用来监听特定对象的创建或销毁、属性的变化

1.监听器接口的分类

1.1、监听对象创建/销毁的监听器接口

Interface ServletRequestListener     监听request对象的创建或销毁
Interface HttpSessionListener        监听session对象的创建或销毁
Interface ServletContextListener     监听servletContext对象的创建或销毁

1.2、监听对象属性的变化

Interface ServletRequestAttributeListener 监听request对象属性变化: 添加、移除、修改
Interface HttpSessionAttributeListener    监听session对象属性变化: 添加、移除、修改

Interface ServletContextAttributeListener 监听servletContext对象属性变化

1.3、session相关监听器

Interface HttpSessionBindingListener   监听对象绑定到session上的事件   
Interface HttpSessionActivationListener(了解) 监听session序列化及反序列化的事件

2.监听器的开发

2.1 步骤：

2.1.1.写一个普通java类，实现相关接口；
2.1.2.配置(web.xml)

2.2 实例

A.监听request对象的创建或销毁

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
public class MyRequestListener implements ServletRequestListener{

    // 对象销毁
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        // 获取request中存放的数据
        Object obj = sre.getServletRequest().getAttribute("cn");
        System.out.println(obj);
        System.out.println("MyRequestListener.requestDestroyed()");
    }

    // 对象创建
    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("MyRequestListener.requestInitialized()");
    }
}

<!-- 监听request对象创建、销毁 -->
    <listener>
        <listener-class>cn.acamy.a_life.MyRequestListener</listener-class>
    </listener>

B. 监听ServletContext对象创建或销毁

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyServletContextListener implements ServletContextListener{

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("MyServletContextListener.contextDestroyed()");
    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("1..........MyServletContextListener.contextInitialized()");
    }

}

<!-- 监听servletContext对象创建、销毁 -->
    <listener>
        <listener-class>cn.acamy.a_life.MyServletContextListener</listener-class>
    </listener>

C.监听Session对象创建、销毁

import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class MySessionListener implements HttpSessionListener{

    // 创建
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        System.out.println("MySessionListener.sessionCreated()");
    }

    // 销毁
    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        System.out.println("MySessionListener.sessionDestroyed()");
    }

}

<!-- 监听session对象创建、销毁 -->
    <listener>
        <listener-class>cn.itcast.a_life.MySessionListener</listener-class>
    </listener>

D.监听session对象属性的变化

import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

public class MySessionAttrListener implements HttpSessionAttributeListener {

    // 属性添加
    @Override
    public void attributeAdded(HttpSessionBindingEvent se) {
        // 先获取session对象
        HttpSession session = se.getSession();
        // 获取添加的属性
        Object obj = session.getAttribute("userName");
        // 测试
        System.out.println("添加的属性：" + obj);
    }

    // 属性移除
    @Override
    public void attributeRemoved(HttpSessionBindingEvent se) {
        System.out.println("属性移除");
    }

    // 属性被替换
    @Override
    public void attributeReplaced(HttpSessionBindingEvent se) {
        // 获取sesison对象
        HttpSession session = se.getSession();
        
        // 获取替换前的值
        Object old = se.getValue();
        System.out.println("原来的值：" + old);
        
        // 获取新值
        Object obj_new = session.getAttribute("userName");
        System.out.println("新值：" + obj_new);
        
    }

}

E.监听此对象绑定到session上的过程

import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

public class Admin implements HttpSessionBindingListener {

    private int id;
    private String name;
    
    public Admin() {
        super();
    }
    public Admin(int id, String name) {
        super();
        this.id = id;
        this.name = name;
    }
    
    
    // 构造函数
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
    // 对象放入session
    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        System.out.println("Admin对象已经放入session");
    }
    // 对象从session中移除
    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        System.out.println("Admin对象从session中移除！");
    }
    
}

注意：
因为监听的对象是自己创建的对象，不是服务器对象,所以不用再进行web.xml配置