Socket closed as soon as passed into new thread

Question

I am trying to create a multi-threaded duplex chat server. I had it working fine before I moved the code into threads, but now I'm getting "SocketException: Socket is closed" whenever a thread tries to access a socket.

My teacher and I can't figure it out. (teacher knows even less Java than I do; he is a C guy). It seems like the socket is open, but as soon as it goes into the thread, it is closed.

What am I doing wrong?

Client code:

public class Client {
    public static void main(String[] args) throws IOException {
        String hostName = "localhost";
        int portNumber = 6969;
        try (
                Socket socket = new Socket(hostName, portNumber);
        ) {
            System.out.println("Chat connected");
            //Sender
            if (!socket.isClosed())
                new Thread(new Sender(socket)).start();
            //Receiver
            if (!socket.isClosed())
                new Thread(new Receiver(socket)).start();
        } catch (SocketException e) {
            System.out.println("Connection terminated unexpectedly");
        }
    }
}

Server code:

public class Server {
    public static void main(String[] args) throws IOException {
        int portNumber = 6969;
        try (
                ServerSocket serverSocket = new ServerSocket(portNumber);
                Socket clientSocket = serverSocket.accept();
        ) {
            //Receiver
            if (!clientSocket.isClosed())
                new Thread(new Receiver(clientSocket)).start();
            //Sender
            if (!clientSocket.isClosed())
                new Thread(new Sender(clientSocket)).start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Receiver thread code:

class Receiver implements Runnable {

    private Socket socket;

    Receiver(Socket s) throws IOException {
        socket = s;
    }

    @Override
    public void run() {
        try (BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()))) {
            String received;
            do {
                received = in.readLine();
                if (received == null) break;
                System.out.println("Them: " + received);
            } while (!received.contains("/dropmic"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Sender thread code:

class Sender implements Runnable {
    private Socket socket;

    Sender(Socket s) throws IOException {
        socket = s;
    }

    @Override
    public void run() {
        try (PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
             BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in))) {
            String sent;
            do {
                System.out.print("You: ");
                sent = stdIn.readLine();
                if (sent == null) break;
                out.println(sent);
            } while (!sent.contains("/dropmic"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Server output:

java.net.SocketException: Socket is closed
    at java.net.Socket.getOutputStream(Socket.java:943)
    at Sender.run(Sender.java:16)
    at java.lang.Thread.run(Thread.java:745)
java.net.SocketException: Socket closed
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
    at java.net.SocketInputStream.read(SocketInputStream.java:170)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    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 Receiver.run(Receiver.java:19)
    at java.lang.Thread.run(Thread.java:745)

Process finished with exit code 0

Client output (Still running):

Chat connected
You: java.net.SocketException: Socket is closed
    at java.net.Socket.getInputStream(Socket.java:903)
    at Receiver.run(Receiver.java:16)
    at java.lang.Thread.run(Thread.java:745)

Answer 1

You are using a try-with-resources statement, and putting the socket as the resources. However, when you create the new thread, this try-with-resources reaches the end of its code and closes the sockets. Put your sockets inside the actual try statement and close them manually to fix this.