11 过滤器(Filter)

什么是过滤器(Filter)

Filter:过滤器，用来过滤网站的数据;

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image-20201121174126385

Filter开发步骤:

1. 导包

   <dependencies>
   <!--        servlet依赖-->
           <dependency>
               <groupId>javax.servlet</groupId>
               <artifactId>javax.servlet-api</artifactId>
               <version>3.1.0</version>
           </dependency>
   <!--        JSP依赖-->
           <dependency>
               <groupId>javax.servlet.jsp</groupId>
               <artifactId>jsp-api</artifactId>
               <version>2.1</version>
           </dependency>
   <!--        JSTL表达式依赖-->
           <dependency>
               <groupId>javax.servlet.jsp.jstl</groupId>
               <artifactId>jstl-api</artifactId>
               <version>1.2</version>
           </dependency>
   <!--        standard标签库-->
           <dependency>
               <groupId>taglibs</groupId>
               <artifactId>standard</artifactId>
               <version>1.1.2</version>
           </dependency>
   <!--        连接数据库-->
           <dependency>
               <groupId>mysql</groupId>
               <artifactId>mysql-connector-java</artifactId>
               <version>5.1.49</version>
           </dependency>
       </dependencies>
   

2. 编写过滤器

   1. 注意: 导包 import javax.servlet.*;​

   2. 实现Filter接口, 重写对应的方法

      package com.karl.filter;
      
      import javax.servlet.*;
      import java.io.IOException;
      
      public class CharacterEncodingFilter implements Filter {
          //初始化:web服务器启动，就以及初始化了，随时等待过滤对象出现!
          public void init(FilterConfig filterConfig) throws ServletException {
              System.out.println("CharacterEncodingFilter初始化");
          }
          // Chain :链
          /*
              1.过滤中的所有代码,在过滤特定请求的机候都会执行
              2．必须要让过滤器继续通行
              chain.doFilter( request,response);
          */
      
          public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
              request.setCharacterEncoding("utf-8");
              response.setCharacterEncoding("utf-8");
              response.setContentType("text/html;charset=utf-8");
              System.out.println("doFilter! 1");
              chain.doFilter(request,response);//让请求继续走，如果不写，程序到这里就被拦截停止!
              System.out.println("doFilter! 2");
          }
      
          //销毁: web服务器关闭的时候，过滤会销毁
          public void destroy() {
              System.out.println("CharacterEncodingFilter销毁");
          }
      }
      

   3. 在 web.xml 中配置 filter

      <filter>
          <filter-name>CharacterEncodingFilter</filter-name>
          <filter-class>com.karl.filter.CharacterEncodingFilter</filter-class>
      </filter>
      <filter-mapping>
          <filter-name>CharacterEncodingFilter</filter-name>
          <url-pattern>/servlet/*</url-pattern>
      </filter-mapping>