客户端在多线程客户端/服务器应用程序上断开连接

Question

This is a multithreading Server/Client application I am doing using swing components, kind of like a chat application. Everything works fine, unless I open 2 or more Clients on the same computer. Then, still everything works fine with the chat and the messages, but if I close one of the Clients, then the other 2 close as well. I know it must have something to do with multithreading but I can't quite figure out exactly what the problem is.

Server.java

import javax.swing.*;
import java.awt.*;
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.A

rrayList;

public class Server extends JFrame {

    private int port;
String host;
private ArrayList<ClientThread> clientThreads;

private JTextArea textArea;

private ServerSocket serverSocket = null;
Socket connection;
private PrintWriter out;
private BufferedReader in;


Server(int port) {
    this.port = port;
    clientThreads = new ArrayList<>();

    // Tar hand om JFrame.
    setLayout(new BorderLayout());
    setVisible(true);
    setLocation(750, 0);
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    textArea = new JTextArea(15, 60);
    JPanel centerPanel = new JPanel();
    centerPanel.add(new JScrollPane(textArea));
    add(centerPanel, BorderLayout.CENTER);
    pack();

    try {
        serverSocket = new ServerSocket(port);
        host = serverSocket.getInetAddress().getLocalHost().getHostAddress();
    } catch (IOException e) {
        System.err.println(e.getMessage());
    }

    // Titeln skrivs ut.
    updateTitle();

    while (true) {
        try {
            connection = serverSocket.accept();
            // Byter titel
            updateTitle();

            // Startar en Thread för den nya socket:en.
            ClientThread task = new ClientThread(connection);
            clientThreads.add(task);
            task.start();
        } catch (IOException ex) {
            // Ignorera.
        }
    }
}

public static void main(String[] args) {
    if (args.length == 0) {
        // Skickar default värde.
        new Server(2000);
    } else if (args.length == 1) {
        // Skickar argumentet
        new Server(Integer.parseInt(args[0]));
    }
}

public class ClientThread extends Thread {

    // Threadklassen till klientsocket:en.
    private Socket clientSocket;
    private String clientHost;

    public ClientThread(Socket connection) {
        this.clientSocket = connection;
        clientHost = clientSocket.getInetAddress().getHostName();
    }

    @Override
    public void run() {
        // Skriver ut att någon har "loggat in" i JTextArea.
        textArea.append("CLIENT: " + clientHost + "CONNECTED" + "\n");
        // Metoden som kollar om klienten skriver en chatmeddelande.

        try {
            synchronized (this) {
                out = new PrintWriter(clientSocket.getOutputStream(), true);
                in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
                while (true) {
                    synchronized (this) {
                        if (in == null) System.out.println("NULL");
                        String line = in.readLine();
                        textArea.append("CLIENT: " + clientHost + " BROADCAST: " + line + "\n");
                        broadcast(line);
                    }
                }


          }
                // Byter titel och stänger allt.
//                if (out != null) out.close();
//                if (in != null) in.close();
//                if (clientSocket != null) clientSocket.close();
            } catch (IOException e){
                    synchronized (this) {
                        e.printStackTrace();
                    // Vad händer när en klient blir disconnected.
                    textArea.append("CLIENT: " + clientHost + " DISCONNECTED" + "\n");
                    for (Thread t : clientThreads) {
                        if (t == this) {
                            t = null;
                        }
                        clientThreads.remove(this);
                    }
                    System.out.println(clientThreads.size());
                    updateTitle();
                }
            }

    }

}

public void broadcast(String message) {
    // Synchronized för att slippa krocka.

        try {
            synchronized (this) {
            // Skriver ut meddelandet i JTextArea och sänder det till alla klienter.
            for (int i = clientThreads.size() - 1; i >= 0; i--) {
                out = new PrintWriter(clientThreads.get(i).clientSocket.getOutputStream(), true);
                out.write(message + "\n");
                out.flush();
            }
            }
        } catch (IOException e) {
            System.err.println(e.getMessage());
        }

}

public void updateTitle() {
    setTitle("HOST: " + host + " | PORT: " + port + " | NUMBER OF CLIENTS: " + 

clientThreads.size());
    }
}

Client.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.*;
import java.net.Socket;
import java.net.SocketTimeoutException;

public class Client extends JFrame {

    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;

    private JTextField inputTextField = new JTextField(60);
    private JTextArea textArea = new JTextArea(15, 60);

    // Tråd som kallas för att läsa rader från servern.
    private Thread backgroundThread = new Thread(new Runnable() {
        @Override
        public void run() {
            String line;
            // Eviga loopen
            try {
                in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                line = in.readLine();
                while (line != null) {
                    textArea.append(line + "\n");
                    line = in.readLine();
                }
            } catch (SocketTimeoutException e2) {
                // Ignorera.
            } catch (IOException e) {
                // Ignorera.
            }

//            try {
//                if (out != null) out.close();
//                if (in != null) in.close();
//                if (socket != null) socket.close();
//            } catch (IOException e) {
                // Ignorera.
//            }
        }
    });

    // Vad som händer när man trycker på enter i JTextField
    private AbstractAction onEnterPressAction = new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            String textToSend = inputTextField.getText();
            inputTextField.setText("");
            try {
                // Skickar ut strängen till servern.
                out = new PrintWriter(new OutputStreamWriter(socket.getOutputStream(), "ISO-8859-1"), true);
                out.write(textToSend + "\n");
                out.flush();
            } catch (SocketTimeoutException e2) {
                // Ignorera
            } catch (IOException e1) {
                e1.printStackTrace();
            }
        }
    };

    public Client(String host, int port) {
        // Konstruktorn.
        // Skapar en Socket.
        try {
            socket = new Socket(host, port);
            socket.setSoTimeout(15000);
        } catch (IOException e) {
            // Kunde inte kopplas till servern.
            JOptionPane.showMessageDialog(null, "Could not connect to server", "Connection Error", JOptionPane.ERROR_MESSAGE);
            System.exit(0);
        }

        // Sätter titelrad
        setTitle("CONNECTED TO SERVER: " + host + " IN PORT: " + port);

        // Startar eviga läsloopen
        backgroundThread.start();

        // Tar hand om JFrame.
        setLayout(new BorderLayout());
        setVisible(true);
        // Sätter en annan operation när JFrame stängs för att stoppa loopen.
        addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });

        // Söder JPanel (JTextField som man skriver meddelande i)
        JPanel southPanel = new JPanel();
        southPanel.add(inputTextField);
        inputTextField.addActionListener(onEnterPressAction);

        // Centrala JPanel (JTextArea som visar alla meddelanden).
        JPanel centerPanel = new JPanel();
        JScrollPane scrollPane = new JScrollPane(textArea);
        centerPanel.add(scrollPane);

        // Adderar och packar.
        add(centerPanel, BorderLayout.CENTER);
        add(southPanel, BorderLayout.SOUTH);
        pack();
        setLocationRelativeTo(null);
    }

    public static void main(String[] args) {
//        if (args.length == 0) {
//            // Skickar default värden.
//            new Client("127.0.0.1", 2000);
//        } else if (args.length == 1) {
//            // Skickar argument och default port värde.
//            new Client(args[0], 2000);
//        } else if (args.length == 2) {
//            // Skickar argumenter.
//            new Client(args[0], Integer.parseInt(args[1]));
//        }
        new Client("192.168.1.66", 2000);
    }
}

EDIT: Here is the error I'm getting, but that might not indicate where the problem is.

2
java.net.SocketException: Connection reset
    at java.net.SocketInputStream.read(SocketInputStream.java:189)
    at java.net.SocketInputStream.read(SocketInputStream.java:121)
    at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
    at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
    at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
    at java.io.InputStreamReader.read(InputStreamReader.java:184)
    at java.io.BufferedReader.fill(BufferedReader.java:161)
    at java.io.BufferedReader.readLine(BufferedReader.java:324)
    at java.io.BufferedReader.readLine(BufferedReader.java:389)
    at com.company.Server$ClientThread.run(Server.java:98)
1
java.net.SocketException: Connection reset
    at java.net.SocketInputStream.read(SocketInputStream.java:134)
    at java.net.SocketInputStream.read(SocketInputStream.java:121)
    at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
    at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
    at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
    at java.io.InputStreamReader.read(InputStreamReader.java:184)
    at java.io.BufferedReader.fill(BufferedReader.java:161)
    at java.io.BufferedReader.readLine(BufferedReader.java:324)
    at java.io.BufferedReader.readLine(BufferedReader.java:389)
    at com.company.Server$ClientThread.run(Server.java:98)

This is just the normal behavior of a client that disconnects, the problem is, after I close the 3rd client (if we assume I opened 3), I go to write something on the 2nd and press Enter, it broadcasts the message to all the rest and then that client disconnects immediately, also with the same error. It wasn't my iteration.

Answer 1

I don't think it has much to do with multithreading, per se.

for (Thread t : clientThreads) {
  if (t == this) {
    t = null;
  }
  clientThreads.remove(this);
}

This isn't the right way to iterate through a list and remove items. You are implicitly creating an iterator in the enhanced for loop; calling remove like this is a concurrent modification with respect to that iterator, because there is no way for the iterator to know that you've called remove on the list.

It's unclear what you're really meaning to do here. The conditional is just setting a local variable to null. You may as well just call clientThreads.remove(this) , no loop required.

The correct way to iterate through a list and remove items is to use an explicit iterator, on which you can call the remove method.

Iterator<Thread> it = clientThreads.iterator();
while (it.hasNext()) {
  Thread t = it.next();
  // Do something with t.
  it.remove();
}

And, of course, you would need to ensure exclusive access to clientThreads while you apply either of these methods.

Answer 2

UPDATE: Turned out the problem was in this line in Server class:

in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

the private BufferedReader in is a private attribute of the Server class, therefore, it was being shared among all ClientThreads. Every time this line got called, it switched all the streams to the current socketInputStream. Instead of using a private member for everyone, you should create BufferedReader for each thread:

BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

Answer 3

You are using the below line while iterating over the list in Server.java class. This is the cause for the same.

clientThreads.remove(this);

Instead of directly removing the thread use thread safe iterators. or use CopyOnWriteArrayList this would fix you this issue.