Thread: Help with Bounded Buffer problem

Hey there,

Been working on an assignment whereby I have to create a bounded buffer in Java using threads. I'll briefly state the main specs for the assignment instead of pasting the whole thing:

- It operates in a FIFO manner (First In First Out)
- We cannot use semaphores & can only use synchronized, wait, notifyAll, etc
- Updates to the buffer must be mutually exclusive
- It must contain the following classes : BoundedBuffer, Producer, Consumer, Watcher

I have a lot of it attempted but I'm having a few issues with the data being produced. I find that the updated status on the buffer data being produced by the Watcher thread seems to be out of sync or inaccurate in regards to what is actually happening. Everything seems to be a bit messy. Also, I'm not sure if my modulo arithmetic is doing the right job to ensure the pointers are being wrapped. Can anybody tell me if I'm doing things the right way and if not, can even nudge me in the right direction I'll post my code below. I understand that it's a lot of code to look through but i'd really be grateful with any help.

BoundedBuffer:

public class BoundedBuffer {
	// Variables
	private int nextIn, nextOut, size, occupied, ins, outs;
	private boolean dataAvailable, roomAvailable;
	private int[] buffer;
 
	// Constructor
	public BoundedBuffer(int size) {
		this.size = size;
		buffer = new int[size]; // Set buffer size
 
		// Initialize variables
		nextIn = nextOut = occupied = ins = outs = 0;
		dataAvailable = false;
		roomAvailable = true;
	}
 
	public synchronized void insertItem(int item) {
		while (roomAvailable == false || occupied == size) {
			try {
				wait();
			} catch (InterruptedException e) {
				System.out.println("Error inserting item.");
			}
		}
		dataAvailable = true;
		notifyAll();
		buffer[nextIn] = item;
		// For debugging purposes
		System.out.println("Item: " + item + " inserted.");
		nextIn = (nextIn + 1) % size;
		occupied++;
		ins++;
	}
 
	public synchronized int removeItem() {
		while (dataAvailable == false || occupied == 0) {
			try {
				wait();
			} catch (InterruptedException e) {
				System.out.println("Error removing item.");
			}
		}
 
		roomAvailable = true;
		notifyAll();
		int item = buffer[nextOut];
		// For debugging purposes
		System.out.println("Item: " + item + " removed.");
		occupied--;
		nextOut = (nextOut + 1) % size;
		outs++;
		return item;
	}
 
	public void printStatus() {
		System.out.println("Items inserted : " + ins);
		System.out.println("Items removed : " + outs);
		System.out.println("Spaces occupied : " + occupied);
		System.out.println("Delta : " + (ins - outs - occupied));
		System.out.println();
	}
 
}

Producer:

public class Producer extends Thread {
	private BoundedBuffer buffer;
	private int item;
 
	public Producer(BoundedBuffer buffer) {
		this.buffer = buffer;
	}
 
	public void run() {
		try {
			while (true) {
				item = (int) (Math.random() * 100);
				buffer.insertItem(item);
 
				// Sleep for 2 seconds
				sleep(2000);
			}
 
		} catch (InterruptedException e) {
			System.out.println("Error with Producer Thread.");
		}
 
	}
}

Consumer:

public class Consumer extends Thread {
	private BoundedBuffer buffer;
 
	public Consumer(BoundedBuffer buffer) {
		this.buffer = buffer;
	}
 
	public void run() {
		try {
			while (true) {
				buffer.removeItem();
 
				// Sleep for 2 seconds
				sleep(2000);
			}
 
		} catch (InterruptedException e) {
			System.out.println("Error with Consumer Thread.");
		}
	}
}

Watcher:

public class Watcher extends Thread {
	private BoundedBuffer buffer;
 
	public Watcher(BoundedBuffer buffer) {
		this.buffer = buffer;
	}
 
	public void run() {
		try {
			while (true) {
				buffer.printStatus();
				sleep(2000);
			}
		} catch (InterruptedException e) {
			System.out.println("Error in Watcher thread.");
		}
	}
}

Assignment1:

 
public class Assignment1 {
	public static void main(String[] args) {
		BoundedBuffer buffer = new BoundedBuffer(20);
		Producer prod = new Producer(buffer);
		Consumer cons = new Consumer(buffer);
		Watcher watch = new Watcher(buffer);
 
		prod.start();
		cons.start();
		watch.start();
 
	}
 
}

Hi friends.

Originally Posted by MangaRao

Hi friends.

Thanks thats great help

Originally Posted by Norm

Try debugging the code by adding printlns that print out the values of variables as they are changed and used. If you know what the code is supposed to do, seeing what it is doing should help you find and fix the problem.

Cheers, I've added in some printlns that print out each variable as soon as any change is made and it seems to be giving the correct results. It's only when I call the printStatus() method in the watcher thread that seems to give slightly unaccurate or out of sync results. For instance it might say the number of occupied spaces is 1 when it really should be 0. It's as if it can be a bit late. Is there a way to stop this?

Also, we are told the threads have to be terminated after 1 minute and we're not allowed to use thread.stop() to do this. Any ideas?

In multi-threading you want to thread to exit gracefully. thread.stop() does not do that. Look for the answer in the api for thread.stop() and why they deprecated the method.