實現跨域時，配置 web.xml時提示：CORSFilter is not a concrete classCORSFilter is not assignable to 'javax,servlet.Filter'是什么原因？-----------------------------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_3_1.xsd"     version="3.1"><!--display-name 顯示名稱--><display-name>wechat-baggio</display-name><servlet>    <servlet-name>WxServlet</servlet-name>    <!--<servlet-class>boilerWeChat.servlet.WxServlet</servlet-class>-->    <servlet-class>boilerWeChat.servlet.WxServlet</servlet-class>    <!--自動啟動-->    <load-on-startup>1</load-on-startup></servlet><!-- url-pattern中配置的/WxServlet用于指定該Servlet的訪問路徑 --><servlet-mapping>    <servlet-name>WxServlet</servlet-name>    <url-pattern>/WxServlet</url-pattern></servlet-mapping><!--<welcome-file-list>-->    <!--<welcome-file></welcome-file>--><!--</welcome-file-list>-->**<filter>    <filter-name>CORSFilter</filter-name>    <filter-class>boilerWeChat.servlet.CORSFilter</filter-class>    /**這里提示錯誤：boilerWeChat.servlet.CORSFilter is not a concrete class；boilerWeChat.servlet.CORSFilteris not assignable to 'javax,servlet.Filter'**/    <init-param>        <param-name>charset</param-name>        <param-value>UTF-8</param-value>    </init-param></filter><filter-mapping>    <filter-name>CORSFilter</filter-name>    <url-pattern>/*</url-pattern></filter-mapping>**</web-app>