Producer Consumer Problem

[AnirbanDelhi Pal]

Dear All,

I have implemented Producer Consumer problem taken hint from How To Program by H. M. Deitel. In book solution has given using object locking mechanism. Like private Lock accessLock = new ReentrantLock();
and declaring as private Condition canWrite = accessLock.newCondition();
private Condition canRead = accessLock.newCondition

Just for a try, I have not used any kind of object locking. Just uses a simple boolean variable just to denote either buffer is empty or full, based on that Producer writes or Consumer reads. And it worked. Why so ? Is not synchronisation has it's job to play in between ??

class 1

package com.example.thread;

public class Buffer {
	private int value;
	private boolean status;
	Buffer(int val){
		value = val;
	}
	public void setVal(int val) {
		this.value = val;
	}
	public int getVal() {
		return this.value;
	}
	public boolean getStatus() {
		return this.status;
	}
	public void setStatus() {
		if(status == true)
			status = false;
		else
			status = true;
	}
}

class 2

package com.example.thread;
import java.util.Random;

public class Producer implements Runnable{

	private Buffer buf;
	private static Random generator = new Random();
	
	public Producer(Buffer buff) {
		buf = buff;
	}
	
	public void run()
	{
		int sum = 0;
		int  i = 1;
		for(;i<11;i++)
		{
			while(buf.getStatus() == true)
			{
				//System.out.println("Producer waiting "+i);
			}
			buf.setVal(i);
			sum += i;
			try {
				Thread.sleep(generator.nextInt(3000));
			}catch(InterruptedException ie){
				ie.printStackTrace();
			}
			System.out.println(" Producer i = "+i+" sum = "+sum+" status = "+buf.getStatus());
			buf.setStatus();
		}
	}
 
}

class 3

package com.example.thread;
import java.util.Random;

public class Consumer implements Runnable{

	private Buffer buf;
	private static Random generator = new Random();
	
	public Consumer(Buffer buff) {
		buf = buff;
	}
	
	public void run()
	{
		int sum = 0;
		int  i = 1;
		int temp = 0;
		for(;i<11;i++)
		{
			try {
				Thread.sleep(generator.nextInt(3000));
			}catch(InterruptedException ie){
				ie.printStackTrace();
			}
			while(buf.getStatus() == false)
			{
				//System.out.println("Consumer waiting ");
			}
			temp = buf.getVal();
			sum += temp;
			System.out.println(" Consumer i = "+temp+" sum = "+sum+" status = "+buf.getStatus());
			buf.setStatus();
		}
	}
}

class 4

package com.example.thread;

public class ProducerConsumer {

	public static void main(String[] args) {

		Buffer buf = new Buffer(-1);
		Producer p = new Producer(buf);
		Consumer c = new Consumer(buf);
		
		Thread t1 = new Thread(p);
		Thread t2 = new Thread(c);
		
		t1.start();
		t2.start();
	}

}

[AnirbanDelhi Pal]

Output :

Producer i = 1 sum = 1 status = false
Consumer i = 1 sum = 1 status = true
Producer i = 2 sum = 3 status = false
Consumer i = 2 sum = 3 status = true
Producer i = 3 sum = 6 status = false
Consumer i = 3 sum = 6 status = true
Producer i = 4 sum = 10 status = false
Consumer i = 4 sum = 10 status = true
Producer i = 5 sum = 15 status = false
Consumer i = 5 sum = 15 status = true
Producer i = 6 sum = 21 status = false
Consumer i = 6 sum = 21 status = true
Producer i = 7 sum = 28 status = false
Consumer i = 7 sum = 28 status = true
Producer i = 8 sum = 36 status = false
Consumer i = 8 sum = 36 status = true
Producer i = 9 sum = 45 status = false
Consumer i = 9 sum = 45 status = true
Producer i = 10 sum = 55 status = false
Consumer i = 10 sum = 55 status = true


If the sum for consumer comes anything other than 55, we can tell synchronisation has failed. but I have tried 5 - 6 times, every time Consumer gives output as 55 at last line.

[Henry Wong]

AnirbanDelhi Pal wrote:
Just for a try, I have not used any kind of object locking. Just uses a simple boolean variable just to denote either buffer is empty or full, based on that Producer writes or Consumer reads. And it worked. Why so ? Is not synchronisation has it's job to play in between ??

Thread safety just means that it will work correctly. The lack of thread safety means that it may not work correctly. It doesn't mean that it will definitely not work.

Try ...

1. Making the loop bigger.
2. Removing the sleep calls during each loop
3. On a different machine, with a different JVM version
3b. On a different machine, with a different processor or OS version
3c. On a different machine, with entirely different processors or operating system.

etc.

Henry

[Martin Vajsar]

Henry Wong wrote:Try ...

1. Making the loop bigger.
2. Removing the sleep calls during each loop
3. On a different machine, with a different JVM version
3b. On a different machine, with a different processor or OS version
3c. On a different machine, with entirely different processors or operating system.

etc.

Henry

I'd just add
4. Running several consumers in parallel.

The sleep in the producer's loop makes everything flow smooth and nice. Adding one or more consumer to the game should bring some race conditions...