve source code from a project as text/plain.";
    }// </editor-fold>
}