ConcretePage.com@WebListener Annotation in Servlet 3
January 14, 2015
web.xml
We are adding some initialization parameter in web.xml. Find the web.xml
<?xml version="1.0" encoding="ISO-8859-1" ?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
version="3.0">
<display-name>Servlet 3</display-name>
<context-param>
<param-name>path</param-name>
<param-value>C:/filePath</param-value>
</context-param>
<context-param>
<param-name>mode</param-name>
<param-value>read</param-value>
</context-param>
</web-app>
Create Listener for ServletContextListener
In our example we are creating listener using ServletContextListener. To create listener, we create a class annotated with @WebListener and implement ServletContextListener. We have to override the methods contextInitialized(ServletContextEvent sce) and contextDestroyed(ServletContextEvent sce) . In our example, we will fetch init parameter from web.xml and will set it in servlet context.MyListener.java
package com.concretepage;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class MyListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
ServletContext sc = sce.getServletContext();
String path = sc.getInitParameter("path");
String mode = sc.getInitParameter("mode");
sc.setAttribute("filePath", path);
sc.setAttribute("fileMode", mode);
System.out.println("Value saved in context.");
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
ServletContext sc = sce.getServletContext();
sc.removeAttribute("path");
sc.removeAttribute("mode");
System.out.println("Value deleted from context.");
}
}
Create Servlet
We are creating a servlet and accessing the attribute set by listener.MyServlet.java
package com.concretepage;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletContext;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(urlPatterns = "/HelloWorldServlet", loadOnStartup = 1)
public class MyServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException{
doGet(request,response);
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException{
response.setContentType("text/html");
ServletContext sc = request.getServletContext();
System.out.println("Path:"+ sc.getAttribute("filePath"));
System.out.println("Mode:"+ sc.getAttribute("fileMode"));
PrintWriter out = response.getWriter();
out.println("Hello World!");
}
}
Output
Our ServletContextListener will be processed at server start and contextInitialized method will run and output will be as below.------------------ ------------------ Value saved in context. ------------------ ------------------
Path:C:/filePath Mode:read
------------------ ------------------ Value deleted from context. ------------------ ------------------
