使用java防止非法请求

public class ImgFilter implementsFilter {


    @Override
    public void init(FilterConfig filterConfig) throwsServletException {
        System.out.println("过滤器开始！！！");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throwsIOException, ServletException {
        //获取当前请求的连接地址和上一个发送请求的地址
        HttpServletRequest httprequest=(HttpServletRequest) request;
        HttpServletResponse httpresponse=(HttpServletResponse) response;
        //获取上一个发送请求的连接
        String referer = httprequest.getHeader("Referer");
        String serverName =httprequest.getServerName();
        System.out.println(referer+"	"+serverName);
        //||!referer.startsWith("http://www.aproject.com")
        if (referer==null ||!referer.contains(serverName)){
            request.getRequestDispatcher("/img/ffqq.png").forward(request,response);
            return;
        }
        //放行
chain.doFilter(request,response);
    }

    @Override
    public voiddestroy() {
        System.out.println("过滤器销毁！！！");
    }
}

<filter>
    <filter-name>ImgFilter</filter-name>
    <filter-class>com.szh.ImgFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>ImgFilter</filter-name>
    <url-pattern>/img/*</url-pattern>
  </filter-mapping>