Socket server stream messages in stream are mixed

Question

I am implementing multiplayer game with server socket mechanism.

• Game is running in one loop mechanism.
• Server only broadcast any message to every player connected.
• Server creates thread for every connected player
• Messages are JSONs and they are being sent every 100ms from client side.
• Messages are being broadcasted by server and read by client without any interval.

This is server loop:

    while (true) {
                try {
                    System.out.println("LISTENING...");
                    Socket clientSocket = serverSocket.accept();
                    System.out.println("GOT CONNECTION");

                    PlayerThread playerThread = new PlayerThread(clientSocket);
                    playerThread.start();
                } catch (IOException e) {
                    LOGGER.log(Level.SEVERE, "Exception occured on server");
                    break;
                }
            }

This is PlayerThread loop:

        while (true) {
            try {
                String inputMessage = in.readUTF();
                System.out.println("GOT: " + inputMessage);
                JsonNode jsonNode = objectMapper.readTree(inputMessage);
                String senderName = jsonNode.get("senderName").asText();

                if (!players.contains(this)) {
                    playerName = senderName;
                    players.add(this);
                }

                for (PlayerThread p : players) {
                    p.getOut().writeUTF(inputMessage);
                }

And finally listening to messages:

public void listen() {
        if (connected) {
            try {
                if (in.available() > 0) {
                    String inputMessage = in.readUTF();
                    System.out.println("GOT MESSAGE: " + inputMessage);
                    handleMessages(inputMessage);
                }
            } catch (IOException e) {
                LOGGER.log(Level.INFO, "Connection lost");
                connected = false;
            }
        } else
            LOGGER.log(Level.INFO, "Player is not connected");
    }

Method above is run in main game loop. It checks if there's something in inputStream and then reads it.

This is how correct message looks like:

GOT MESSAGE: {"type":"UPDATE_STATE","content":null,"senderName":"GRACZ 52","posX":10.0,"posY":5.0}

It works ok when there are 2 players, but the more players connect the more probably is to get message like this (or similiar broken messages):

GOT MESSAGE: 0} U{"type":"UPDATE_STATE","content":null,"senderName":"GRACZ 65","posX":10.0,"posY":5.0}

or

GOT MESSAGE: {"type":"UPDATE_STATE","content":null,"senderName":"GRACZ 24","pos

There different errors in message like Y letter, half-message or multiple messages in one row. Why such thing happen? It looks like when there are more players and they write into output stream in server side, this stream is not read yet so they are appending and appending. But it doesn't explain why there are broken and most imporant, how to resolve it?

I can move reading stream to another thread because in.readUTF() locks process but I wanted to keep it synchronized in main game loop and I don't think this will help (am I wrong?)

Answer 1

You need to synchronize your write loop on an object that's common between all PlayerThreads so that messages don't interleave.

         synchronized(/*Should be a "global" server object*/) {
           for (PlayerThread p : players) {
                p.getOut().writeUTF(inputMessage);
            }
         }