过滤器 filter

配置文件信息
<?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_2_5.xsd" version="2.5">
  <display-name></display-name>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
  <filter>
    <filter-name>myfirst</filter-name>
    <filter-class>com.itheima.web.filter.HelloFilter</filter-class>
  </filter>
   <filter-mapping>
    <filter-name>myfirst</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  
  
  <filter>
    <display-name>FilterDemo2</display-name>
    <filter-name>FilterDemo2</filter-name>
    <filter-class>com.itheima.web.filter.FilterDemo2</filter-class>
    <init-param>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
    <init-param>
      <param-name>cgx</param-name>
      <param-value>aj</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>FilterDemo2</filter-name>
    <url-pattern>*.jsp</url-pattern>
  </filter-mapping>
  
 
</web-app>

编写一个过滤器，实现过滤器接口 filter

package com.itheima.web.filter;

import java.io.IOException;
import java.util.Enumeration;

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 FilterDemo2 implements Filter {

    private FilterConfig config;
    public void destroy() {
        
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("FilterDemo2放行前的代码");
        
        
        //2.获取初始化参数    FilterConfig对象可以获取，   这个对象当容器调用 init(FilterConfig fConfig)时，会自动为FilterConfig赋值
        
        /** <init-param>
              <param-name>encoding</param-name>
              <param-value>UTF-8</param-value>
            </init-param>*/
        String encodingValue = config.getInitParameter("encoding");//用于获取初始化参数
        System.out.println(encodingValue);
        
        
        //2.1获取多个初始化参数  getInitParameterNames() 
        /**
         *  <init-param>
              <param-name>encoding</param-name>
              <param-value>UTF-8</param-value>
            </init-param>
            <init-param>
              <param-name>cgx</param-name>
              <param-value>aj</param-value>
            </init-param>
         * 
         */
        Enumeration<String> enumss = config.getInitParameterNames();//encoding ,cgx
        while(enumss.hasMoreElements()){
            String paramName = enumss.nextElement();//得到一个参数 <param-name>encoding</param-name>
            String paramValue = config.getInitParameter(paramName);//<param-value>UTF-8</param-value>
            System.out.println(paramName+":"+paramValue);
        }
        chain.doFilter(request, response);//放行
        System.out.println("FilterDemo2放行回来后的代码");
    }

    
    public void init(FilterConfig fConfig) throws ServletException {
        this.config = fConfig;
    }

}

 过滤器的执行顺序和mapping在web.xml里的位置有关，放前面的先执行。