过滤器 Filter

Posted 2022-06-03 Al_tair

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过滤器 Filter

过滤器 Filter

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过滤器 Filter

概述

Filter 过滤器它是 JavaWeb 的三大组件之一(Servlet 程序、Listener 监听器、Filter 过滤器)

Filter 过滤器作用：拦截请求，过滤响应

应用场景：权限检查，日记操作，事务管理

Filter 过滤器是 JavaEE 的规范，是接口

public interface Filter 
    void init(FilterConfig var1) throws ServletException;
    void doFilter(ServletRequest var1, ServletResponse var2, FilterChain var3) throws IOException, ServletException;
    void destroy();

框架图

示例代码

登录页面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>管理后台登录</title>
</head>
<body>
    <h1>管理后台登录</h1>
    <form action="<%=request.getContextPath() %>/loginCheckServlet" method="post">
        u：<input type="text" name="username"/> <br/>
        p：<input type="password" name="password"/> <br/><br/>
        <input type="submit" value="用户登录"/>
    </form>
</body>
</html>

登录检测

@WebServlet(name = "LoginCheckServlet",urlPatterns = "/loginCheckServlet")
public class LoginCheckServlet extends HttpServlet 
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
        doGet(request,response);
    

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        if("".equals(username) || "".equals(password))
            System.out.println("登录失败");
            response.sendRedirect(getServletContext().getContextPath() + "/login.jsp");
            return;
        
        HttpSession session = request.getSession();
        session.setAttribute("name",username);
        session.setAttribute("password",password);
        response.sendRedirect(getServletContext().getContextPath() + "/manage/admin.jsp");
        // request.getRequestDispatcher("/manage/admin.jsp").forward(request,response);

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">
    <filter>
        <filter-name>FilterDemo</filter-name>
        <filter-class>com.al_tair.filter.FilterDemo</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>/manage/*</url-pattern>
    </filter-mapping>
</web-app>

public class FilterDemo implements Filter 

    @Override
    public void init(FilterConfig filterConfig) throws ServletException 
        System.out.println("过滤器初始化成功");
    

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException 
        System.out.println("doFilter");
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        if(request.getSession().getAttribute("name") != null)
            filterChain.doFilter(servletRequest,servletResponse);
        else
            response.sendRedirect(request.getContextPath() + "/login.jsp");
    

    @Override
    public void destroy() 
        System.out.println("过滤器初销毁成功");

登陆成功

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>管理后台</title>
</head>
    <body>
    <h1>管理后台</h1>
    <h2>登录成功</h2>
    </body>
</html>

Filter生命周期

public class FilterDemo implements Filter 
    // 当web工程启动时，会执行构造器和init方法
    // 创建Filter对象也是单例模式，并且常驻内存直到web工程关闭
    @Override
    public void init(FilterConfig filterConfig) throws ServletException 
	// 如果匹配到Filter的url-pattern就会调用该方法
    // 请求转发可以跳过过滤器
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException 
        // FilterChain 接口的 doFilter 方法用于通知 Web 容器把请求交给 Filter 链中的下一个 Filter 去处理，如果当前调用此方法的 Filter 对象是Filter 链中的最后一个 Filter，那么将把请求交给目标 Servlet 程序去处理；如果没有调用该方法默认为拦截，不予通过
        filterChain.doFilter(servletRequest,servletResponse); 
    
	// 当web工程停止时，会执行destroy方法
    @Override
    public void destroy()

FilterConfig

FilterConfig 是 Filter 过滤器的配置类，Tomcat 每次创建 Filter 的时候，也会创建一个 FilterConfig 对象，这里包含了 Filter 配置文件的配置信息

FilterConfig 对象作用是获取 filter 过滤器的配置内容

FilterConfig接口

public interface FilterConfig 
    // 获取配置的名称
    String getFilterName();
    // 获取ServletContext对象，以便于与Servlet进行数据交互
    ServletContext getServletContext();
    // 获取初始化参数值
    String getInitParameter(String var1);
    // 获取多个初始化参数值
    Enumeration<String> getInitParameterNames();

<filter>
    <filter-name>FilterConfig_</filter-name>
    <filter-class>XXX</filter-class>
    <init-param> <!-- 封杀 ip 段 -->
        <param-name>ip</param-name>
        <param-value>128.12</param-value>
    </init-param>
    <init-param>
        <param-name>port</param-name>
        <param-value>8888</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>FilterConfig_</filter-name>
    <url-pattern>/abc/*</url-pattern>
</filter-mapping>

FilterChain 过滤器链

FilterChain: 在处理某些复杂业务时，一个过滤器不够，可以设计多个过滤器共同完成过滤任务，形成过滤器链

框架图

代码示例

AFilter

public class AFilter implements Filter 
    @Override
    public void init(FilterConfig filterConfig) throws ServletException 

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException 
        System.out.println("AFilter doFilter");
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println("AFilter doFilter end");
    

    @Override
    public void destroy()

BFilter

public class BFilter implements Filter
    @Override
    public void init(FilterConfig filterConfig) throws ServletException 

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException 
        System.out.println("BFilter doFilter");
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println("BFilter doFilter end");
    

    @Override
    public void destroy()

web.xml

<?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">
    <!--FilterChain 过滤器链 -->
    <filter>
        <filter-name>AFilter</filter-name>
        <filter-class>com.al_tair.filter.AFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>AFilter</filter-name>
        <url-pattern>/index.jsp</url-pattern>
    </filter-mapping>
    <filter>
        <filter-name>BFilter</filter-name>
        <filter-class>com.al_tair.filter.BFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>BFilter</filter-name>
        <url-pattern>/index.jsp</url-pattern>
    </filter-mapping>
</web-app>

<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
  <h1>FilterChain 过滤器链 完成</h1>
  </body>
</html>

显示顺序

注意事项和细节

多个 filter 和目标资源在一次 http 请求，在同一个线程中，使用同一个 request 对象
多个 filter 执行顺序，和 web.xml 配置顺序保持一致.

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