服务器-客户端模型中的Java网络

Question

I have a question regarding waiting on messages in a network using a server-client model.

You can distinguish the received messages in two different types:

1. Ad-hoc messages, for example "user has logged in", which do not relate to anything else in the client.

2. Responsive messages, for example when you yourself are trying to log in.

My question is about the latter, namely I use this scheme:

1. Send login request to server (in form of "login username password-encrypted")

2. (Client) Wait until server responds or until timeout of five seconds happens.

3. (Server) Respond either with "loginconfirmed" or "logindenied".

So I am wondering what is the best way to code this, I have some old code here, but I think the coupling in here is way too high and I have already rewritten part of the framework around this code (It also resides in a graphical class, which makes no sense at all and I shouldn't have done that):

public void initLoginPanel() {
    setLoginStatus(LoginStatus.TIMEOUT);    //standard value
    LoginPanelOld loginPanel = new LoginPanelOld();
    int result = JOptionPane.showConfirmDialog(this, loginPanel, "Please log in", JOptionPane.OK_CANCEL_OPTION);
    if (result == JOptionPane.OK_OPTION) {
        String login = loginPanel.getLoginValue();
        if (!login.isEmpty() && login.split(" ").length == 1) {
            Network.getInstance().waitFor("loginconfirmed");
            Network.getInstance().waitFor("logindenied");
            Network.getInstance().send("login " + login);
            initSimpleTextPanel("Logging in...");
            try {
                //TODO synchronize on some "login waiter object"
                synchronized(this) {
                    wait();
                }
                switch (loginStatus) {
                    case CONFIRMED:
                        JOptionPane.showMessageDialog(this, "Login succesful.");
                        break;
                    case DENIED:
                        JOptionPane.showMessageDialog(this, "Login denied.");
                        initLoginPanel();
                        break;
                    case TIMEOUT:
                        JOptionPane.showMessageDialog(this, "Could not connect to the server.");
                        initLoginPanel();
                        break;
                    default:
                        JOptionPane.showMessageDialog(this, "An unexpected error has occured.");
                        initLoginPanel();
                        break;
                }
                removeAll();
                revalidate();
                repaint();
            } catch (InterruptedException ex) {
                Logger.getLogger(MainPanelOld.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
        else {
            JOptionPane.showMessageDialog(this, "You have entered invalid details.");
            initLoginPanel();
        }
    }
    else if (result == JOptionPane.CANCEL_OPTION || result == JOptionPane.CLOSED_OPTION) {
        initLoginPanel();
    }
}

I use synchronisation, but I don't really like it, so if possible I would want to avoid it. However the problem kind of is that the network runs in a different thread, and a responsive can come in any moment, however there are only five seconds in which the login process wants a response.

Rewritten code without networking:

public class LoginProcedure implements Procedure {
    private final GUI gui;

    private LoginPanel loginPanel;

    public LoginProcedure(final GUI gui) {
        this.gui = gui;
    }

    @Override
    public void start() {
        loginPanel = new LoginPanel(this);
        gui.setPanel(loginPanel);
    }

    @Override
    public void finish() {
        Controller.getInstance().finishProcedure();
    }

    public void tryLogin(final String username, final char[] password) {
        if (username.isEmpty()) {
            loginPanel.setErrorMessage("Please enter your username.");
            return;
        }
        if (password.length == 0) {
            loginPanel.setErrorMessage("Please enter your password.");
            return;
        }
        gui.setPanel(new LoadingPanel());
    }
}

In the new code it should start with the networking after the gui.setPanel(new LoadingPanel()); .

So once more to repeat the main question: How can I make the Networking class (a class reading responses from the server and processing them) only react to "loginconfirmed" and "logindenied" in a given timeout at a given moment, and redirect the event to the given LoginProcedure object?

Answer 1

I would design it in such a way as to enable a sync/async messaging system.

• Each exchangeable message must have a UUID, and a 'originID' property.
• An async message is simply sent from the emitter to the receiver.
• A synchronous message have its content sent to the receiver, and enters a 'waiting reply' queue. The caller thread is paused, or pings the queue from time to time.
• The receiver parses the original message, prepares a reply and fills the originID property with the original message's ID.
• The reply message is sent to the emitter.
• The emitter parses it, recognizes that it's a reply, and signals the 'waiting reply' queue.
• The original thread is allowed to proceed, with the reply in hand.
• If no reply arrives, then a TimeOutException may be raised.