1.filter的作用

• 在HttpServletRequest请求到达Servlet之前，拦截客户的HttpServletRequest
• 根据需要检查HttpServletRequest的相关信息，修改头信息和数据
• 在HttpServletResponse到达客户端之前可以进行拦截
• 在HttpServletResponse到达客户端之前可以进行检查，也可以修改头信息和数据

2.实现方式

• 定义一个类，继承自javax.servlet.Filter类
• 实现javax.servlet.Filter的init（），destroy（），doFilter()方法
• 配置filter在web.xml或者filter的代码中

3.具体实现

package com.filter;
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import java.io.IOException;

public class LogFilter  implements Filter  {

    private FilterConfig config;
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.config=filterConfig;
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        ServletContext context=config.getServletContext();
       long time= System.currentTimeMillis();
        System.out.println("过滤开始");
        System.out.println(time);
        HttpServletRequest request=(HttpServletRequest)servletRequest;
        HttpSession session=request.getSession(true);
        String loginpage=config.getInitParameter("loginpage");
        if(session.getAttribute("username")==null&&!request.getContextPath().endsWith(loginpage)){
            servletRequest.getRequestDispatcher(loginpage).forward(servletRequest,servletResponse);
        }else{
            System.out.println(request.getContextPath());
            filterChain.doFilter(servletRequest,servletResponse);
            long atime= System.currentTimeMillis();
            System.out.println(time);
            System.out.println("过滤结束");
            System.out.println("请求被处理时间是："+request.getContextPath()+"("+(atime-time)+")");
        }

    }

    @Override
    public void destroy() {
        config=null;
    }
}

4.在web.xml中的配置方法

<filter>
    <filter-name>logs</filter-name>
    <filter-class>com.filter.LogFilter</filter-class>
    <init-param>
        <param-name>loginpage</param-name>
        <param-value>/login.jsp</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>logs</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

说明：filter.doFilter调用是用来区分拦截的前后的

doFilter的使用可以进行权限的验证，日志的记录等方法