Example of Using Servlet Filters in a Dynamic Web Application

The following article demonstrates an Example of Using Servlet Filters in a Dynamic Web Application.

In order to create a Dynamic Web Application that makes use of Servlet Filters, follow these steps.

Firstly, open your IDE (e.g. Eclipse, IntelliJ, etc.) and create a new Dynamic Web Application project. Give it a name and select the appropriate web server runtime.
Then, create a new Servlet Filter by right-clicking on the project name in Package Explorer and selecting New > Servlet Filter. Give it a name (e.g. LoggingFilter) and click Finish.
After that, edit the LoggingFilter class to perform some pre-processing on the request data and post-processing on the response data. The following code shows an example.

import java.io.IOException;
import java.util.logging.Logger;
import javax.servlet.*;

public class LoggingFilter implements Filter {

  private static final Logger logger = Logger.getLogger(LoggingFilter.class.getName());

  @Override
  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
    throws IOException, ServletException {
    
    // Log the incoming request
    logger.info("Received request: " + request.toString());
    
    chain.doFilter(request, response);
    
    // Log the outgoing response
    logger.info("Sent response: " + response.toString());
  }

  @Override
  public void init(FilterConfig filterConfig) throws ServletException {
    // Initialization code goes here (optional)
  }

  @Override
  public void destroy() {
    // Cleanup code goes here (optional)
  }
}

Map the filter to a servlet or URL pattern in the web.xml deployment descriptor file. The following code shows it.

<web-app>
  ...
  <filter>
    <filter-name>LoggingFilter</filter-name>
    <filter-class>com.example.LoggingFilter</filter-class>
  </filter>
  
  <filter-mapping>
    <filter-name>LoggingFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  ...
</web-app>

This maps the LoggingFilter to all requests that come into the server.

Create a servlet that will respond to a request. The following code demonstrates it.

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

public class MyServlet extends HttpServlet {

  @Override
  protected void doGet(HttpServletRequest request, HttpServletResponse response) 
    throws ServletException, IOException {
    
    response.getWriter().write("Hello, world!");
  }
}

After that, map the servlet to a URL pattern in the web.xml deployment descriptor file. The following example shows it.

<web-app>
  ...
  <servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>com.example.MyServlet</servlet-class>
  </servlet>
  
  <servlet-mapping>
    <servlet-name>MyServlet</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
  ...
</web-app>

This maps the MyServlet to the “/hello” URL.

Finally, deploy the application to your web server and access the URL in a web browser. You should see the “Hello, world!” message displayed in the browser.

Then, check the logs (e.g. in the console or a log file) and you should see log messages generated by the LoggingFilter for the incoming request and outgoing responses.

That’s it! You have created a Dynamic Web Application that makes use of a Servlet Filter.

Further Reading