How to Kill Threads in Server for Disconnected Clients in Java

Question

I was Developing a Multi threaded Server/Client Socket in java. Whenever a client Connects to the Server, a Thread for it will be created. but when a client got disconnected or its process closed by user, its thread will still remain in Server. how can I know a socket is disconnected then I can kill the corresponding thread in server? by the way, I have a checking system in Server which checks if the client is already registered with an id or not. so for example when a client with ID=12 connects and suddenly disconnects then wants to connect again, the server will not let the client get connected because it's server keeps its socket information in an ArrayList and tells the client that a client with ID=12 is already running in the server. please help me how to fix this

Here is my Server Code:

import java.net.ServerSocket;
import java.net.Socket;

public class Server {

    public static void main(String[] args) {
        try {
            ServerSocket serverSocket = new ServerSocket(9898);
            System.out.println("Server Created!");
            while (true) {
                Socket socket = serverSocket.accept();
                Thread t = new Thread(() -> ClientManager.getInstance().run(socket));
                t.start();
                System.out.println("Thread for Socket " + socket.getPort() + " Created");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

and this is my Client Manager Code:

import java.io.IOException;
import java.net.Socket;
import java.util.ArrayList;
import java.util.HashMap;

public class  ClientManager {

    private ArrayList<SocketHub> socketHubs = new ArrayList<>();
    private ArrayList<SocketPhone> socketPhones = new ArrayList<>();

    private static ClientManager instance;

    public static ClientManager getInstance() {
        if (instance == null) {
            instance = new ClientManager();
        }
        return instance;
    }

    public void run(Socket socket) {
        String identity = null;
        String id = null;

        try {
            boolean receiveData = false;
            while (!receiveData) {
                int i = socket.getInputStream().available();
                while (i > 0) {
                    byte[] buffer = new byte[i];
                    socket.getInputStream().read(buffer);
                    identity = new String(buffer);
                    String[] idAndIdendity = identity.split("\\s");
                    identity = idAndIdendity[0];
                    id = idAndIdendity[1];
                    System.out.println("Identity & ID Received: " + identity + " " + id);
                    receiveData = true;
                    break;
                }
            }
            if (identity.toLowerCase().equals("hub")) {
                if (findSocket(id) != null) {
                    System.out.println("The Hub is Already Registered!");
                    socket.getOutputStream().write("You are already Registered!".getBytes());
                }
                else {
                    SocketHub socketHub = new SocketHub(socket, id);
                    socketHubs.add(socketHub);
                    new Thread(socketHub).start();
                    System.out.println("Hub Socket " + id + " Created!");
                }
            }
            else if (identity.toLowerCase().equals("phone")) {
                SocketHub socketHub = findSocket(id);
                SocketPhone socketPhone = new SocketPhone(socket, socketHub);
                socketHub.setSocketPhone(socketPhone);
                Thread t = new Thread(socketPhone);
                socketPhones.add(socketPhone);
                t.start();
                System.out.println("Phone Socket " + id + " Created!");
                System.out.println(socketPhone.getSocketPhone() + " is Connected to "+ socketHub.getSocketHub());
            }
            System.out.println("=====================================");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private SocketHub findSocket(String id) {
        for (SocketHub socket : socketHubs) {
            if (socket.getId().equals(id))
                return socket;
        }
        return null;
    }
}

Answer 1

There are a few minor "mistakes" in your code.

Why would you write received data in a buffer first to only create a new string with it?

Create a BufferedReader with the InputStreamReader from the InputStream of the Socket.

bufferedReader = new BufferedReader(new InputStreamReader(client.getInputStream()));

Use the method bufferedReader.readLine() or bufferedReader.read() to get the string right away.

To your question - read() will return -1, readLine() will return null if the connection was closed remotely by the client (the stream ended). - You can just check those conditions and stop the thread thread.interrupt().

If the remote client does follow the teardown phase of the TCP protocol you will get a SocketException when trying to read()/readLine()/write() from the sockets inputStream - in this example from the bufferedReader(). - Catch the exception and close the thread thread.interrupt().