Hi!

Below I do start Thread t1 before t2. t1 tries to call print1() on a
Printer object and t1 print2() but only the first will success since it
obtains a lock on the Printer object. I'm surprised that although t1 is
started first the code prints "2". If I put a sleep(5) between the
starting of the threads t1 wins the race.

Can somebody explain?

public class SyncThreadTest
{
    public static void main( String[] args )
    {
        new SyncThreadTest().go();
    }

    private void go()
    {
        Thread t1 = new Thread( new Runner( 1 ) );
        Thread t2 = new Thread( new Runner( 2 ) );
        System.out.println(t1.toString());
        t1.start();

        System.out.println(t2.toString());
        t2.start();
    }

    class Runner implements Runnable
    {
        int i = 0;

        public Runner( int i )
        {
            this.i = i;
        }

        public void run()
        {
//            Printer p = Printer.getInstance();
            Printer p = new Printer();
//            synchronized( p )
//            synchronized( Printer.class )
            synchronized( p.MUTEX )
            {
                while( true )
                {
                    System.out.println(toString());
                    if( i == 1 ) p.print1();
                    else p.print2();

                    try
                    {
                        Thread.sleep( 500 );
                    }
                    catch( InterruptedException e )
                    {
                    }
                }
            }
        }
    }
}

class Printer
{
    private static final Printer instance = new Printer();

    public static final Object MUTEX = new Object();

    public static Printer getInstance()
    {
        return instance;
    }

    public synchronized void print1()
    {
        System.out.println( 1 );
    }

    public synchronized void print2()
    {
        System.out.println( 2 );
    }
}