套接字编程中的代码卡在readLine（）中

Question

I am new to socket programming. I copied a java server client code from a tutorial where the client sends a string to the server and the server capitalizes the string and sends it back to the client.

I have a JTextField where I input the string to be capitalized and a JTextArea where the capitalized string output is shown.

But the problem is when I give input in the JTextField and press enter the code hangs. By debuging I have found that the code is hang when client tries to read the capitalized string sent by server with readLine().

I do not know how to get rid of it. I have read several similar questions in Stack Overflow and tried the solutions but no hope.

Server code:

public class CapitalizeServer {

/**
 * @param args the command line arguments
 * @throws java.io.IOException
 */
public static void main(String[] args) throws IOException {
    System.out.println("The Capitalization Server is running");

    int clientNumber = 0;
    try (ServerSocket listener = new ServerSocket(1234)) {
        while(true) {
            new Capitalizer(listener.accept(), clientNumber++).start();
        } 
    }
} 

private static class Capitalizer extends Thread {

    Socket socket;
    int clientNumber;

    public Capitalizer(Socket socket, int clientNumber) {
        this.socket = socket;
        this.clientNumber = clientNumber;
        log("New connection with client# " + clientNumber + " at " + socket);  
    }

    @Override
    public void run() {
        try {

            // Decorate the streams so we can send characters
            // and not just bytes.  Ensure output is flushed
            // after every newline.
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

            // Send a welcome message to the client.
            out.println("Hello, you are client # " + clientNumber + ".");
            out.println("Enter a line with only a period to quit");

            // Get messages from the client, line by line; return them capitalized
            while(true) {
                String input = in.readLine();
                if(input == null || input.equals(".")) {
                    break;
                }

                //System.out.println("Coming input: " + input);
                out.println(input.toUpperCase());
                //System.out.println("UpperCase: " + input.toUpperCase());
            }

        } catch (IOException ex) {
            log("Error handling client# " + clientNumber + ": " + ex );

        } finally {
            try {
                socket.close();
            } catch (IOException ex) {
                log("Could not close a socket. What's going on?");
            }

            log("Connection with client# " + clientNumber + " closed");
        }
    }

    /**
     * Logs a simple message.  In this case we just write the
     * message to the server applications standard output.
     */
    private void log(String message) {
        System.out.println(message);
    }
}

}

Client code:

public class CapitalizeClient {

private BufferedReader in;
private PrintWriter out;
private JFrame frame = new JFrame("Capitalize Client");
private final JTextField dataField = new JTextField(40);
private final JTextArea messageArea = new JTextArea(8,60);    
/**
 * Constructs the client by laying out the GUI and registering a
 * listener with the textField so that pressing Enter in the
 * listener sends the textField contents to the server.
 */
public CapitalizeClient() {

    // Layout GUI
    messageArea.setEditable(false);
    frame.getContentPane().add(dataField, "North");
    frame.getContentPane().add(new JScrollPane(messageArea), "Center");

    // Add Listeners
    dataField.addActionListener(new ActionListener() {

        /**
         * Responds to pressing the enter key in the textField
         * by sending the contents of the text field to the
         * server and displaying the response from the server
         * in the text area.  If the response is "." we exit
         * the whole application, which closes all sockets,
         * streams and windows.
         */
        @Override
        public void actionPerformed(ActionEvent e) {
            out.println(dataField.getText());
            String response;

            try {
    // ******* The code stucks here *************
                response = in.readLine();

                if(response == null) {
                    System.exit(0); 
                } 
            } catch (IOException ex) {
                response = "Error: " + ex;
            }
            messageArea.append(response + "\n");
            dataField.selectAll();
        } 
    });
}

public void connectToServer() throws IOException {

    // Get the server address from a dialog box.
    String serverAddress = JOptionPane.showInputDialog(frame,  "Enter IP Address of the Server:" ,
        "Welcome to the Capitalization Program", JOptionPane.QUESTION_MESSAGE); 

    // Make connection and initialize streams
    Socket socket = new Socket(serverAddress, 1234);
    in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    out = new PrintWriter(socket.getOutputStream(), true);

    // Consume the initial welcoming messages from the server
    for(int i=0; i < 3; i++) {
        messageArea.append(in.readLine() + "\n");
    }  
}

public static void main(String[]args) throws IOException {
    CapitalizeClient client = new CapitalizeClient();
    client.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    client.frame.pack();
    client.frame.setVisible(true);
    client.connectToServer();
}

}

Answer 1

your problem is on client side. Replace the code of method

public void connectToServer() throws IOException {

    // Get the server address from a dialog box.
    String serverAddress = JOptionPane.showInputDialog(frame, "Enter IP Address of the Server:",
            "Welcome to the Capitalization Program", JOptionPane.QUESTION_MESSAGE);

    // Make connection and initialize streams
    Socket socket = new Socket(serverAddress, 1234);
    in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    out = new PrintWriter(socket.getOutputStream(), true);

    // Consume the initial welcoming messages from the server
    for (int i = 0; i < 2; i++) {
        messageArea.append(in.readLine() + "\n");
    }
}

You need to iterate through the loop only twice, as your welcome message is two lines not three.

The communication always needs to be synchronous. If you send two lines from server you need to read two lines on client before you send something.