过滤器Filter（17/4/8）

1：是JavaWeb三大组件之一：Servlet、Lisener（2个感知监听器不需要配置）、Filter:2：过滤器它会在一组资源（jsp、servlet、css、html等等）的前面执行！它可以让请求达到目标资源，也可以不让请求达到！*过滤器有拦截请求的能力！相当于小区中的保安3：过滤器的写法写一个类实现Filter接口在web.xml中进行配置4:Filter 接口void init（FilterConfig）创建后马上执行；Filter在服务器启动时就创建void destory（）销毁之前执行！在关闭服务器之前销毁void doFilter（ServletRequest，ServletResponse，FilterChain）每次过滤时都会执行Filter是单例的！即只能实例化一次5：web.xml中的配置

<filter>
<filter-name>AFilter</filter-name>
<filter-class>filter.AFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>AFilter</filter-name>

//这里可以是/web/*；或者是/AServlet

<url-pattern>/AServlet</url-pattern>

</filter-mapping>

6：代码

1）servlet

package servlet;import java.io.IOException;/*** Created by YuWenHui on 2017/4/8 0008.*/public class AServlet extends javax.servlet.http.HttpServlet {protected void doPost(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {}protected void doGet(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {System.out.println("AServlet.....");}}

2）filter

package filter;import javax.servlet.*;import java.io.IOException;/*** Created by YuWenHui on 2017/4/8 0008.*/public class AFilter implements Filter {public void destroy() {System.out.println("destroy。。。");}public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {System.out.println("doFilter。。。");

//这里将请求传递下去，如果不符合条件这另作处理chain.doFilter(req, resp);}public void init(FilterConfig config) throws ServletException {System.out.println("init。。。");}}

3）配置文件

<?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"><filter><filter-name>AFilter</filter-name><filter-class>filter.AFilter</filter-class></filter><filter-mapping><filter-name>AFilter</filter-name><url-pattern>/AServlet</url-pattern></filter-mapping><servlet><servlet-name>AServlet</servlet-name><servlet-class>servlet.AServlet</servlet-class></servlet><servlet-mapping><servlet-name>AServlet</servlet-name><url-pattern>/AServlet</url-pattern></servlet-mapping></web-app>