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---------------------------------------------

     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">

wechat-baggio

    WxServlet
    
    boilerWeChat.servlet.WxServlet
    
    1



    WxServlet
    /WxServlet



    

**
    CORSFilter
    boilerWeChat.servlet.CORSFilter
    /**这里提示错误：boilerWeChat.servlet.CORSFilter is not a concrete class；boilerWeChat.servlet.CORSFilteris not assignable to 'javax,servlet.Filter'**/
    
        charset
        UTF-8
    


    CORSFilter
    /*
**

---------------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(){}

}