socket的问题,为什么服务器端没有发消息,客户端就不能给客户端发消息呢?

客户端的主要代码如下
 Socket socket = null;
          try{
            socket =  new Socket("127.0.0.1", 9999);
          }catch(Exception e){
              System.out.println(e.getMessage());
              System.out.println("不能连到这个端口");
          }
           BufferedReader in = new BufferedReader(new InputStreamReader(socket.
                   getInputStream()));
           PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
           String aa = in.readLine();
           System.out.println(aa);        boolean flag = true;
        while(flag){
            BufferedReader inUser = new BufferedReader(new InputStreamReader(System.in));
            String userIn = inUser.readLine();
            if(userIn.equals("bye"))
                flag = false;
            out.println(userIn);
            String aa1 = in.readLine();
            System.out.println(aa1);
        }
        socket.close();
服务端的主要代码如下ServerSocket server = new ServerSocket(9999);
                System.out.println("9999 这个端口正在监听");
                Socket socket = server.accept();
                System.out.println("aasdfasfd");
                BufferedReader in = new BufferedReader(new InputStreamReader(
                        socket.getInputStream()));
                PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
                out.println(" 您已经连接成功");\\如果把这条语名屏蔽掉,双方就不能正常通信了,怎么回事呢?
                boolean flag = true;
                while (flag) {
                    String userIn = in.readLine();
                    System.out.println(userIn);
                    if (userIn.equals("bye"))
                        flag = false;                    out.println("信息已收到");                }                socket.close();是不是必须要求 服务端先给客户端一条消息,客户端才能发送消息,还是怎么回事,望指教.