Filter过滤器

Filter过滤器是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，也就是接口
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：

权限检查
日记操作
事务管理
等等

原理
在这里插入图片描述

package at.guitu.com.FIlter;


import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import java.io.IOException;

public class Filter implements javax.servlet.Filter { 
   

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

    }

    @Override
    //拦截请求
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { 
   
        HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
        HttpSession session = httpServletRequest.getSession();
        Object user = session.getAttribute("user");
        //拦截
        if(user == null){ 
   
            servletRequest.getRequestDispatcher("/login.jsp").forward(servletRequest,servletResponse);
            return;
        }
        //不拦截，让程序继续往下走访问用户目标资源
        else{ 
   
            filterChain.doFilter(servletRequest,servletResponse);
        }
    }

    @Override
    public void destroy() { 
   

    }
}

配置文件

<filter>
        <filter-name>Filter</filter-name>
        <filter-class>at.guitu.com.FIlter.Filter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>Filter</filter-name>
        <!--拦截/admins/目录下的所有文件，都要经过Filter拦截器-->
        <url-pattern>/admin/*</url-pattern>
    </filter-mapping>