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I have two Java projects that communicate with sockets through strings. One is a client and the other is a server. The server accepts the connection trough the "ServerSocket" and creates a new "Session" (Thread) with a freshly created "Socket".

Meanwhile, the client only has a "Socket", once that socket is connected, the client creates a "ClientSession" (Thread, pretty similar to "Session).

What I want is the server to ask the client his username through the "USERNAME" string, and the client to answers with his username. But the answer never come back. I think it's maybe a synchronisation problem with the BufferedReader and the PrintWriter lines fired at the wrong place.

-----------Server Code---------------

Login Class:

public class Login implements Runnable {

private ArrayList<Session> sessions;
private int port;
private Thread thread;
private ServerSocket serverSocket;
private Game game;

public Login(int port, Game game) throws Exception {
    this.sessions = new ArrayList();
    this.port = port;
    this.serverSocket = new ServerSocket(port);
    this.game = game;
    this.thread = new Thread(this);
    this.thread.start();
}

@Override
public void run() {
    while(true) {
        Socket socket;
        try {
            System.out.println("[Server Network] Waiting for a new player to log in.");
            socket = serverSocket.accept();
            sessions.add(new Session(socket,this));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

public void addPlayer(Player p) {
    this.game.addPlayer(p);
}

}

Session Class:

public class Session implements Runnable {

private String username;
private Thread thread;
private Socket socket;
private BufferedReader br;
private OutputStreamWriter os;
private PrintWriter out;
private Login login;

private boolean ready;

public Session(Socket socket, Login login) throws IOException {
    this.login = login;
    this.socket = socket;
    this.ready = false;

    this.br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    this.os = new OutputStreamWriter(socket.getOutputStream());
    this.out = new PrintWriter(os);

    System.out.println("[Server network] Session created for client with ip: "+socket.getInetAddress().getHostAddress());

    this.thread = new Thread(this);
    this.thread.start();
}

public void send(String m) {
    System.out.println("[Server network] Sending message: "+m);
    out.write(m);
    out.flush();
    System.out.println("[Server network] Message sent: "+m);
}


@Override
public void run() {
    while (true) {
        if (!ready) {
            try {
                this.send("USERNAME");
                this.username = br.readLine();
                this.ready = true;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        try {
            String request = br.readLine();
            System.out.println(request);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

}

-----------Client Code-------------

Game Class:

public class Game {

private Window window;

private LocalPlayer localPlayer;
private ArrayList<Player> players;
private Map map;

private String username;

private String serverIpAddress;
private ClientSession clientSession;
private static int port = 23123;

public Game(Window window, Map map) throws UnknownHostException, IOException {
    System.out.println("Game Launched.");

    //Asks for the server ip address and creates a session.
    //this.serverIpAddress = JOptionPane.showInputDialog(null,"Please enter the server ip address.",JOptionPane.QUESTION_MESSAGE);
    this.serverIpAddress = "localhost";

    //this.username = JOptionPane.showInputDialog(null,"What is your username?",JOptionPane.QUESTION_MESSAGE);
    this.username = "GriffinBabe";

    this.clientSession = new ClientSession(new Socket(serverIpAddress,port),this);

    this.window = window;
    this.localPlayer = new LocalPlayer(new Warrior(),this.username,'R');
    this.map = map;

}

public LocalPlayer getLocalPlayer() {
    return this.localPlayer;
}

public Map getMap() {
    return map;
}

}

ClientSession Class:

public class ClientSession implements Runnable {

private Socket socket;
private Thread thread;
private BufferedReader br;
private OutputStreamWriter os;
private PrintWriter out;
private Game game;

public ClientSession(Socket socket, Game game) throws IOException {
    this.game = game;
    this.socket = socket;

    this.br = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
    this.os = new OutputStreamWriter(this.socket.getOutputStream());
    this.out = new PrintWriter(os);

    System.out.println("[Client network] Session created for server with ip: "+socket.getInetAddress().getHostAddress());

    this.thread = new Thread(this);
    this.thread.start();
}

public void send(String m) {
    System.out.println("[Client network] Sending message: "+m);
    out.write(m);
    out.flush();
    System.out.println("[Client network] Message sent: "+m);
}

@Override
public void run() {
    while(true) {
        try {
            String request = br.readLine();
            System.out.println(request);
            switch (request) {
            case "USERNAME":
                send(game.getLocalPlayer().getUsername());
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

}

Here are the Console logs:

Server (launched first, of course):

Server Console log

Client :

Client Console log

2 Answers 2

3

Your server sends "USERNAME" and then waits. The client reads the next line, and blocks until it receives it. So you have a deadlock, since the server never sends an EOL character. If the client reads a line, the server should send a line.

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2 Comments

Thank you. That was really that simple... Just in case, Is there another way to read a String and not a Line with the BufferedReader ?
You can read as many characters you want. But if you have no idea of the length of the String that the server is supposed to send, you don't know how many you must read. If your protocol always starts with the server sending USERNAME, you can just read 8 characters.
2

In your send methods, try changing out.write(m) to out.println(m).

The readLine() method will keep reading until it hits a line terminator or end of stream, whichever comes first. The println() method will automatically put a line terminatorat the end of the string you are trying to send.

https://docs.oracle.com/javase/7/docs/api/java/io/PrintWriter.html#println()

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