java synchronisation

[bangor_smith]

Hi,

I am bit stuck here, I have to have edit this code so that a train must wait in front of the tunnel lock for a train to come in the opposite direction to exit before it can enter the tunnel, i have to use java built in synchronisation condition. Before the try statement I have tried looking at while statements, if statements and still cant get it to give me the following output:

Train 1 arrives
Train 1 ascends
Train 1 Exits Tunnel
Train 2 Arrives
train 2 descends
train 2 exits tunnel
train 3 and so on


Any help would be greatful

public synchronized void useTunnelLock(Train train)
{


System.out.println(train + " " + train.getDirection());
System.out.println(train + " exiting Tunnel");
while(train.getDirection() == ASCENDING())
{
this.wait();
}
notifyAll();
try
{
// occupy tunnel lock for 5s
Thread.sleep(5000);
}
catch (InterruptedException ex)
{
ex.printStackTrace();
}

// swap direction of tunnel lock
direction = (direction == ASCENDING)? DESCENDING : ASCENDING;
}

[sunde887]

Im not terribly practiced with the use of threads, however; I believe that making the method synchronized makes it so a thread acquires a lock, and no other thread can access it until the thread is unlocked. If you synchronize the method it should make all other threads wait for the first thread to finish with the method before they can enter the method.

What is your original code? What modifications have you made?

As stated, I am not exceptionally experienced with threading and this is simply my understanding. Im sure other posters here will validify/falsify my advice.

[bangor_smith]

Correct in what you say, but seeing the thread has to wait till the thread coming in the opposite direction has arrived before it can go is the problem im having ie

Thread 1 which is ascending starts and then exits the lock, at this point thread 3 or 4 which is descending should start, but without a statement stopping it, it is possible for thread 2 which is also ascending to start before thread 3 or 4 descends

[JosAH]

Correct in what you say, but seeing the thread has to wait till the thread coming in the opposite direction has arrived before it can go is the problem im having ie

Thread 1 which is ascending starts and then exits the lock, at this point thread 3 or 4 which is descending should start, but without a statement stopping it, it is possible for thread 2 which is also ascending to start before thread 3 or 4 descends

The Tunnel is the shared resource here so the Trains should try to acquire a lock on the Tunnel. If all ascending (or descending) Trains can pass through the Tunnel without locking you effectively have two Tunnels: an ascending one and a descending one. An ascending Train has to acquire the descending Tunnel lock and vice versa.

kind regards,

Jos

[bangor_smith]

And how would that look not exact code but something for me to work on as im lost

[JosAH]

And how would that look not exact code but something for me to work on as im lost

I'd use Semaphores (a class) for both directions or BlockingQueues (an interface). Read the API documentation for the java.util.concurrent package. Implementing wait() and notify() is done by those objects.

kind regards,

Jos