过滤器

做过滤器分为两步：

1.做类
实现javax.servlet.Filter接口
在doFilter()方法中编写过滤逻辑

2.做配置
在web.xml中配置<filter>和<filter-mapping>元素

1.做类：

1.1实现javax.servlet.Filter接口

在doFilter()方法中编写过滤逻辑

package com.itnba.maya.test;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;

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.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class Testfilter implements Filter {
    private ArrayList<String> list =new ArrayList<>();

    @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 {
        // TODO 自动生成的方法存根
        HttpServletRequest req = (HttpServletRequest )request;//转换类型，最好很次都转
        HttpServletResponse res =(HttpServletResponse)response;
        HttpSession session=req.getSession();
        String path1 = req.getRequestURI();//长路径
        String path2 = req.getContextPath();//短路径
        String target =path1.substring(path2.length());
        if(list.contains(target)==false){//判断是否是可以直接访问的路径
            if(session.getAttribute("user")==null)//判断是否登陆
            {
                res.sendRedirect("show.jsp");
            }
            else{
                chain.doFilter(req, res);    
            }
        }
        else
        {
            chain.doFilter(request, res);
        }
    }

    @Override
    public void init(FilterConfig cfg) throws ServletException {
        // TODO 自动生成的方法存根
        String val=cfg.getInitParameter("allowpage");//获取init param中的内容
        String[] arr=val.split(",");
        list.addAll(Arrays.asList(arr));

    }

}

2.做配置
在web.xml中配置<filter>和<filter-mapping>元素

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
  version="3.1"
  metadata-complete="true">

    <filter>
        <filter-name>testFilter</filter-name>
        <filter-class>com.itnba.maya.test.Testfilter</filter-class><!--过滤器类的位置--!>
        <init-param>
              <param-name>allowpage</param-name><!--在过滤器类中的 init(FilterConfig cfg)方法调用，获取下面的value值--!>
              <param-value>/show.jsp,/show</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>testFilter</filter-name>
        <url-pattern>/*</url-pattern><!--需要过滤的路径 --!>
    </filter-mapping>

    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.xhtml</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>

</web-app>