尝试用Filter 实现跨域时，提示is not assignable to 'javax,servlet.Filter'

实现跨域时，配置 web.xml时提示：
CORSFilter is not a concrete class
CORSFilter 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 显示名称-->
<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>
    <!--自动启动-->
    <load-on-startup>1</load-on-startup>
</servlet>
<!-- url-pattern中配置的/WxServlet用于指定该Servlet的访问路径 -->
<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>
    /**这里提示错误：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>

---------------CORSFilter.java -----------------------------------
public abstract class CORSFilter implements Filter {

public void  doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
    throws IOException,ServletException{
    HttpServletResponse res=(HttpServletResponse)response;
    res.setHeader("Access-Control-Allow-Origin", "*");
    res.setHeader("Access-Control-Allow-Methods", "POST, GET, PUT, OPTIONS, DELETE");
    res.setHeader("Access-Control-Max-Age", "3600");
    res.setHeader("Access-Control-Allow-Headers", "x-requested-with, Content-Type");
    res.setHeader("Access-Control-Allow-Credentials", "true");
    chain.doFilter(request, response);
}
public void init(FilterConfig filterConfig){}

public void destory(){}

}

---

public abstract class CORSFilter implements Filter 你使用了抽象类去实现Filter肯定不行，请将abstract去掉