用來監聽特定對象的創建或銷毀、屬性的變化

1.監聽器接口的分類

1.1、監聽對象創建/銷毀的監聽器接口

Interface ServletRequestListener     監聽request對象的創建或銷毀
Interface HttpSessionListener        監聽session對象的創建或銷毀
Interface ServletContextListener     監聽servletContext對象的創建或銷毀

1.2、監聽對象屬性的變化

Interface ServletRequestAttributeListener 監聽request對象屬性變化: 添加、移除、修改
Interface HttpSessionAttributeListener    監聽session對象屬性變化: 添加、移除、修改

Interface ServletContextAttributeListener 監聽servletContext對象屬性變化

1.3、session相關監聽器

Interface HttpSessionBindingListener   監聽對象綁定到session上的事件   
Interface HttpSessionActivationListener(了解) 監聽session序列化及反序列化的事件

2.監聽器的開發

2.1 步驟：

2.1.1.寫一個普通java類，實現相關接口；
2.1.2.配置(web.xml)

2.2 實例

A.監聽request對象的創建或銷毀

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

    // 對象銷毀
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        // 獲取request中存放的數據
        Object obj = sre.getServletRequest().getAttribute("cn");
        System.out.println(obj);
        System.out.println("MyRequestListener.requestDestroyed()");
    }

    // 對象創建
    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("MyRequestListener.requestInitialized()");
    }
}

<!-- 監聽request對象創建、銷毀 -->
    <listener>
        <listener-class>cn.acamy.a_life.MyRequestListener</listener-class>
    </listener>

B. 監聽ServletContext對象創建或銷毀

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()");
    }

}

<!-- 監聽servletContext對象創建、銷毀 -->
    <listener>
        <listener-class>cn.acamy.a_life.MyServletContextListener</listener-class>
    </listener>

C.監聽Session對象創建、銷毀

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

public class MySessionListener implements HttpSessionListener{

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

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

}

<!-- 監聽session對象創建、銷毀 -->
    <listener>
        <listener-class>cn.itcast.a_life.MySessionListener</listener-class>
    </listener>

D.監聽session對象屬性的變化

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

public class MySessionAttrListener implements HttpSessionAttributeListener {

    // 屬性添加
    @Override
    public void attributeAdded(HttpSessionBindingEvent se) {
        // 先獲取session對象
        HttpSession session = se.getSession();
        // 獲取添加的屬性
        Object obj = session.getAttribute("userName");
        // 測試
        System.out.println("添加的屬性：" + obj);
    }

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

    // 屬性被替換
    @Override
    public void attributeReplaced(HttpSessionBindingEvent se) {
        // 獲取sesison對象
        HttpSession session = se.getSession();
        
        // 獲取替換前的值
        Object old = se.getValue();
        System.out.println("原來的值：" + old);
        
        // 獲取新值
        Object obj_new = session.getAttribute("userName");
        System.out.println("新值：" + obj_new);
        
    }

}

E.監聽此對象綁定到session上的過程

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;
    }
    
    
    // 構造函數
    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;
    }
    
    // 對象放入session
    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        System.out.println("Admin對象已經放入session");
    }
    // 對象從session中移除
    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        System.out.println("Admin對象從session中移除！");
    }
    
}

注意：
因為監聽的對象是自己創建的對象，不是服務器對象,所以不用再進行web.xml配置