1、Listener 用于监听 Java web程序中的事件，例如创建、修改、删除Session、request、context等，并触发响应的事件。
2、 Listener 对应观察者模式，事件发生的时候会自动触发该事件对应的Listeer。 Listener 主要用于对 Session、request、context 进行监控。servlet2.5 ~ 5.0规范中共有 8 种Listener 。
2.实现1、不同功能的Listener 需要实现不同的 Listener 接口，一个Listener也可以实现多个接口，这样就可以多种功能的监听器一起工作。
2、8种监听器可以分为三类：
1）监听 Session、request、context 的创建于销毁，分别为
HttpSessionLister、ServletContextListener、ServletRequestListener
2）监听对象属性变化，分别为：
HttpSessionAttributeLister、ServletContextAttributeListener、ServletRequestAttributeListener
3）监听Session 内的对象，分别为HttpSessionBindingListener 和 HttpSessionActivationListener。与上面六类不同，这两类 Listener 监听的是Session 内的对象，而非 Session 本身，不需要在 web.xml中配置。
2、实现web.xml的Listener配置。
1）
2）
package servlet.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyListener implements ServletContextListener {
public void contextDestroyed(ServletContextEvent sce) { }
public void contextInitialized(ServletContextEvent sce) { }
}
HttpSessionLister、ServletContextListener、ServletRequestListener
package servlet.listener;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class ListenerTest implements HttpSessionListener, ServletContextListener, ServletRequestListener {
Log log = LogFactory.getLog(getClass());
// 创建 session
public void sessionCreated(HttpSessionEvent se) {
HttpSession session = se.getSession();
log.info("新创建一个session, ID为: " + session.getId()); }
// 销毁 session
public void sessionDestroyed(HttpSessionEvent se) {
HttpSession session = se.getSession();
log.info("销毁一个session, ID为: " + session.getId()); }
// 加载 context
public void contextInitialized(ServletContextEvent sce) {
ServletContext servletContext = sce.getServletContext();
log.info("即将启动" + servletContext.getContextPath()); }
// 卸载 context
public void contextDestroyed(ServletContextEvent sce) {
ServletContext servletContext = sce.getServletContext();
log.info("即将关闭" + servletContext.getContextPath()); }
// 创建 request
public void requestInitialized(ServletRequestEvent sre) {
HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
String uri = request.getRequestURI();
uri = request.getQueryString() == null ? uri : (uri + "?" + request.getQueryString());
request.setAttribute("dateCreated", System.currentTimeMillis());
log.info("IP " + request.getRemoteAddr() + " 请求 " + uri); }
// 销毁 request
public void requestDestroyed(ServletRequestEvent sre) {
HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
long time = System.currentTimeMillis() - (Long) request.getAttribute("dateCreated");
log.info(request.getRemoteAddr() + "请求处理结束, 用时" + time + "毫秒. "); }
}
HttpSessionAttributeLister、ServletContextAttributeListener、ServletRequestAttributeListener
package servlet.listener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class SessionAttributeListenerTest implements HttpSessionAttributeListener {
Log log = LogFactory.getLog(getClass());
// 添加属性
public void attributeAdded(HttpSessionBindingEvent se) {
HttpSession session = se.getSession();
String name = se.getName();
log.info("新建session属性：" + name + ", 值为：" + se.getValue());
}
// 删除属性
public void attributeRemoved(HttpSessionBindingEvent se) {
HttpSession session = se.getSession();
String name = se.getName();
log.info("删除session属性：" + name + ", 值为：" + se.getValue());
}
// 修改属性
public void attributeReplaced(HttpSessionBindingEvent se) {
HttpSession session = se.getSession();
String name = se.getName();
Object oldValue = se.getValue();
log.info("修改session属性：" + name + ", 原值：" + oldValue + ", 新值：" + session.getAttribute(name));
}
}
HttpSessionBindingListener 和 HttpSessionActivationListener
package servlet.listener;
import java.io.Serializable;
import java.util.Date;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionActivationListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
import javax.servlet.http.HttpSessionEvent;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class PersonInfo implements HttpSessionActivationListener, HttpSessionBindingListener, Serializable {
private static final long serialVersionUID = -4780592776386225973L;
Log log = LogFactory.getLog(getClass());
private String name;
private Date dateCreated;
// 从硬盘加载后
public void sessionDidActivate(HttpSessionEvent se) {
HttpSession session = se.getSession();
log.info(this + "已经成功从硬盘中加载。sessionId: " + session.getId());
}
// 即将被钝化到硬盘时
public void sessionWillPassivate(HttpSessionEvent se) {
HttpSession session = se.getSession();
log.info(this + "即将保存到硬盘。sessionId: " + session.getId()); }
// 被放进session前
public void valueBound(HttpSessionBindingEvent event) {
HttpSession session = event.getSession();
String name = event.getName();
log.info(this + "被绑定到session "" + session.getId() + ""的" + name + "属性上");
// 记录放到session中的时间
this.setDateCreated(new Date()); }
// 从session中移除后
public void valueUnbound(HttpSessionBindingEvent event) {
HttpSession session = event.getSession();
String name = event.getName();
log.info(this + "被从session "" + session.getId() + ""的" + name + "属性上移除"); }
@Override
public String toString() {
return "PersonInfo(" + name + ")"; }
public String getName() {
return name; }
public void setName(String name) {
this.name = name; }
public Date getDateCreated() {
return dateCreated; }
public void setDateCreated(Date dateCreated) {
this.dateCreated = dateCreated; } }
监听器Listener就是在application,session,request三个对象创建、销毁或者往其中添加修改删除属性时自动执行代码的功能组件。
Listener是Servlet的监听器，可以监听客户端的请求，服务端的操作等。
Listener实现了javax.servlet.ServletContextListener 接口的服务器端程序，它也是随web应用的启动而启动，只初始化一次，随web应用的停止而销毁。主要作用是：做一些初始化的内容添加工作、设置一些基本的内容、比如一些参数或者是一些固定的对象等等。
