动手学servlet(六) 过滤器和监听器

 过滤器（Filter）

  过滤器是在客户端和请求资源之间，起一个过滤的作用，举个例子，比如我们要请求admin文件夹下的index.jsp这个页面，那么我们可以用一个过滤器，判断登录用户是不是管理员

-请求资源可以是servlet，jsp页面，html页面等

-在请求资源的前后执行

-请求和响应可以通过filter修改

  过滤器常用在这几个方面：Session管理，权限验证，日志记录，字符编码转换

package servletdemo;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class MyFilter implements Filter {

    @Override
    public void destroy() {
        // TODO 自动生成的方法存根
        
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        System.out.println("filter...");
        chain.doFilter(request, response);
        // TODO 自动生成的方法存根
        
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO 自动生成的方法存根
        
    }

}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>javaeedemo2</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
 <filter>
 <filter-name>MyFilter</filter-name>
 <filter-class>servletdemo.MyFilter</filter-class>
 </filter>
 <filter-mapping>
 <filter-name>MyFilter</filter-name>
 <url-pattern>/*</url-pattern>
 </filter-mapping>
</web-app>

  注意：这里ulr-parten是起一个过滤的作用，如果是/*，那么所有的资源请求都要经过这个过滤器，再假如要对admin文件夹下的文件过滤，就可以写成/admin/*

  我们新建一个index.jsp页面进行访问，每次刷新页面，过滤器都会起作用

  再看一个例子，使用Filter进行登录权限验证（访问admin下面的内容需要登录）

package servletdemo;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import sun.misc.Regexp;

public class MyFilter implements Filter {

    @Override
    public void destroy() {
        // TODO 自动生成的方法存根
        
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req=(HttpServletRequest)request;
        HttpServletResponse res=(HttpServletResponse)response;
        
        HttpSession session=req.getSession();
        if(session.getAttribute("userlogin")==null)
        {
            System.out.println("please login...");
        }
        else {
            req.getRequestDispatcher("../login.html").forward(request, response);
        }
        chain.doFilter(request, response);
        // TODO 自动生成的方法存根
        
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO 自动生成的方法存根
        
    }

}

监听器（Listener）

  监听器用于监听JAVA WEB程序中的事件，例如创建，修改，删除Session，request，context等，并触发相应的事件

  下面我们就针对刚才提到的三个共享变量来学习监听器

1、ServletContextListener

package servletdemo;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyListener implements ServletContextListener {

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {        
        // TODO 自动生成的方法存根
        System.out.println("contextDestroyed......");
        
    }

    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("contextInitialized......");
    }

}

运行WEB工程，无论是哪个页面首先启动，该监听器都会执行，所以一切全局的操作就可以写在这里

2、ServletContextAttributeListener

package servletdemo;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyListener implements ServletContextAttributeListener {

    @Override
    public void attributeAdded(ServletContextAttributeEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("attributeAdded......");
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("attributeRemoved......");
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("attributeReplaced......");
    }


    

}

package servletdemo;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class TestListenerServlet
 */
@WebServlet("/TestListenerServlet")
public class TestListenerServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public TestListenerServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        ServletContext ctx=this.getServletContext();
        ctx.setAttribute("ctx_name", "ctx_value");
        //替换
        ctx.setAttribute("ctx_name", "ctx_value2");
        //删除
        ctx.removeAttribute("ctx_name");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

运行servlet，该监听器的事件会执行，效果如下

3、HttpSessionListener

package servletdemo;


import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class MyListener implements HttpSessionListener {

    @Override
    public void sessionCreated(HttpSessionEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("sessionCreated......");
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("sessionDestroyed......");
    }




    

}

package servletdemo;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class TestListenerServlet
 */
@WebServlet("/TestListenerServlet")
public class TestListenerServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public TestListenerServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        HttpSession session=request.getSession();
        session.setAttribute("session_name", "session_value");
        //销毁
        session.invalidate();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

运行servlet，该监听器的事件会执行，效果如下

4、HttpSessionAttributeListener

package servletdemo;


import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

public class MyListener implements HttpSessionAttributeListener {

    @Override
    public void attributeAdded(HttpSessionBindingEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("attributeAdded......");
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("attributeRemoved......");
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent arg0) {
        // TODO 自动生成的方法存根
        System.out.println("attributeReplaced......");
    }

    

}

package servletdemo;

import java.io.IOException;


import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class TestListenerServlet
 */
@WebServlet("/TestListenerServlet")
public class TestListenerServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public TestListenerServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session=request.getSession();
        session.setAttribute("session_name", "session_value");
        //替换
        session.setAttribute("session_name", "session_value2");
        //删除
        session.invalidate();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

运行servlet，该监听器的事件会执行，效果如下

还有一个HttpRequestListener和HttpRequestAttributeListener，一运行servlet就会创建一个request,所以对应的监听器的add事件就会执行

还有监听器的配置文件代码：

<listener>
<listener-class>servletdemo.MyListener</listener-class>
</listener>