Filter过滤器

• Filter过滤器它是JavaWeb的三大组件之一。三大组件分别是：Servlet程序、Listener监听器、Filter过滤器。

• Filter过滤器它是JavaEE的规范。也是接口。

• Filter过滤器它的作用是：拦截请求，过滤响应。

• 拦截请求常见的应用场景有：

  • 权限检查
  • 日记操作
  • 事务管理
  • 等

• 要求：在你的web工程下，有一个admin目录。这个admin目录下的所有资源（html页面、jpg图片、jsp文件等等）都必须是用户登录之后才允许访问。

import com.wz.userManager.pojo.User;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
@WebFilter(filterName = "UserCenterFilter",urlPatterns = {"/pages/*"})
public class UserCenterFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest)servletRequest;
        HttpServletResponse resp= (HttpServletResponse)servletResponse;
        HttpSession session = req.getSession();
        User username = (User) session.getAttribute("user");
        if(username==null){
            servletRequest.getRequestDispatcher("/login.jsp").forward(servletRequest,servletResponse);
        }else{
            filterChain.doFilter(servletRequest,servletResponse);
        }

    }

    @Override
    public void destroy() {

    }
}