Filter 入门详解

当访问服务器的资源时，过滤器可以将请求拦截下来，完成一些特殊的功能。

常用在登录验证、统一编码处理、敏感字符过滤

编写Filter

实现Filter 接口，重写以下三个方法

init 初始化——>doFilter 拦截——>destory 销毁

public class CharacterEncodingFilter implements Filter {

    /*
    1、过滤器中所有代码在过滤特定请求时都会执行
    2、必须要让过滤器filterChain.doFilter()将req和resp转交给后面的过滤器，继续通行
     */
    
    //初始化:web服务器启动就进行了初始化
    public void init(FilterConfig filterConfig) throws ServletException { 	  
        System.out.println("CharacterEncodingFilter Init..."); 
    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("UTF-8");
        servletResponse.setCharacterEncoding("UTF-8");
        System.out.println("Filter run start......");
        //放行
        filterChain.doFilter(servletRequest,servletResponse);  
        System.out.println("Filter run stop......");

    }

	//销毁:web服务器关闭时，Filter销毁
    public void destroy() { 
        System.out.println("CharacterEncodingFilter Destmroy..."); }
    }
}

注册Filter

在WEB.XML编写以下配置

<filter>
    <filter-name>CharacterEncodingFilter</filter-name>
    <filter-class>com.zh1z3ven.filter.CharacterEncodingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>CharacterEncodingFilter</filter-name>
	<!--在此设定的路由下的请求都会经过Filter处理-->
    <url-pattern>/servlet/*</url-pattern>
</filter-mapping>

代码实践

如果用户不携带登录状态想访问 success.jsp 就行拦截处理

方法一

在success.jsp 处理跳转

<%--在jsp中实现鉴权。如果用户未携带session就跳转到login页面--%>
<%
    Object user_session = request.getSession().getAttribute("USER_SESSION");
    if (user_session==null){
        response.sendRedirect("/login.jsp");
    }
%>

方法二

编写 LoginFilter，使用Filter实现拦截

package com.bright;

import javax.servlet.*;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class LoginFilter 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;

        Object username = req.getSession().getAttribute("username");

        if (username == null) {
            resp.sendRedirect("/view/error.jsp");
        }
        filterChain.doFilter(req,resp);
    }

    @Override
    public void destroy() {

    }
}

配置 web.xml

<filter>
    <filter-name>LoginFilter</filter-name>
    <filter-class>com.bright.LoginFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>LoginFilter</filter-name>
    <url-pattern>/view/success.jsp</url-pattern>
</filter-mapping>

最后

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