Question

i need to write a java program but i need some advice before starting on my own.

The program i will be writing is to do the following:

• Simulate a shop takes advanced order for donuts

• The shop would not take further orders, once 5000 donuts have been ordered

Ok i am kind of stuck thinking if i should be writing the java-class to act as a Monitor or should i use Java-Semaphore class instead?

Please advice me. Thanks for the help.

Answer 1

Any java object can work as a monitor via the wait/notify methods inherited from Object:

Object monitor = new Object();

// thread 1    
synchronized(monitor) {
    monitor.wait();
}

// thread 2
synchronized(monitor) {
    monitor.notify();
}

Just make sure to hold the lock on the monitor object when calling these methods (don’t worry about the wait, the lock is released automatically to allow other threads to acquire it). This way, you have a convenient mechanism for signalling among threads.

It seems to me like you are implementing a bounded producer-consumer queue. In this case:

1. The producer will keep putting items in a shared queue.
2. If the queue size reaches 5000, it will call wait on a shared monitor and go to sleep.
3. When it puts an item, it will call notify on the monitor to wake up the consumer if it’s waiting.
4. The consumer will keep taking items from the queue.
5. When it takes an item, it will call notify on the monitor to wake up the producer.
6. If the queue size reaches 0 the consumer calls wait and goes to sleep.

For an even more simplified approach, have a loop at the various implementation of BlockingQueue, which provides the above features out of the box!