Listener可以监听容器中某一执行动作，并根据其要求做出相应的响应。 常用的Web事件的监听接口如下：
ServletContextListener：用于监听Web的启动及关闭 ServletContextAttributeListener：
用于监听ServletContext范围内属性的改变 ServletRequestListener：
用于监听用户请求 ServletRequestAttributeListener：
用于监听ServletRequest范围属性的改变 HttpSessionListener：
用于监听用户session的开始及结束 HttpSessionAttributeListener：
用于监听HttpSession范围内的属性改变：
栗子：参考https://www.journaldev.com/1945/servletcontextlistener-servlet-listener-example#servlet-listener-example
import java.sql.Connection;
public class DBConnectionManager {
private String dbURL;
private String user;
private String password;
private Connection con;
public DBConnectionManager(String url, String u, String p){
this.dbURL=url;
this.user=u;
this.password=p;
//create db connection now
}
public Connection getConnection(){
return this.con;
}
public void closeConnection(){
//close DB connection here
}
}
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class AppContextAttributeListener implements ServletContextAttributeListener {
public void attributeAdded(ServletContextAttributeEvent servletContextAttributeEvent) {
System.out.println("ServletContext attribute added::{"+servletContextAttributeEvent.getName()+","+servletContextAttributeEvent.getValue()+"}");
}
public void attributeReplaced(ServletContextAttributeEvent servletContextAttributeEvent) {
System.out.println("ServletContext attribute replaced::{"+servletContextAttributeEvent.getName()+","+servletContextAttributeEvent.getValue()+"}");
}
public void attributeRemoved(ServletContextAttributeEvent servletContextAttributeEvent) {
System.out.println("ServletContext attribute removed::{"+servletContextAttributeEvent.getName()+","+servletContextAttributeEvent.getValue()+"}");
}
}
@WebListener
public class AppContextListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent servletContextEvent) {
ServletContext ctx = servletContextEvent.getServletContext();
String url = ctx.getInitParameter("DBURL");
String u = ctx.getInitParameter("DBUSER");
String p = ctx.getInitParameter("DBPWD");
//create database connection from init parameters and set it to context
DBConnectionManager dbManager = new DBConnectionManager(url, u, p);
ctx.setAttribute("DBManager", dbManager);
System.out.println("Database connection initialized for Application.");
}
public void contextDestroyed(ServletContextEvent servletContextEvent) {
ServletContext ctx = servletContextEvent.getServletContext();
DBConnectionManager dbManager = (DBConnectionManager) ctx.getAttribute("DBManager");
dbManager.closeConnection();
System.out.println("Database connection closed for Application.");
}
}
import javax.servlet.ServletRequest;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class MyServletRequestListener implements ServletRequestListener {
public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
ServletRequest servletRequest = servletRequestEvent.getServletRequest();
System.out.println("ServletRequest destroyed. Remote IP="+servletRequest.getRemoteAddr());
}
public void requestInitialized(ServletRequestEvent servletRequestEvent) {
ServletRequest servletRequest = servletRequestEvent.getServletRequest();
System.out.println("ServletRequest initialized. Remote IP="+servletRequest.getRemoteAddr());
}
}
@WebListener
public class MySessionListener implements HttpSessionListener {
public void sessionCreated(HttpSessionEvent sessionEvent) {
System.out.println("Session Created:: ID="+sessionEvent.getSession().getId());
}
public void sessionDestroyed(HttpSessionEvent sessionEvent) {
System.out.println("Session Destroyed:: ID="+sessionEvent.getSession().getId());
}
}
@WebServlet("/MyServlet")
public class MyServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
System.out.println("begin to handle request....");
ServletContext ctx = request.getServletContext();
ctx.setAttribute("User", "Pankaj");
String user = (String) ctx.getAttribute("User");
ctx.removeAttribute("User");
HttpSession session = request.getSession();
session.invalidate();
PrintWriter out = response.getWriter();
out.write("Hi "+user);
}
}
web.xml:
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<display-name>ServletListenerExample</display-name>
<context-param>
<param-name>DBUSER</param-name>
<param-value>pankaj</param-value>
</context-param>
<context-param>
<param-name>DBPWD</param-name>
<param-value>password</param-value>
</context-param>
<context-param>
<param-name>DBURL</param-name>
<param-value>jdbc:mysql://localhost/mysql_db</param-value>
</context-param>
<listener>
<listener-class>com.journaldev.listener.AppContextListener</listener-class>
</listener>
<listener>
<listener-class>com.journaldev.listener.AppContextAttributeListener</listener-class>
</listener>
<listener>
<listener-class>com.journaldev.listener.MySessionListener</listener-class>
</listener>
<listener>
<listener-class>com.journaldev.listener.MyServletRequestListener</listener-class>
</listener>
</web-app>
启动时：
ServletContext attribute added::{DBManager,com.journaldev.db.DBConnectionManager@18140878}
Database connection initialized for Application.
ServletContext attribute added::{org.apache.jasper.compiler.TldLocationsCache,org.apache.jasper.compiler.TldLocationsCache@45cc5809}
浏览器输入：http://localhost:8084/ServletListenerExample/MyServlet
输出：
ServletRequest initialized. Remote IP=127.0.0.1
begin to handle request....
ServletContext attribute added::{User,Pankaj}
ServletContext attribute removed::{User,Pankaj}
Session Created:: ID=349003AB937F7C54BCCB4DE4506F5958
Session Destroyed:: ID=349003AB937F7C54BCCB4DE4506F5958
ServletRequest destroyed. Remote IP=127.0.0.1
关闭时：Database connection closed for Application.