How to send messages of offline user after getting online in java websocket?

Question

I am trying to write a chat using webcocket, for offline users I have used java Queue, If user is offline I keep messages in a Queue and when user gets online I check if Queue is empty and if it's not, using a loop i remove each messages from Queue. The problem is it just sends last message to user even though all messages are in queue, here is my onOpen method:

@ServerEndpoint(value = "/chat/{room}/{user}", encoders = ChatMessageEncoder.class, decoders = ChatMessageDecoder.class)
 public class ChatEndpoint {


private final Logger log = Logger.getLogger(getClass().getName());
private static final Map<String, Queue<ChatMessage>> userMessageBuffer = new HashMap<>();

@OnOpen
public void open(final Session session,
        @PathParam("room") final String room,
        @PathParam("user") final String userId) {

    log.info("session openend and bound to room: " + room);
    // session.getUserProperties().put("room", room);
    session.getUserProperties().put("user", userId);

    Queue<ChatMessage> userMsgs = userMessageBuffer.get(userId);
    ChatMessage sendChat = new ChatMessage();

    if (userMsgs != null && !userMsgs.isEmpty()) {

        for (int i = 0; i < userMsgs.size(); i++) {

            sendChat = userMsgs.remove();
            System.out.println("size!!!!!! " + sendChat.getMessage());
            try {

                    session.getBasicRemote().sendObject(sendChat);

            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (EncodeException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }
    }

}}

Does any one knows where is the problem?

Answer 1

I did not check the complete code, but certainly

      for (int i = 0; i < userMsgs.size(); i++)

is your problem. Because you are using i++ and checking userMsgs.size() as a condition in for loop. So you are increasing i by 1 and decreasing userMsgs size by 1, effectively you will be able access only half of the elements in the queue.

say you have 8 elements in the queue, initially (Imagine this like Admiral General Aladeen explaining about round shaped missile)

               i=0 and userMsgs.size()=8
               i=1 and userMsgs.size()=7
               i=2 and userMsgs.size()=6
               i=3 and userMsgs.size()=5
               i=4 and userMsgs.size()=4 // comes out of loop.

you should use a while loop instead,

  while(!userMsgs.isEmpty()){
   .....
  }

You said you are able to send only last message to the user, may be that is because you have only 2 messages in the queue. I know it's rare but that should be the case according your code.