Filter(过滤器/拦截器)的作用

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Filter(过滤器/拦截器)的作用

1、filter用于拦截用户请求，在服务器作出响应前，可以在拦截后修改request和response，这样实现很多开发者想得到的功能。

2、filter实现
×编写一个继承Filter接口的类
×在工程的web.xml文件描述此过滤器
<?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">

<filter>
<filter-name>CharsetEcodingFilter2</filter-name>
<filter-class>cn.itcast.filter.CharsetEcodingFilter2</filter-class>
</filter>
<filter-mapping>
<filter-name>CharsetEcodingFilter2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
3、常见用法
×处理全站中文乱码问题
×实现自动登录
×过滤敏感词汇
×压缩网页
×选择性让浏览器缓存

这几种功能的实现采用同样的原理，那就是使用包装模式或动态代理增强request或response对象的功能。

Filter工作流程图：

范例代码：

-------------web.xml-----------:

<filter><filter-name>mutiDomainFilter</filter-name><filter-class>javacommon.filter.MutiDomainFilter</filter-class></filter>

------------MutiDomainFilter.java----------:

public class MutiDomainFilter implements Filter {@Overridepublic void destroy() {// TODO Auto-generated method stub}@Overridepublic void doFilter(ServletRequest req, ServletResponse res,FilterChain filterChain) throws IOException, ServletException {// TODO Auto-generated method stub// 自动编码处理HttpServletRequest request = (HttpServletRequest) req;HttpServletResponse response = (HttpServletResponse) res;String reqUri = request.getRequestURI();try {// 过滤非二级域名urlString modulePath = UrlUtil.getProdModulePath(request);if (modulePath == null) {<strong>filterChain.doFilter(request, response);</strong>return;}// 更改url路径String qryString = request.getQueryString();StringBuilder sb = new StringBuilder();sb.append(modulePath).append(reqUri == null ? "" : reqUri).append(qryString == null ? "" : qryString);System.out.println("changed url:" + sb.toString());<strong>request.getRequestDispatcher(sb.toString()).forward(request,response);</strong>} catch (SecurityException e) {} finally {}}@Overridepublic void init(FilterConfig arg0) throws ServletException {// TODO Auto-generated method stub}}

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