Why is my waiting thread not waking up even though it's being notified?

Question

I have a client server tic-tac-toe game which is attempting to run a different thread (in different terminals) for each player, which I have built in eclipse.

My goal is to make each player make their move, .notify() the other player and then .wait() for the other player to make their move, and alternate that process until the game is done.

toSync is the object used for synchronization

public static final Object toSync = new Object()

and it is found in the Player class (which is extended by both XPlayer and OPlayer).

The lines which seem to be causing the problem are commented in XPlayer and Oplayer:

Both Xplayer and OPlayer have main methods, so that they can be run concurrently. X makes the first move then uses a socket to communicate this move to the server.
The server passes this move to O, who then makes its own move and passes it back to the server. This alternates until the game is done.

Making the first move as the x player works fine, but once the intial move is made, o is supposed to display the board and then prompt the user for their move. This however does not happen: x makes its move, and o is supposedly notified, but in fact never wakes up. The curly brace ending the while loop which is commented in OPlayer is never reached (which I know to be true by the debugging I've done so far).

Class XPlayer:

import java.io.*;

public class XPlayer 
extends Player
implements Runnable
{
    public static volatile boolean xTurn = true;

    public XPlayer() throws IOException
    {
        super();
        mark = LETTER_X;
    }

    public void run()
    {
        try
        {
            System.out.println("Okay " + name + ", You will be the X-Player");

            synchronized(toSync)
            {
                Cell move = makeMove();
                out.println(move.toString());
                board.addMark
                    (move.row(),move.col(),move.mark());
                board.display();

                xTurn = false;
                toSync.notifyAll();    //THIS IS THE LINE THAT ISNT WORKING!!

                System.out.println(WAITING);
            }
            synchronized(toSync)
            {
                while (!xTurn)
                    {toSync.wait();}
            }

            while (!board.isOver())
            {
                synchronized(toSync)
                {
                    String line;
                    do {line = in.readLine();}
                    while (line == null);   

                    Cell opponentMove = Cell.split(line);
                    board.addMark
                        (opponentMove.row(),opponentMove.col(), opponentMove.mark());

                    String move = makeMove().toString();
                    out.println(move);
                    xTurn = false;
                    toSync.notifyAll();

                    while (!xTurn)
                        {toSync.wait();}
                }
            }

            endGame();

            sock.close();
            in.close();
            stdin.close();
            out.close();

        } catch (InterruptedException ie)
        {
            System.out.println("IE IN XPLAYER! " + ie.getMessage());
            System.exit(1);

        } catch (IOException ioe) 
        {
            System.out.println("IOE IN XPLAYER! " + ioe.getMessage());
            System.exit(1);
        }
    }
    public static void main(String[] args)
    {
        try
        {
            XPlayer x = new XPlayer();
            Thread t = new Thread(x);
            t.start();

        } catch(IOException ioe)
        {
            System.err.println
                ("IOE IN XPLAYER MAIN " + ioe.getMessage());
            System.exit(1);
        }
    }

Class OPlayer:

import java.io.*;

public class OPlayer 
extends Player
implements Runnable
{

    public OPlayer() throws IOException
    {
        super();
        mark = LETTER_O;
    }

    public void run() 
    {
        try
        {       
            synchronized(toSync)
            {
                System.out.println("Okay " + name + ", You will be the O-Player");

                System.out.println(WAITING);
                while(!XPlayer.xTurn)
                    {toSync.wait();}    // THIS IS THE LINE THAT ISN'T WAKING UP

                while (!board.isOver())
                {
                    String line;

                    do {line = in.readLine();}
                    while (line == null);   

                    Cell opponentMove = Cell.split(line);
                    board.addMark
                        (opponentMove.row(),opponentMove.col(),opponentMove.mark());

                    Cell move = makeMove();
                    out.println(move.toString());
                    board.addMark(move.row(),move.col(),move.mark());
                    board.display();
                    XPlayer.xTurn = true;
                    toSync.notifyAll();

                    System.out.println(WAITING);
                    while (XPlayer.xTurn)
                        {toSync.wait();}    
                }
            }

            endGame();

            sock.close();
            in.close();
            stdin.close();
            out.close();

        } catch (InterruptedException ie)
        {
            System.out.println("IE IN OPLAYER " + ie.getMessage());
            System.exit(1);

        } catch (IOException ioe)
        {
            System.err.println("IOE IN OPLAYER " + ioe.getMessage());
            System.exit(1);
        }
    }

    public static void main(String[] args)
    {
        try
        {
            OPlayer o = new OPlayer();
            Thread t = new Thread(o);
            t.start();

        } catch(IOException ioe)
        {
            System.err.println("IOE IN OPLAYER MAIN" + ioe.getMessage());
            System.exit(1);
        }
    }
}

As indicated by the code, the toSync.notifyAll() call in XPlayer is not waking up the OPlayer thread, and I am stuck in a deadlock once the first move has been made by the XPlayer

I believe only those 2 classes are needed to resolve the problem but just in case, here are the classes Player Board and TTTServer: Class Player:

import java.net.*;
import java.io.*;

public class Player
implements Constants
{   
    protected static final Object toSync = new Object();

    protected Socket sock;
    protected BufferedReader stdin;
    protected BufferedReader in;
    protected PrintWriter out;

    protected String name;
    protected char mark;
    protected Board board;

    public Player() throws IOException
    {
        sock = new Socket("localhost",1298);
        stdin = new BufferedReader(new InputStreamReader(System.in));
        in = new BufferedReader(new 
                InputStreamReader(sock.getInputStream()));
        out = new PrintWriter(sock.getOutputStream(),true);

        System.out.println(WELCOME);
        System.out.println("Please Enter your name:");
        name = stdin.readLine();
        board = new Board();
    }

    public Cell makeMove() throws IOException
    {
        board.display();

        int row = -1;
        int col = -1;
        do 
        {
            while (row < 0 || row > 2)
            {   
                System.out.println
                    (name + ", what row would you like your next move to be in?");
                row = Integer.parseInt(stdin.readLine());
                if (row < 0 || row > 2)
                    {System.out.println("Invalid entry! Try again...");}
            }
            while (col < 0 || col > 2)
            {   
                System.out.println
                    (name + ", what column would you like your next move to be in?");
                col = Integer.parseInt(stdin.readLine());
                if (col < 0 || col > 2)
                    {System.out.println("Invalid entry! Try again...");}
            }

            if (board.getMark(row, col) != SPACE_CHAR)
                {System.out.println("That spot is already taken Try again...");}

        } while (board.getMark(row,col) != SPACE_CHAR);

        return new Cell(row,col,mark);
    }

    public void endGame()
    {
        if (board.xWins() == 1)     {System.out.println(END + XWIN);}
        if (board.oWins() == 1)     {System.out.println(END + OWIN);}
        else            {System.out.println(END + " It was a tie!!");}
    }
}

Class TTTServer:

import java.net.*;
import java.io.*;

public class TTTServer
implements Constants
{
    public static void main(String[] args)
    {
        try
        {
            ServerSocket ss = new ServerSocket(1298,2);
            System.out.println("The Server is running...");
            Socket sock;

            Board board = new Board();

            sock = ss.accept();
            sock = ss.accept();

            BufferedReader in = new BufferedReader(new 
                    InputStreamReader(sock.getInputStream()));
            PrintWriter out = new PrintWriter(sock.getOutputStream(),true);

            do
            {
                String moveString;

                do {moveString = in.readLine();}
                while (moveString == null); 

                Cell move = Cell.split(moveString);
                board.addMark(move.row(), move.col(), move.mark());

                out.println(moveString);

            } while(!board.isOver());

            in.close();
            out.close();
            ss.close();
            sock.close();

        } catch(IOException ioe)
        {
            System.out.println("IOE IN TTTSERVER " + ioe.getMessage());
            System.exit(1);
        }

    }
}

Class Board:

public class Board 
implements Constants
{
    /**
     * A 2D char array stores the game board and 
     * the total number of marks
     */
    private char theBoard[][];
    private int markCount;
    /**
     * Default constructor initializes the array and fills it with
     * SPACE_CHARs from the Constants interface
     */
    public Board() 
    {
        markCount = 0;
        theBoard = new char[3][];
        for (int i = 0; i < 3; i++) {
            theBoard[i] = new char[3];
            for (int j = 0; j < 3; j++)
                theBoard[i][j] = SPACE_CHAR;
        }
    }
    /**
     * Getter for the mark at the location specified by the arguments
     * 
     * @param row
     * @param column
     * 
     * @return mark
     */
    public char getMark(int row, int col) 
        {return theBoard[row][col];}
    /**
     * Getter for the number of moves which have been made thus far
     * 
     * @return markCount
     */
    public int getMarkCount()       {return markCount;}
    /**
     * @return true if the game is over, otherwise false
     */
    public boolean isOver()
    {
        if (xWins() == 1 || oWins() == 1 || isFull())
            {return true;}

        return false;
    }
    /**
     * @return true if the board has been completely filled with 
     * X_CHARs and O_CHARs from Constants interface, else false
     */
    public boolean isFull() 
        {return markCount == 9;}
    /**
     * Runs checkWinner on LETTER_X from Constants interface
     * 
     * @return true if X has won, else false
     */
    public int xWins() 
        {return checkWinner(LETTER_X);}
    /**
     * Runs checkWinner on LETTER_O from Constants interface 
     * 
     * @return true if O has won, else false
     */
    public int oWins() 
        {return checkWinner(LETTER_O);}
    /**
     * Uses the formatting helper methods to display the board 
     * in the console
     */
    public void display() 
    {
        displayColumnHeaders();
        addHyphens();
        for (int row = 0; row < 3; row++) {
            addSpaces();
            System.out.print("    row " + row + ' ');
            for (int col = 0; col < 3; col++)
                System.out.print("|  " + getMark(row, col) + "  ");
            System.out.println("|");
            addSpaces();
            addHyphens();
        }
    }
    /**
     * Add the mark in the last argument to the location specified by the 
     * first two arguments
     * 
     * @param row
     * @param column
     * @param mark
     */
    public void addMark(int row, int col, char mark) 
    {
        theBoard[row][col] = mark;
        markCount++;
    }
    /**
     * Clears the board by replacing all marks with 
     * SPACE_CHARs from the Constants interface
     */
    public void clear() 
    {
        for (int i = 0; i < 3; i++)
            for (int j = 0; j < 3; j++)
                theBoard[i][j] = SPACE_CHAR;
        markCount = 0;
    }
    /**
     * Checks if the player with the argument mark has won the game
     * 
     * @param mark
     * 
     * @return true if the game was won, else false
     */
    int checkWinner(char mark) {
        int row, col;
        int result = 0;

        for (row = 0; result == 0 && row < 3; row++) {
            int row_result = 1;
            for (col = 0; row_result == 1 && col < 3; col++)
                if (theBoard[row][col] != mark)
                    row_result = 0;
            if (row_result != 0)
                result = 1;
        }


        for (col = 0; result == 0 && col < 3; col++) {
            int col_result = 1;
            for (row = 0; col_result != 0 && row < 3; row++)
                if (theBoard[row][col] != mark)
                    col_result = 0;
            if (col_result != 0)
                result = 1;
        }

        if (result == 0) {
            int diag1Result = 1;
            for (row = 0; diag1Result != 0 && row < 3; row++)
                if (theBoard[row][row] != mark)
                    diag1Result = 0;
            if (diag1Result != 0)
                result = 1;
        }
        if (result == 0) {
            int diag2Result = 1;
            for (row = 0; diag2Result != 0 && row < 3; row++)
                if (theBoard[row][3 - 1 - row] != mark)
                    diag2Result = 0;
            if (diag2Result != 0)
                result = 1;
        }
        return result;
    }

    /**
     * The final three helper methods are called by display 
     * to format the board properly in the console
     */
    void displayColumnHeaders() {
        System.out.print("          ");
        for (int j = 0; j < 3; j++)
            System.out.print("|col " + j);
        System.out.println();
    }

    void addHyphens() {
        System.out.print("          ");
        for (int j = 0; j < 3; j++)
            System.out.print("+-----");
        System.out.println("+");
    }

    void addSpaces() {
        System.out.print("          ");
        for (int j = 0; j < 3; j++)
            System.out.print("|     ");
        System.out.println("|");
    }
}

Answer 1

Here's your mistake:

Both Xplayer and OPlayer have main methods, so that they can be run concurrently.

If you're running two main() methods, they're not running "concurrently"; they're entirely separate processes. That means no shared threads, variables, objects, notifications etc. If you want to share state, you need to start everything from a single main() method:

class StarterClass {
    public static void main(String[] args)
    {
        // start XPlayer thread
        try
        {
            XPlayer x = new XPlayer();
            Thread t = new Thread(x);
            t.start();

        } catch(IOException ioe)
        {
            System.err.println
                ("IOE IN XPLAYER MAIN " + ioe.getMessage());
            System.exit(1);
        }

        // start OPlayer thread
        try
        {
            OPlayer o = new OPlayer();
            Thread t = new Thread(o);
            t.start();

        } catch(IOException ioe)
        {
            System.err.println("IOE IN OPLAYER MAIN" + ioe.getMessage());
            System.exit(1);
        }
    }
}

If your intent is to have each Player run as a separate client while alternating turns, thread synchronization is the wrong tool for the job. You'll need to implement custom messaging between your server and your clients to keep them in sync.