title: How to check a HttpServletResponse object is valid
body: <p>i have a thread which is responsible for sending message and that thread holds a HashMap which hold userId as key and  HttpServletResponse object for sending message. My question is how to check the HttpServletResponse object is valid before using it to send message? because the user may log out. It is giving me error if i try to send message a user who has been logged out. or can i delete the entry of that Map from outside(when the user log out).</p>