防盗链图片的filter的问题。 - 编程论坛

防盗链图片的filter
public class ImageRedirectFilter implements Filter {

public void init(FilterConfig config) throws ServletException {
}

public void doFilter(ServletRequest req, ServletResponse res,
   FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;

// 禁止缓存
response.setHeader("Cache-Control", "no-store");
response.setHeader("Pragrma", "no-cache");
response.setDateHeader("Expires", 0);

// 链接来源地址
String referer = request.getHeader("referer");
System.out.println("refer is"+referer);

if (referer == null || !referer.contains(request.getServerName())) {

   /**
    * 如果链接地址来自其他网站，则返回错误图片
    */
   request.getRequestDispatcher("/error.gif").forward(request,response);

} else {

   /**
    * 图片正常显示
    */
   chain.doFilter(request, response);
}

}

public void destroy() {
}
}
/////////////////////////////////////////////////////
web.xml:

<filter-mapping>
<filter-name>imageRedirectFilter</filter-name>
    <url-pattern>/upload/images/*</url-pattern>
</filter-mapping>

问题：红色的那句并不能请求转移而显示error.gif的图片。
必须将红色的那句写成：request.getRequestDispatcher("error.jsp").forward(request,response);
在error.jsp中再显示error.gif图片才可以，为什么？
error.jsp内容如下：
<html>
  <head>
    <base href="<%=basePath%>">

    <title>My JSP 'error.jsp' starting page</title>
   </head>

  <body>
  <%=path %><br>
  <%=basePath %><br>
    <img src="error.gif">
    </body>
</html>