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一、Listener监听器

1、概念简介

JavaWeb三大组件：Servlet，Listener，Filter。监听器就是指在应用程序中监听相关对象状态变化的组件。

2、事件源对象

指被监听对象。

• ServletContext

ServletContextListener生命周期监听，它有两个方法，出生时调用contextInitialized()，销毁时调用contextDestroyed();

ServletContextAttributeListener属性监听，它有三个方法，添加属性attributeAdded()，替换属性attributeReplaced()，移除属性时attributeRemoved()。

• HttpSession

HttpSessionListener生命周期监听：它有两个方法，出生时调用sessionCreated()，销毁时调用sessionDestroyed()；

HttpSessioniAttributeListener属性监听：它有三个方法，添加属性attributeAdded()，替换属性attributeReplaced()，移除属性attributeRemoved()。

• ServletRequest

ServletRequestListener生命周期监听：它有两个方法，出生时调用requestInitialized()，销毁时调用requestDestroyed()；

ServletRequestAttributeListener属性监听：它有三个方法，添加属性attributeAdded()，替换属性attributeReplaced()，移除属性attributeRemoved()。

3、编码案例

• 相关监听器

TheContextListener

public class TheContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        System.out.println("初始化：TheContextListener");
        ServletContext servletContext = servletContextEvent.getServletContext() ;
        servletContext.setAttribute("author","cicada");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        System.out.println("销毁：TheContextListener");
    }
}

TheRequestListener

public class TheRequestListener implements ServletRequestListener {
    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
        System.out.println("初始化：TheRequestListener");
    }
    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent) {
        System.out.println("销毁：TheRequestListener");
    }
}

TheSessionListener

public class TheSessionListener implements HttpSessionListener {
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        System.out.println("初始化：TheSessionListener");
    }
    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        System.out.println("销毁：TheSessionListener");
    }
}

RequestAttributeListener

public class RequestAttributeListener implements ServletRequestAttributeListener {
    @Override
    public void attributeAdded(ServletRequestAttributeEvent evt) {
        System.out.println("Request添加属性："+evt.getName()+";"+evt.getValue());
    }
    @Override
    public void attributeRemoved(ServletRequestAttributeEvent evt) {
        System.out.println("Request移除属性："+evt.getName()+";"+evt.getValue());
    }
    @Override
    public void attributeReplaced(ServletRequestAttributeEvent evt) {
        System.out.println("Request替换属性："+evt.getName()+";"+evt.getValue());
    }
}

• web.xml配置文件

<!-- 监听器相关配置 -->
<listener>
    <listener-class>com.node05.servlet.listener.TheContextListener</listener-class>
</listener>
<listener>
    <listener-class>com.node05.servlet.listener.TheSessionListener</listener-class>
</listener>
<listener>
    <listener-class>com.node05.servlet.listener.TheRequestListener</listener-class>
</listener>
<listener>
    <listener-class>com.node05.servlet.listener.RequestAttributeListener</listener-class>
</listener>
<session-config>
    <!-- 设置session失效时间为1分钟 -->
    <session-timeout>1</session-timeout>
</session-config>

• 测试接口

public class ListenerServletImpl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        // 1、获取TheContextListener初始化数据
        ServletContext servletContext = this.getServletContext() ;
        String author = String.valueOf(servletContext.getAttribute("author")) ;
        System.out.println("TheContextListener Author:"+author);
        // 2、Request属性设置
        request.setAttribute("mood","smile");
        request.setAttribute("mood","agitated");
        // 3、Session创建，1分钟失效，调用销毁
        HttpSession session = request.getSession(true) ;
        session.setAttribute("casually","casually");
        response.getWriter().print("Hello:Listener");
    }
}

二、Filter过滤器

1、过滤器简介

客户端请求Servlet时，先执行相关Filter，如果Filter通过，则继承执行请求的Servlet；如果Filter不通过，则不会执行用户请求的Servlet。过滤器可以动态地拦截请求和响应。

2、Filter接口

Filter接口定义了三个核心方法。

• init()

应用程序启动时，服务器实例化Filter对象，并调用其init方法，读取web.xml配置，完成对象的初始化加载。

• doFilter()

实际的过滤操作，请求达到服务器时，Servlet容器将先调用过滤器的doFilter方法。

• destroy()

容器在销毁过滤器前调用该方法，释放过滤器占用的资源。

3、编码案例

• 编写过滤器

public class ThePrintLogFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String myName = filterConfig.getInitParameter("myName") ;
        System.out.println("myName："+myName);
    }
    @Override
    public void doFilter(ServletRequest servletRequest,
                         ServletResponse servletResponse,
                         FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest)servletRequest ;
        HttpServletResponse response = (HttpServletResponse)servletResponse ;
        String name = request.getParameter("name") ;
        if (!name.equals("cicada")){
            response.getWriter().print("User Error !");
            return ;
        }
        chain.doFilter(servletRequest,servletResponse);
    }
    @Override
    public void destroy() {
        System.out.println("ThePrintLogFilter destroy()");
    }
}

• web.xml配置文件

<!-- 过滤器相关配置 -->
<filter>
    <filter-name>thePrintLogFilter</filter-name>
    <filter-class>com.node05.servlet.filter.ThePrintLogFilter</filter-class>
    <init-param>
        <param-name>myName</param-name>
        <param-value>cicada</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>thePrintLogFilter</filter-name>
    <url-pattern>/filterServletImpl</url-pattern>
</filter-mapping>

• 测试接口

public class FilterServletImpl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().print("Hello:Filter");
    }
}

三、Interceptor拦截器

Spring框架中的拦截器Interceptor类似于Servlet中的过滤器Filter，主要用于拦截用户请求并作相应的处理。例如通过拦截器可以进行权限验证、记录请求信息的日志、判断用户是否登录等。请求转发不执行拦截、过滤；重定向执行拦截和过滤。

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