35.监听器listener

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本文深入解析Servlet监听器的各类接口，包括监听对象创建/销毁、属性变化及session相关事件。通过具体实现类示例，展示如何监听request、session和servletContext对象的状态变更，以及属性的添加、移除和修改。

监听器接口：

一、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

二、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

三、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

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()");
	}
}

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()");
	}

}

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

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

2.属性类

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);
		
	}

}

<listener>
		<listener-class>cn.itcast.b_attr.MySessionAttrListener</listener-class>
	</listener>

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！");
	}
	
}