Using a Java Servlet Filter to intercept the response HTTP status code with Netbeans 7 and Maven

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Version 2.3 of the Java servlet spec introduced the concept of filters. According to the documentation from Oracle’s site: “A filter dynamically intercepts requests and responses to transform or use the information contained in the requests or responses”. Today I’ll show you how to build a simple filter to intercept the response HTTP response code using annotations introduced in the Servlet 3.0 specification.


With Netbeans 7 create a new Maven Java Web Application called: Intercept

Delete the index.jsp file under the Web Pages folder. Right-click on the project and add a new servlet called: MainServlet

Since we are using the new Servlet 3 annotations we don’t need to set a whole lot of properties.

Maven generates a decent MainServlet.java file for us, I just removed the comments for the output. My file looks like this:

package com.giantflyingsaucer.intercept;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(name = "MainServlet", urlPatterns = {"/"})
public class MainServlet extends HttpServlet {

    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        try {

            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet MainServlet</title>");  
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Servlet MainServlet</h1>");
            out.println("</body>");
            out.println("</html>");
        } finally {            
            out.close();
        }
    }

    // <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 "Short description";
    }// </editor-fold>
}

Right-click on the project and add a Filter called: InterceptFilter

We will add the following two lines to the doFilter method.

HttpServletResponse hsr = (HttpServletResponse) response;
System.out.println("HTTP Status: " + hsr.getStatus());

My doFilter method looks like this:

@Override
public void doFilter(ServletRequest request, ServletResponse response,
        FilterChain chain)
        throws IOException, ServletException {

    if (debug) {
        log("InterceptFilter:doFilter()");
    }

    doBeforeProcessing(request, response);

    HttpServletResponse hsr = (HttpServletResponse) response;
    System.out.println("HTTP Status: " + hsr.getStatus());        

    Throwable problem = null;
    try {
        chain.doFilter(request, response);
    } catch (Throwable t) {
        problem = t;
        t.printStackTrace();
    }

    doAfterProcessing(request, response);

    if (problem != null) {
        if (problem instanceof ServletException) {
            throw (ServletException) problem;
        }
        if (problem instanceof IOException) {
            throw (IOException) problem;
        }
        sendProcessingError(problem, response);
    }
}

Clean and Build the project and deploy it to Apache Tomcat. Access the URL with a browser and take a look at your catalina.out file and you should see the HTTP response code.

Note: You shouldn’t need to do any changes to the web.xml file for this project to work.

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