详细了解java监听器和过滤器

public class OneListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("Initialized............");
    }
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("Destroyed.............");
    }
}

<listener>
    <listener-class>school.xauat.listener.OneListener</listener-class>
</listener>

public class OneListener implements ServletContextAttributeListener {
    @Override
    public void attributeAdded(ServletContextAttributeEvent scae) {
        System.out.println("ServletContextAttribute is added......");
    }
    @Override
    public void attributeRemoved(ServletContextAttributeEvent scae){
        System.out.println("ServletContextAttribute is removed......");
    }
    @Override
    public void attributeReplaced(ServletContextAttributeEvent scae){
        System.out.println("ServletContextAttribute is replaced......");
    }
}

<servlet>
    <servlet-name>OneServlet</servlet-name>
    <servlet-class>school.xauat.controller.OneServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>OneServlet</servlet-name>
    <url-pattern>/one</url-pattern>
</servlet-mapping>
<listener>
    <listener-class>school.xauat.listener.OneListener</listener-class>
</listener>

public class OneServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //通过请求对象获取全局作用域对象
        ServletContext application=request.getServletContext();
        //向全局作用域对象中添加共享数据
        application.setAttribute("key",100);
        //更改全局作用域对象中的共享数据
        application.setAttribute("key",500);
        //删除全局作用域对象中的共享数据
        application.removeAttribute("key");
    }
}

public class OneFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        //通过拦截的请求对象来得到请求包中的参数信息，从而得到来访用户的真实年龄
        String age=servletRequest.getParameter("age");
        //根据这个年龄帮助我们的Http服务器判断本次请求的合法性
        if(Integer.valueOf(age)<70){
            //将拦截请求对象和相应对象交换给Tomcat，由Tomcat继续调用资源文件
            filterChain.doFilter(servletRequest,servletResponse);
        }else{
            //过滤器代替Http服务器拒绝本次请求
            servletResponse.setContentType("text/html;charset=utf-8");
            PrintWriter out=servletResponse.getWriter();
            out.print("<center><font style='color:red;font-size:40px'>不合适！！！！</font></center>");
        }
    }
}

<!--将过滤器类文件交给Tomcat-->
<filter>
    <filter-name>OneFilter</filter-name>
    <filter-class>school.xauat.filter.OneFilter</filter-class>
</filter>
<!--通知Tomcat在调用何种资源文件是需要被当前过滤器拦截-->
<filter-mapping>
    <filter-name>OneFilter</filter-name>
    <url-pattern>/mm.jpg</url-pattern>
</filter-mapping>

public class OneFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");
        filterChain.doFilter(servletRequest,servletResponse);
    }
}

<servlet>
        <servlet-name>OneServlet</servlet-name>
        <servlet-class>school.xauat.controller.OneServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>OneServlet</servlet-name>
        <url-pattern>/one</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>TwoServlet</servlet-name>
        <servlet-class>school.xauat.controller.TwoServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>TwoServlet</servlet-name>
        <url-pattern>/two</url-pattern>
    </servlet-mapping>
    <!--注册Filter类-->
    <filter>
        <filter-name>OneFilter</filter-name>
        <filter-class>school.xauat.filter.OneFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>OneFilter</filter-name>
        <!--通知Tomcat在调用所有资源文件之前都需要调用OneFilter进行拦截-->
        <url-pattern>/*</url-pattern>
    </filter-mapping>

public class OneServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //通过请求对象获取请求体中的请求参数
        String userName=request.getParameter("userName");
        System.out.println("OneServlet----->"+userName);
    }
}

public class TwoServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //通过请求对象获取请求体中的请求参数
        String userName=request.getParameter("userName");
        System.out.println("TwoServlet---->"+userName);
    }
}