090_过滤器 Filter

什么是过滤器

过滤器：用来过滤网站的数据

• 处理中文乱码
• 登录验证……

Filter开发步骤

创建一个普通maven项目

添加框架支持

pom.xml添加依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.qing</groupId>
    <artifactId>javaweb-filter</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <!--Servlet依赖-->
        <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
        </dependency>
        <!--JSP依赖-->
        <!-- https://mvnrepository.com/artifact/javax.servlet.jsp/javax.servlet.jsp-api -->
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>javax.servlet.jsp-api</artifactId>
            <version>2.3.3</version>
        </dependency>
        <!--JSTL表达式依赖-->
        <!-- https://mvnrepository.com/artifact/javax.servlet.jsp.jstl/jstl-api -->
        <dependency>
            <groupId>javax.servlet.jsp.jstl</groupId>
            <artifactId>jstl-api</artifactId>
            <version>1.2</version>
        </dependency>
        <!--standard标签库-->
        <!-- https://mvnrepository.com/artifact/taglibs/standard -->
        <dependency>
            <groupId>taglibs</groupId>
            <artifactId>standard</artifactId>
            <version>1.1.2</version>
        </dependency>
    </dependencies>
</project>

编写过滤器

导包不要错，Filter接口导javax.servlet.*

实现Filter接口，重写对应的方法

package com.qing.filter;

import javax.servlet.*;
import java.io.IOException;

public class CharacterEncodingFilter implements Filter {
    // 初始化：web服务器启动时，就初始化，随时等待过滤对象出现
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("CharacterEncodingFilter初始化");
    }

    // Chain 链
    /*
    1. 过滤器中的所有代码，在过滤特定请求时都会执行
    2. 必须要让过滤器继续通行，即必须写filterChain.doFilter(servletRequest,servletResponse);
     */
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html;charset=UTF-8");

        System.out.println("CharacterEncodingFilter执行前");
        filterChain.doFilter(servletRequest,servletResponse);// 让请求继续通行，如果不写，程序到这里就被拦截停止
        System.out.println("CharacterEncodingFilter执行后");
    }

    // 销毁：web服务器关闭的时候，过滤器会销毁
    public void destroy() {
        System.out.println("CharacterEncodingFilter销毁");
    }
}

在web.xml中配置过滤器

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

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

  
    <!--配置过滤器-->
    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.qing.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <!--只要是/servlet的任何请求，都会经过这个过滤器-->
        <url-pattern>/servlet/*</url-pattern>
        <!--所有的请求，都会经过这个过滤器-->
        <!--<url-pattern>/*</url-pattern>-->
    </filter-mapping>
</web-app>

过滤器执行顺序

进入过滤器（doFilter()）->执行Servlet（filterChain.doFilter()）->回到过滤器（doFilter()）

Filter实现权限拦截

用户登录后，将用户数据放入session

package com.qing.servlet;

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

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 ("admin".equals(username)) {// 登录成功
            req.getSession().setAttribute("USER_SESSION",req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");
        } else {// 登录失败
            resp.sendRedirect("/error.jsp");
        }
    }

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

用户登录后的其他业务都验证session

package com.qing.filter;

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

public class CheckFilter implements Filter {
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        if (request.getSession().getAttribute("USER_SESSION") == null) {
            response.sendRedirect("/error.jsp");
        }

        filterChain.doFilter(servletRequest,servletResponse);
    }

    public void destroy() {

    }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.qing.servlet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/servlet/login</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>LogoutServlet</servlet-name>
        <servlet-class>com.qing.servlet.LogoutServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LogoutServlet</servlet-name>
        <url-pattern>/servlet/logout</url-pattern>
    </servlet-mapping>

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

    <!--配置过滤器-->
    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.qing.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <!--只要是/servlet的任何请求，都会经过这个过滤器-->
        <url-pattern>/servlet/*</url-pattern>
        <!--所有的请求，都会经过这个过滤器-->
        <!--<url-pattern>/*</url-pattern>-->
    </filter-mapping>
    <filter>
        <filter-name>CheckFilter</filter-name>
        <filter-class>com.qing.filter.CheckFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CheckFilter</filter-name>
        <!--只要是/sys的任何请求，都会经过这个过滤器-->
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>

    <!--配置监听器-->
    <listener>
        <listener-class>com.qing.listener.OnlineCountListener</listener-class>
    </listener>

    <!--设置session的默认失效时间-->
    <session-config>
        <!--15分钟后session自动失效，以分钟为单位-->
        <session-timeout>1</session-timeout>
    </session-config>
</web-app>