How do I use Thread.join() in this situation?

Question

I'm writing a simple roulette game that can take multiple players at a time. I'm trying to deserialize a .dat file players.dat, however I'm getting a FileNotFoundException that I suspect is due to my main thread running concurrently to another thread.

My main file (note that readObj is simply a convenience method that uses ObjectInputStream to read the object back):

    Collection<Player> mPlayerList = null;

    try {
        connectToServer();
    } catch (IOException e) {
        System.err.println("IO exception when trying to write/connect to server");
        e.printStackTrace();
    }

    // read input stream and attempt to deserialize
            // stack trace points to the error being from here
    try {
        mPlayerList = (Collection<Player>) readObj("players.dat");
    } catch (IOException e) {
        System.err.println("IO exception when trying to read master player list");
        e.printStackTrace();    
        System.exit(1);     
    } catch (ClassNotFoundException e) {
        System.err.println("Class not found exception when trying to read master player list");
        e.printStackTrace();    
        System.exit(1);                 
    }

In my first try statement, I attempt to connect to a server running off of my computer:

public class GameEngineServerStub {

public static void main(String[] args) throws IOException {

    GameEngineImpl myEngine = new GameEngineImpl();

    int portNumber = 4444;
    boolean listening = true;

    // While statement loops forever, waiting for a client to connect.
    // Once client connects, accept, create a new Thread, pass the socket to it, and requests it to start
    try (ServerSocket serverSocket = new ServerSocket(portNumber)) {
        while (listening) { 
            new GameEngineServerStubThread(serverSocket.accept(), myEngine).start();
        }
    } catch (IOException e) {
        System.err.println("Could not listen on port 4444");
        System.exit(-1);
    }

}

 }

The server launches this thread, which is responsible for requesting a Collection<Player> object from another file, and attempting to serialize it.

public void run() {

    try (
        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

    ) {

        Collection<Player> currentPlayers = engine.getAllPlayers();

        try {
            ObjectOutputStream objOut = new ObjectOutputStream(new FileOutputStream("players.dat"));
            objOut.writeObject(currentPlayers);
        } catch (IOException e) {
            System.err.println("IO Exception when trying to get master player list!");
            e.printStackTrace();
            System.exit(1);                     
        }

    } catch(IOException e) {
        e.printStackTrace();
    }

}
}

I assume that I will need to pause my main thread, to wait for my serialization thread to finish, but I'm unsure of how to do so.

Answer 1

I'm unsure of some aspect of your question, but there are a series of things you could do depending on your situation

Your server and client are running on different JVMs

This case you need an inter process locks which is hard to do, you could try JCIP maybe you could look at here to get some ideas

Your server and client are running on the same JVM but in different threads

This case it's much easier, you can use the number of java's locks to achieve this, for example the easiest would be to create a ReentrantReadWriteLock and pass it to both threads and make the server hold the write lock until the serialization is done, and make the client block on the read lock. You could read more about locks and locking in java here and here