/* Java version of mutexes */

class mutex
{
    boolean lock = false;

    public synchronized void lock() throws InterruptedException
    {
	if (lock) wait();   
	lock = true;
    }

    public synchronized void unlock() throws InterruptedException
    {
	lock = false;
	notify();  // wakes up one thread waiting on this object
	// an alternative is to use notifyAll() with a while loop in lock()
    }
} // mutex

// Please don't confuse a thread and an instance of mutex.   In ada,
// the closes counterpart to the mutex class will be a protected type.

class mythread implements Runnable
{
    static mutex MU = new mutex(); // shared mutex - "static" is critical
    turnbroker TB;  // not shared, but is a pointer to shared object

    int id;
    public mythread(int i, turnbroker T) { id = i; TB=T; }
  
    public void run()  // required by Runnable interface
    {
     try
     {
	for(int i=0;i<10;i++)
	    {
		TB.synch(); // synch with fairness broker
		MU.lock();
		System.out.println(id+" is inside critical section");
		MU.unlock();
		System.out.println(id+" is doing asynch work.");
		Thread.sleep(id*2000); // delay for milliseconds
	    }
     }
     catch (Exception e) {System.out.println(e);}
    }
}//mythread


//////////// Synchronization monitor - similar to "Fairnessmonitor" in Ada
class turnbroker
{
    int totalclients;
    int clients = 0;
    public turnbroker(int t) {totalclients = t;}
    
    public synchronized void synch() throws InterruptedException
    {
	clients++;
	if (clients<totalclients) wait();  // why if and not while?
	else 
	    {
		clients = 0;
		notifyAll();  // why is this not just "notify"?
	    }
    }//synch
}//turnbroker


public class mutexes
{
    public static void main(String[] args) throws Exception
    {
	Thread t1, t2, t3;
	turnbroker TB = new turnbroker(3);
	t1 = new Thread(new mythread(1,TB));
	t2 = new Thread(new mythread(2,TB));
	t3 = new Thread(new mythread(3,TB));
	t1.start();
	t2.start();
	t3.start();
	t1.join(); // join waits for a thread to terminate
	t2.join();
	t3.join();
    } // main
}