/**
 * A simple program to demonstrate data races
 *
 * Author: John C. Linford (jlinford@vt.edu)
 * Date: 25 August 2005
 * License: Public Domain
 */

public class DataRace {

    private int[] shared = new int[1];
    private Thread t1, t2;

    public DataRace() {

        //
        // Create a first thread to increment shared
        //
        t1 = new Thread() {
            public void run() {
                int copy = 0; // Stores a copy of shared to detect races

                while(t1 != null) {
                    // Make a copy of shared in a safe way
                    synchronized(shared) {
                        copy = shared[0];
                    }

                    // Increment shared unsafely (possible race)
                    shared[0] = shared[0] + 1;

                    // Check for a race condition in a safe way
                    synchronized(shared) {
                        System.out.println("[t1] -- Shared: " + shared[0] + ".");
                        if(shared[0] - 1 != copy) {
                            System.err.println("[t1] -- Data Race!  Had " + shared[0] + " when I expected " + (copy + 1) + ".");
                            t1 = t2 = null;
                        }
                    }
                }
            }
        };

        //
        // Create a second thread to increment shared
        //
        t2 = new Thread() {
            public void run() {
                int copy = 0; // Stores a copy of shared to detect races

                while(t2 != null) {
                    // Make a copy of shared in a safe way
                    synchronized(shared) {
                        copy = shared[0];
                    }

                    // Increment shared unsafely (possible race)
                    shared[0] = shared[0] + 1;

                    // Check for a race condition in a safe way
                    synchronized(shared) {
                        System.out.println("[t2] -- Shared: " + shared[0] + ".");
                        if(shared[0] - 1 != copy) {
                            System.err.println("[t2] -- Data Race!  Had " + shared[0] + " when I expected " + (copy + 1) + ".");
                            t1 = t2 = null;
                        }
                    }
                }
            }
        };

        t1.start();
        t2.start();
    }
    
    public static void main(String[] args) {
        new DataRace();
    }
}