• Filter：过滤器，用来过滤网站的数据（处理中文乱码，登录验证…）

• Filter开发步骤

  • 导包配置maven
  • 编写过滤器（导包：javax.servlet.Filter）

  public class CharacterEncodingFilter implements Filter {
  
      //初始化
      public void init(FilterConfig filterConfig) throws ServletException {
          System.out.println("CharacterEncodingFilter已经初始化");
      }
  
      //chain：链
      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("CharacterEncodingFilter执行前");
          chain.doFilter(request,response); //让请求继续走，如果不写程序到这里自动停止
          System.out.println("CharacterEncodingFilter执行后");
      }
  
      //销毁：web服务器关闭时，过滤器会被销毁
      public void destroy() {
          System.out.println("CharacterEncodingFilter已经销毁");
      }
  }
  

  • web.xml配置过滤器

  <filter>
      <filter-name>CharacterEncodingFilter</filter-name>
      <filter-class>com.java.filter.CharacterEncodingFilter</filter-class>
  </filter>
  <!--过滤/servlet路径下的所有请求-->
  <filter-mapping>
      <filter-name>CharacterEncodingFilter</filter-name>
      <url-pattern>/servlet/*</url-pattern>
  </filter-mapping>
  

• 总结：

  1. 过滤器中所有的代码，在过滤特定请求的时候都会执行
  2. 必须要让过滤器继续通行
  3. chain.doFilter(request,response);必须写

案例：用户登录之后才能进入主页，用户注销之后则不能进入

• 分析
  • 1.用户登录之后向Session中放入用户数据信息
  • 2.进入主页的时候要判断用户是否已经登录（过滤器）

登录

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取前端请求的参数
        String username = req.getParameter("username");

        if (username.equals("admin")){ //登录成功
            req.getSession().setAttribute(Constant.USER_SESSION,req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");
        }else { //登录失败
            resp.sendRedirect("/error.jsp");
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

登出

public class LogoutServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String user_session = (String) req.getSession().getAttribute(Constant.USER_SESSION);
        if (user_session != null){
            req.getSession().removeAttribute(Constant.USER_SESSION);
            resp.sendRedirect("/login.jsp");
        }else {
            resp.sendRedirect("/login.jsp");
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

过滤器

public class SysFilter implements Filter {

    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        if (request.getSession().getAttribute(Constant.USER_SESSION)==null) {

            response.sendRedirect("/error.jsp");
        }
        filterChain.doFilter(servletRequest,servletResponse);
    }

    public void destroy() {

    }
}

提取常量工具类

public class Constant {
    public final static String USER_SESSION = "USER_SESSION";
}