一、FilterChain 过滤器链

多个Filter对同一个资源进行拦截就可以形成过滤器链，同一个资源是指的匹配路径一致
多个Filter过滤器是根据配置的顺序来执行，谁在前，先执行谁

二、Filter的生命周期

1、生命周期流程

和servlet 生命周期类似：实例化-》初始化->服务-》销毁

Filter的生命周期
服务器启动时，会创建Filter对象， 并调用init方法，只调用一次
当访问资源的时，路径与Filter的拦截路径匹配，会执行Filter的doFilter方法
当服务器关闭时，会调用Filter 的destroy方法进行销毁

2、代码展示

package com.heihei;

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() {
		System.out.println("Filter destroy ");
		
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("拦截开始");
		
		//放行
		chain.doFilter(request, response);
		System.out.println("结束拦截,资源过滤后，回来了");
		
	}

	@Override
	public void init(FilterConfig arg0) throws ServletException {
		System.out.println("Filter init ");
		
	}

}

三、FilterConfig对象说明

Filter初始化方法init上的一个参数 ，就是FilterConfig对象

public interface Filter {
    void init(FilterConfig var1) throws ServletException;

    void doFilter(ServletRequest var1, ServletResponse var2, FilterChain var3) throws IOException, ServletException;

    void destroy();
}

MyFilterConfig

package com.heihei;

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 MyFilterConfig implements Filter{

	private FilterConfig config;
	@Override
	public void init(FilterConfig config) throws ServletException {
		this.config = config;
		
	}

	@Override
	public void doFilter(ServletRequest req, ServletResponse rsp, FilterChain chain)
			throws IOException, ServletException {
		//通过FilterConfig对象获取配置文件中的初始化信息
		String coding = config.getInitParameter("encoding");
		System.out.println(coding);
		req.setCharacterEncoding(coding);
		chain.doFilter(req, rsp);
		
	}
	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		
	}

}

ServletDemo

package com.heihei;

import java.io.IOException;

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

public class ServletDemo extends HttpServlet{
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println("ServletDemo");
	}
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		// TODO Auto-generated method stub
		super.doPost(req, resp);
	}

}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http:///2001/XMLSchema-instance" xmlns="http:///xml/ns/javaee" xsi:schemaLocation="http:///xml/ns/javaee http:///xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  
  <filter>
  	<filter-name>MyFilterConfig</filter-name>
  	<filter-class>com.heihei.MyFilterConfig</filter-class>
  	<init-param>
		<param-name>encoding</param-name>
		<param-value>UTF-8</param-value>  	
  	</init-param>
  </filter>
  
  <filter-mapping>
  	<filter-name>MyFilterConfig</filter-name>
  	<url-pattern>/*</url-pattern>
  </filter-mapping>
  
  
  <servlet>
  	<servlet-name>ServletDemo</servlet-name>
  	<servlet-class>com.heihei.ServletDemo</servlet-class>
  </servlet>
  <servlet-mapping>
  	<servlet-name>ServletDemo</servlet-name>
  	<url-pattern>/ServletDemo</url-pattern>
  </servlet-mapping>
</web-app>

四、Filter配置

1、匹配规则

在web.xm文件中，<url-pattern>标签的匹配规则：
完全匹配	 以"/demo"，不包含通配符
目录匹配		以"/"开头，以"*"结束
扩展名匹配：	 *.xxxx，不能写成 /*.xxx ，比如 *.do

2、servlet-name 标签

也可以直接通过配置<servlet-name>标签，来关联到servlet对应的servlet-mapping配置，从而达到拦截的目的
<filter-mapping>
  	<filter-name>MyFilterConfig</filter-name>
	<servlet-name>ServletDemo</servlet-name>
  </filter-mapping>

3、dispatcher 标签

<dispatcher>标签，默认是request请求，可以写多个dispatcher标签，比如：
 <filter-mapping>
  	<filter-name>MyFilterConfig</filter-name>
  	****
	<dispatcher>REQUEST</dispatcher>
	<dispatcher>FORWARD</dispatcher>
  </filter-mapping>

REQUEST:默认值,只过滤客户端请求和重定向

FORWARD:只过滤转发请求