Filter过滤器

Filter：过滤器，用来过滤网站的数据

• 处理中文乱码
• 登陆验证…
  使用过滤器实现登陆拦截，这样当我们点击注销后，登陆就无法直接进入结果页面了
  
  

过滤器的使用

• 导包

  servlet下的Filter包

1 编写过滤器

public class CharacterEncodingFilter implements Filter {
    
    @Override
    //初始化，和web服务器一起启动，随时等待过滤器对象出现
    public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println("已经初始化");
    }

    @Override
    /**
     * chain
     * 1.过滤中的所有代码，在过滤特定请求的时候都会执行
     * 2.必须要让过滤器继续执行  chain.doFilter(req,resp);
     */
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {
    
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        System.out.println("执行前");
        chain.doFilter(req,resp);   //如果不写，程序到此拦截停止
        System.out.println("执行后");
    }

    @Override
    //web服务器关闭时一同销毁
    public void destroy() {
    
        System.out.println("已经销毁");
    }
}

2 在web.xml中配置Filter

<servlet>
    <servlet-name>show</servlet-name>
    <servlet-class>servlet.ShowServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>show</servlet-name>
    <url-pattern>/servlet/show</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>show</servlet-name>
    <url-pattern>/show</url-pattern>
</servlet-mapping>

<filter>
    <filter-name>filter1</filter-name>
    <filter-class>filter.CharacterEncodingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>filter1</filter-name>
    <!--只要是/servlet的任何请求，会经过这个过滤器 -->
    <url-pattern>/servlet/*</url-pattern>
    <!--<url-pattern>/*</url-pattern>全过滤-->
</filter-mapping>

3. 用户登陆之后，向Session中放入用户的数据

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")){
    
            req.getSession().setAttribute(Constant.USER_SESSION,req.getSession().getId());
            resp.sendRedirect("/sys/result.jsp");
        }else {
    
            resp.sendRedirect("/error.jsp");
        }
    }

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

4. 进入主页的时候判断用户是否已经登陆

public class LogoutServlet extends HttpServlet {
    

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

5.登陆失败或者输入失败

public class ShowServlet extends HttpServlet {
    

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        resp.getWriter().write("你好，世界");
    }

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