捕获这个错误,把OutputStream关闭就是了.
try {  OutputStream outp = response.getOutputStream();
            while ( (len = RAFile.read(b)) > 0) {
              outp.write(b, 0, len);
}
        }
        catch (Exception ex) {
          System.out.println("下载出错,错误信息为:" + ex.getMessage());
        }
        finally {
          if (RAFile != null) {
            System.out.println("关闭打开的文件");
            RAFile.close();
          } System.out.println("关闭response的输出流");
           outp.close();
          return;
        }