关于使用socket通信，android客户端，java写的服务器，传输数据时总出现异常

客户端：

 public void run() {
 // TODO Auto-generated method stub
 try {
        s = new Socket(ip, port);

        DataOutputStream os = new DataOutputStream(s.getOutputStream());
        os.writeUTF("可以准备接收了吗？");


        ObjectOutputStream out = new ObjectOutputStream(s.getOutputStream());
        out.writeObject(u);
        out.flush();
        out.close();

        System.out.println("用户名是："+u.getID());


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

服务器：

 try{
            ServerSocket ss = new ServerSocket(9999);
            System.out.println("服务器正在监听！");
            Socket s = ss.accept();
            System.out.println("建立连接！");

            DataInputStream is = new DataInputStream(s.getInputStream());
            String str = is.readUTF();
            System.out.println(str);

            ObjectInputStream ois = new ObjectInputStream(s.getInputStream());
            User u = (User)ois.readObject();

            ois.close();

            System.out.println("用户ID ："+u.getID());


        }catch(Exception e){
            e.printStackTrace();
        }

异常：

java.net.SocketException: Connection reset
 at java.net.SocketInputStream.read(Unknown Source)
 at java.net.SocketInputStream.read(Unknown Source)
 at java.net.SocketInputStream.read(Unknown Source)
 at java.io.DataInputStream.readUnsignedShort(Unknown Source)
 at java.io.DataInputStream.readUTF(Unknown Source)
 at java.io.DataInputStream.readUTF(Unknown Source)
 at com.server.EasyServer.main(EasyServer.java:24)
 （在os.writeUTF一行）