JavaWeb：Listener监听器ServletContextListener、ServletRequestListener、HttpSessionListener

一、 Context、Request、Session作用域与生命周期

在介绍JavaWeb：Listener监听器ServletContextListener、ServletRequestListener、HttpSessionListener之前，需要对 Context、Request、Session 对象的作用域和生命周期有基础的了解。这里先回顾下三者的生命周期。

类别	init 初始化	destroy销毁	作用域
Context	web项目服务启动后	web项目关闭	整个application项目
Request	每个请求进来时	每个请求结束时	只在单个Request对象线程中
Session	当Request请求创建Session时	当Session失效过期时	只在具体的Session对象范围内。

初步简单的回顾下 Context、Request、Session 对象的作用域和生命周期。
接下来是介绍Listener监听器。

二、JavaWeb内的Listener们

主要介绍以下6个Listener监听器：
ServletContextListener、ServletContextAttributeListener
ServletRequestListener、ServletRequestAttributeListener
HttpSessionListener、HttpSessionAttributeListener

这里按两个维度理解这几个Listener监听器。

① 生命周期维度

ServletContextListener、ServletRequestListener、HttpSessionListener 这三个都是按照各自的生命周期中init初始化和destroy销毁时触发对应的方法。
如下图：

ServletContextListner

ServletRequestListener

HttpSessionListener

这三者的接口中都是init初始化/create创建方法和 destroyed销毁方法，都是在对应生命周期的对应时刻触发。

② 字段变更维度

ServletContextAttributeListener、ServletRequestAttributeListener、HttpSessionAttributeListener 这三个是安装各自在字段变更时触发对应的 Added()、Removed()、Replaced() 方法。

ServletContextAttributeListener

ServletRequestAttributeListener

HttpSessionAttributeListener

这三者的接口中都是在字段属性发生变更时触发对应的 Added()、Removed()、Replaced() 方法。

三、测试代码

① MyContextListener 类

public class MyContextListener implements ServletContextListener, ServletContextAttributeListener {
    @Override
    public void attributeAdded(ServletContextAttributeEvent scae) {
        System.out.println("context [added] name: " + scae.getName() + " = " + scae.getValue());
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent scae) {
        System.out.println("context [removed] name: " + scae.getName() + " = " + scae.getValue());

    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent scae) {
        System.out.println("context [replaced] name: " + scae.getName() + " = " + scae.getValue());

    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("context [init] name: " + sce.getServletContext());
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("context [destroyed] name: " + sce.getServletContext());
    }
}

② MyRequestListener类

public class MyRequestListener implements ServletRequestListener,ServletRequestAttributeListener  {
    @Override
    public void attributeAdded(ServletRequestAttributeEvent srae) {
        System.out.println("request [added] name: " + srae.getName() + " = " + srae.getValue());
    }

    @Override
    public void attributeRemoved(ServletRequestAttributeEvent srae) {
        System.out.println("request [removed] name: " + srae.getName() + " = " + srae.getValue());
    }

    @Override
    public void attributeReplaced(ServletRequestAttributeEvent srae) {
        System.out.println("request [replaced] name: " + srae.getName() + " = " + srae.getValue());
    }

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("request [destroyed] name: " +  sre.getServletRequest());
    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("request [init] name: " + sre.getServletRequest());
    }
}

③ MySessionListener类

public class OnlineCountListener implements HttpSessionListener, HttpSessionAttributeListener {
    @Override
    public void attributeAdded(HttpSessionBindingEvent se) {
        System.out.println("http session attribute add : " + se.getName() + " = " + se.getValue());
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent se) {
        System.out.println("http session attribute removed : " + se.getName() + " = " + se.getValue());
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent se) {
        System.out.println("http session attribute replaced : " + se.getName() + " = " + se.getValue());
    }

    @Override
    public void sessionCreated(HttpSessionEvent se) {
        System.out.println("http session [created] :" + se.getSession().getId());
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        System.out.println("http session [destroyed] :" + se.getSession().getId());
    }
}

④ 用于访问的OnlineServlet类

public class OnlineServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    //  context测试
        getServletContext().setAttribute("context", "context");
        getServletContext().setAttribute("context", "context");
        getServletContext().removeAttribute("context");

    //  reqeust测试
        req.setAttribute("req", "req");
        req.setAttribute("req", "req");
        req.removeAttribute("req");

    //  session测试
        HttpSession session = req.getSession();
        session.setAttribute("a", "a");
        session.setAttribute("a", "b");
        session.removeAttribute("a");
        session.setMaxInactiveInterval(3);

        resp.getWriter().write(" sessionId: " + session.getId());
    }
}

⑤ web.xml配置

    
        com.jx.listener.MyRequestListener
    
    
        com.jx.listener.MyContextListener
    
    
        com.jx.listener.MySessionListener
    

    
        online
        com.jx.listener.OnlineServlet
    
    
        online
        /online

① 运行结果

启动项目-> 访问Servlet -> 关闭项目

context [init] name: org.apache.catalina.core.ApplicationContextFacade@149c182
context [add] name: org.apache.jasper.compiler.TldLocationsCache = org.apache.jasper.compiler.TldLocationsCache@12012fb
request [init] name: org.apache.catalina.connector.RequestFacade@1ed2fb0
request [replaced] name: org.apache.catalina.ASYNC_SUPPORTED = true
context [add] name: context = context
context [replaced] name: context = context
context [removed] name: context = context
request [add] name: req = req
request [replaced] name: req = req
request [removed] name: req = req
http session [created] :27270467C78BF21720DF220EE4CF7428
http session attribute [add] : sessionid :  = 27270467C78BF21720DF220EE4CF7428
http session attribute [add] : a = a
http session attribute [replaced] : a = a
http session attribute [removed] : a = b
request [destroyed] name: org.apache.catalina.connector.RequestFacade@1ed2fb0
http session [destroyed] :27270467C78BF21720DF220EE4CF7428
http session attribute [removed] : sessionid :  = 27270467C78BF21720DF220EE4CF7428
context [destroyed] name: org.apache.catalina.core.ApplicationContextFacade@149c182

分析下步骤：

context 先初始化 [init]
context 启动后会自动往attribute里塞东西，所以触发了 [add]
访问Servlet ，所以触发了 request [init]
request 创建后系统触发了 request [replaced] --系统的
代码里触发了 context [add] ，代码： getServletContext().setAttribute("context", "context");
代码里触发了 context [replaced] ，代码： getServletContext().setAttribute("context", "context");
代码里触发了 context [removed]，代码： getServletContext().removeAttribute("context");
代码里触发了 request [add]，代码： req.setAttribute("req", "req");
代码里触发了 request [replaced]，代码： req.setAttribute("req", "req");
代码里触发了 request [removed]，代码： req.removeAttribute("req");
代码里触发了 http session [created]，代码： HttpSession session = req.getSession();
代码里触发了 http session attribute [add] ，代码： session.setAttribute("sessionid : ", session.getId());
代码里触发了 http session attribute [add] ，代码： session.setAttribute("a", "a");
代码里触发了 http session attribute [replaced]，代码： session.setAttribute("a", "b");
代码里触发了 http session attribute [removed]，代码： session.removeAttribute("a");
request访问结束，所以触发了request [destroyed]
设置了session过期时间，所以触发了http session [destroyed]，代码：session.setMaxInactiveInterval(3);
session的destroy方法里触发了http session attribute [removed]方法。
关闭了项目，触发了 context [destroyed]

结束~