JavaWeb-10-Filter(过滤器)

1.概述

Filter:用来过滤网站的数据

• 处理中文乱码
• 登录验证
• …

2.编写过滤器

• 创建Filter(javax.servlet下的接口)的实现类

  

• 过滤器用法(以解决页面乱码为例)

  public class CharacterEncodingFilter implements Filter {
      //初始化,在web服务器启动就已经初始化了 随时等待监听
      public void init(FilterConfig filterConfig) throws ServletException {
          System.out.println("进入CharacterEncodingFilter init 方法");
      }
  
      /*
       filterChain 过滤链  (不止一个过滤器)
       1.过滤器中的所有代码,再过滤特定请求时都会执行
       2.必须要让过滤器继续转接,
      */
      public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterChain) throws IOException, ServletException {
          req.setCharacterEncoding("utf-8");
          resp.setCharacterEncoding("utf-8");
          resp.setContentType("text/html;charset=utf-8");
  
          System.out.println("CharacterEncodingFilter执行前");
          filterChain.doFilter(req,resp);//将请求继续转接,不写,程序将被拦截停止
          System.out.println("CharacterEncodingFilter执行后");
      }
  
      //销毁 web服务器关闭时销毁
      public void destroy() {
          System.out.println("CharacterEncodingFilter 销毁");
      }
  }
             

• web.xml的配置

  //模拟乱码的servlet 的注册信息,注意给了两条映射
  // /show    没有过滤器的路径     
  // /servlet/show  	经过过滤器的路径
  <servlet>
      <servlet-name>show</servlet-name>
      <servlet-class>com.cmy.servlet.ShowServlet</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>show</servlet-name>
      <url-pattern>/show</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
      <servlet-name>show</servlet-name>
      <url-pattern>/servlet/show</url-pattern>
  </servlet-mapping>
  //设置指定路径/servlet/所有路径  通过过滤器
  <filter>
      <filter-name>CharacterEncodingFilter</filter-name>
      <filter-class>com.cmy.filter.CharacterEncodingFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>CharacterEncodingFilter</filter-name>
      <url-pattern>/servlet/*</url-pattern>
  </filter-mapping>
             

• 效果演示

  

3.过滤器的应用

在web应用的文件夹下,只要知道路径大多可以直接访问

但是,现在只想让登录了的用户才能访问/sys/content.jsp,其他直接输入该地址的游客都重定向到/error/error.jsp界面,就需要过滤器

实现

• 在登录时,为该登录用户将信息保存至session,这里模拟admin,123456为数据库中的存在用户

  //Constant.USER_SESSION 为常量类的USER_SESSION属性 "USER_SESSION" 为了修改方便
  public class LoginServlet extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          String username = req.getParameter("username");
          String password = req.getParameter("password");
          if (username.equals("admin")&&password.equals("123456")){
              req.getSession().setAttribute(Constant.USER_SESSION,req.getSession().getId());
              resp.sendRedirect("/sys/content.jsp");
          }else {
              resp.sendRedirect("/error/error.jsp");
          }
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
             

• 为/sys/*的访问设置过滤器,保证用户必须要有Constant.USER_SESSION属性(登录过才有),否则重定向到错误界面

  public class SysFilter implements Filter {
      public void init(FilterConfig filterConfig) throws ServletException {
  
      }
  
      public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterChain) throws IOException, ServletException {
          HttpServletRequest request = (HttpServletRequest) req;
          HttpServletResponse response = (HttpServletResponse) resp;
          if (request.getSession().getAttribute(Constant.USER_SESSION)==null){
              response.sendRedirect("/error/error.jsp");
          }
          filterChain.doFilter(request,response);
      }
      public void destroy() {
  
      }
  }
             

• 在web.xml注册过滤器,所有访问/sys/*的请求,都将进入SysFilter.java,精选过滤判断,游客将被重定向,登录过拥有session属性的可以直接访问

  <filter>
      <filter-name>SysFilter</filter-name>
      <filter-class>com.cmy.filter.SysFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>SysFilter</filter-name>
      <url-pattern>/sys/*</url-pattern>
  </filter-mapping>