<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
    <listener>
        <listener-class>com.itheima09.servlet.listener.ServletListener</listener-class>
    </listener>
    
    <filter>
        <filter-name>actionFilter</filter-name>
        <filter-class>com.itheima09.servlet.filter.DispatcherFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>actionFilter</filter-name>
        <url-pattern>*.action</url-pattern>
    </filter-mapping>
    <welcome-file-list>
        <welcome-file>login.jsp</welcome-file>
    </welcome-file-list>
</web-app>

package com.itheima09.servlet.listener;

import java.util.HashMap;
import java.util.Map;

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

public class ServletListener implements ServletContextListener{
    /**
     * 在tomcat销毁的时候执行
     */
    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        arg0.getServletContext().setAttribute("mappings", null);
    }
    /**
     *    在tomcat启动的时候执行 
     */
    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("userAction", "com.itheima09.action.UserAction");
        arg0.getServletContext().setAttribute("mappings", map);
    }
}

package com.itheima09.servlet.filter;

import java.io.IOException;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;

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

import com.itheima09.servlet.utils.ServletUtils;

public class DispatcherFilter implements Filter{
    
    private ServletContext servletContext;

    @Override
    public void destroy() {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse arg1,
            FilterChain arg2) throws IOException, ServletException {
        /**
         * 1、从 application域中获取map
         */
        HttpServletRequest request = (HttpServletRequest)arg0;
        HttpServletResponse response = (HttpServletResponse)arg1;
        Map<String, String> map = (HashMap<String, String>)this.servletContext.getAttribute("mappings");
        /**
         * 2、获取浏览器中的url,把url解析出来
         *     http://localhost:8080/itheima09_servlet_super/userAction.action
         *     ---->userAction
         */
        //mapping = userAction
        String mapping = ServletUtils.parse(request.getRequestURI());
        String value = map.get(mapping); //value就是action的类的全名
        try {
            Class class1 = Class.forName(value);
            Method method = class1.getMethod("execute", HttpServletRequest.class,HttpServletResponse.class);
            //调用了action中的方法
            String jspName = (String)method.invoke(class1.newInstance(), request,response);
            request.getRequestDispatcher(jspName).forward(request, response);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub
        this.servletContext = arg0.getServletContext();
    }
    
}

package com.itheima09.servlet.utils;

import org.junit.Test;

public class ServletUtils {
    /**
     * http://localhost:8080/itheima09_servlet_super/userAction.action
     * @param url
     * @return
     */
    public static String parse(String url){
        String[] array = url.split("/");
        String mapping = array[array.length-1].substring(0,array[array.length-1].indexOf("."));
        return mapping;
    }
    
    @Test
    public void test(){
        System.out.println(ServletUtils.parse("http://localhost:8080/itheima09_servlet_super/userAction.action"));
    }
}