JavaWeb 监听器

按监听的对象划分

1.用于监听应用程序环境对象（ServletContext）的事件监听器，实现ServletContextListener接口

public class MyServletContextListener implements ServletContextListener {

    public void contextInitialized(ServletContextEvent servletcontextevent) {
        String initParam = servletcontextevent.getServletContext().getInitParameter("initParam");
        System.out.println("contextInitialized : initParam = "+initParam);
    }

    public void contextDestroyed(ServletContextEvent servletcontextevent) {
        System.out.println("contextDestroyed");
    }

}

2.用于监听用户会话对象（HttpSession）的事件监听器，实现HttpSessionListener接口

public class MyHttpSessionListener implements HttpSessionListener {

    public void sessionCreated(HttpSessionEvent httpsessionevent) {
        System.out.println("sessionCreated");
    }

    public void sessionDestroyed(HttpSessionEvent httpsessionevent) {
        System.out.println("sessionDestroyed");
    }

}

3.用于监听请求消息对象（ServletRequest）的事件监听器，实现ServletRequestListener接口

public class MyServletRequestListener implements ServletRequestListener {

    public void requestDestroyed(ServletRequestEvent servletrequestevent) {
        System.out.println("requestDestroyed ");
    }

    public void requestInitialized(ServletRequestEvent servletrequestevent) {
        String name = servletrequestevent.getServletRequest().getParameter("name");
        System.out.println("requestInitialized name:"+name);
    }

}

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按监听的事件划分

1.监听域对象自身的创建和销毁的事件监听器

即按监听对象划分的那几种

2.监听域对象的属性的增加和删除的事件监听器，实现ServletContextAttributeListener，HttpSessionAttributeListener 或 ServletRequestAttributeListener接口

public class MyServletContextAttributeListener implements ServletContextAttributeListener {

    public void attributeAdded(ServletContextAttributeEvent servletcontextattributeevent) {
        System.out.println("ServletContext_attributeAdded:"+servletcontextattributeevent.getName());
    }

    public void attributeRemoved(ServletContextAttributeEvent servletcontextattributeevent) {
        System.out.println("ServletContext_attributeRemoved:"+servletcontextattributeevent.getName());

    }

    public void attributeReplaced(ServletContextAttributeEvent servletcontextattributeevent) {
        System.out.println("ServletContext_attributeReplaced:"+servletcontextattributeevent.getName());

    }

}

HttpSession 和 ServletRequest 同理，只是方法参数类型不同

3.监听绑定到HttpSession域中的某个对象的状态的事件监听器

这种情况不需要专门设计一个作为监听器的类，可以作为一个实体类，然后继承需要的接口：

public class User implements HttpSessionBindingListener,HttpSessionActivationListener,Serializable {

    private String username;
    
    public void valueBound(HttpSessionBindingEvent httpsessionbindingevent) {
        System.out.println("valueBound Name:"+httpsessionbindingevent.getName());
    }

    public void valueUnbound(HttpSessionBindingEvent httpsessionbindingevent) {
        System.out.println("valueUnbound Name:"+httpsessionbindingevent.getName());
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    //钝化
    public void sessionWillPassivate(HttpSessionEvent httpsessionevent) {
        System.out.println("sessionWillPassivate "+httpsessionevent.getSource());
    }
    //活化
    public void sessionDidActivate(HttpSessionEvent httpsessionevent) {
        System.out.println("sessionDidActivate "+httpsessionevent.getSource());
    }
}

绑定和解除绑定：实现HttpSessionBindingListener接口
钝化和活化：实现HttpSessionActivationListener和Serializable接口

实现Serializable接口是因为钝化时需要将seesion序列化存储到文件或者数据库，活化时需要反序列化

参阅：
慕课网：JAVA Web开发技术应用——监听器