调用输出流后无法写入cookie

使用response的getOutputStream后无法添加cookie

ServletOutputStream os = response.getOutputStream();

    String str = CheckCodeUtil.outputVerifyImage(100, 50, os, 4);
    os.close();
    Cookie cookie = new Cookie("checkCode", "str55");
    //System.out.println("checkCode: " + str);
    cookie.setMaxAge(60*60*24*7);
    response.addCookie(cookie);
    System.out.println("I was executed");

这两者之间是有什么冲突吗，即使关掉输出后仍无法写入cookie