multithreading

who will dance with Nancy ? Find in below code !

Gyanendra Concurrency, Java

My new post is going to take little more time so thought of to post solution of this long pending problem posted by me on November 1, 2014

public class DecideDancePair {

DecideDancePair ddp1;
StringBuffer ddp2 = new StringBuffer();

public static void main(String[] args) {

new Thread(new Runnable() {

@Override
public void run() {
synchronized (DecideDancePair.class) {
System.out.println("Its weekend .... Hey Guys !! this is Nancy , who will dance with me? ");
}

}
}).start();

new Thread(new Runnable() {

@Override
public void run() {

synchronized (new DecideDancePair().ddp1) {
System.out.println("Hey Nancy !! this is Peter , I will dance with you.");
}

}
}).start();

new Thread(new Runnable() {

@Override
public void run() {
synchronized (new DecideDancePair().ddp2) {
System.out.println("Hey Nancy !! this is Rozer , I will dance with you.");
}

}
}).start();

}

}

==============================

Soln :

Nancy will always go for dance because she having Lock over a object which is not NULL. same with Rozer , he is also having a non-NULL lock. while poor Peter is caught with NULL object Lock so that piece of code will throw below exception and Hence Nancy and Rozer will be in dance 🙂 !!!

Code execution will looks like :


Its weekend .... Hey Guys !! this is Nancy , who will dance with me?
Exception in thread "Thread-1" Hey Nancy !! this is Rozer , I will dance with you.
java.lang.NullPointerException
	at BasicConcepts.DecideDancePair$2.run(DecideDancePair.java:26)
	at java.lang.Thread.run(Thread.java:745)

Shared queue processing with Pub and Sub Threads

Gyanendra Concurrency, Design pattern, Java , , 
import java.util.Random;

public class PubSubDemo {

	public static void main(String[] args) {
		SharedQ sharedQ = new SharedQ();

		new Thread(new Consumer(sharedQ), "Consumer").start();
		// Delay publishing
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		new Thread(new Producer(sharedQ), "Producer").start();
	}

}

class Producer implements Runnable {
	SharedQ sharedQPro = null;

	public Producer(SharedQ sharedQ) {
		this.sharedQPro = sharedQ;
	}

	@Override
	public void run() {
		for (int i = 0; i < 3; i++) {
			produce();
		}
	}

	public void produce() {
		synchronized (sharedQPro.sharedQueue) {

			for (int i = 0; i < 3; i++) {
				sharedQPro.sharedQueue[i] = (new Random().nextInt(20));
				System.out.println(" ### Produced ### " + sharedQPro.sharedQueue[i]);
			}
			sharedQPro.sharedQueueIsEmpty = false;
			sharedQPro.sharedQueue.notify();
			try {
				sharedQPro.sharedQueue.wait();
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}

}

class Consumer implements Runnable {
	SharedQ sharedQCon = null;

	public Consumer(SharedQ sharedQ) {
		this.sharedQCon = sharedQ;
	}

	@Override
	public void run() {
		for (int i = 0; i < 3; i++) {
			consume();
		}
	}

	public void consume() {
		synchronized (sharedQCon.sharedQueue) {
			if (sharedQCon.sharedQueueIsEmpty) {
				try {
					sharedQCon.sharedQueue.wait();
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}

			}

			System.out.println();

			for (int i = 0; i < 3; i++) {
				System.out.println(" *** Consumed *** " + sharedQCon.sharedQueue[i]);
				sharedQCon.sharedQueue[i] = 0;
			}
			sharedQCon.sharedQueue.notify();
			System.out.println(" --------------------------");
			try {
				sharedQCon.sharedQueue.wait();
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}

		}
	}

}

class SharedQ {

	int[] sharedQueue = new int[3];
	boolean sharedQueueIsEmpty = true;

}

/*
Output of pub sub :

 ### Produced ### 1
 ### Produced ### 0
 ### Produced ### 3

 *** Consumed *** 1
 *** Consumed *** 0
 *** Consumed *** 3
 --------------------------
 ### Produced ### 12
 ### Produced ### 0
 ### Produced ### 14

 *** Consumed *** 12
 *** Consumed *** 0
 *** Consumed *** 14
 --------------------------
 ### Produced ### 13
 ### Produced ### 8
 ### Produced ### 17

 *** Consumed *** 13
 *** Consumed *** 8
 *** Consumed *** 17
 --------------------------
*/

Sequence generator tasks by Threads

Gyanendra Concurrency, Java, software architecture , , ,

Many times we face scenario where we need to execute two sequential tasks to perform a operation, like Tagging / Stamping serial Coupons , Producing serial Tickets at different Vending Machines. Below code ( generating Odd and Even in sequence) can be used for such requirement.

PS: This is a basic implementation, the same can be done using other Threading concepts.

				
package multithreading;

// print ODD and Even number alternatively

public class SquenceGeneratorTasks {

	public static boolean oddPrinted = false;

	public static void main(String[] args) {
		 Object lock =new Object();
		Thread oddThread = new Thread(new Runnable() {

			@Override
			public void run() {
				int i = 1;
				while (i <= 20) {

					synchronized (lock) {
						if (!oddPrinted) { // make sure printing starting with Odd number and each Odd is followed with a Even number
							System.out.println(i);
							i = i + 2;
							oddPrinted = true;
							lock.notify();
							try {
								lock.wait();
							} catch (InterruptedException e) {
								// TODO Auto-generated catch block
								e.printStackTrace();
							}
						}
					}

				}
			}
		});

		
		Thread evenThread = new Thread(new Runnable() {

			@Override
			public void run() {
				
				System.out.println(" Though you have delayed Odd thread but printing gonna start with Odd number only !!");
				
				int i = 2;
				while (i <= 20) {
					synchronized (lock) {
						if (oddPrinted) {
							System.out.println(i);
							i = i + 2;
							oddPrinted = false;
							lock.notify();
							try {
								lock.wait();
							} catch (InterruptedException e) {
								// TODO Auto-generated catch block
								e.printStackTrace();
							}
						}
					}
				}
			}
		});

		evenThread.start();
		
		// even if We apply some delay to Odd thread printing will start with ODD number only and 
		// Sequence of Odd-Even-Odd maintained
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		oddThread.start();

	}

}						
/*
Though you have delayed Odd thread but printing gonna start with Odd number only !!
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20 */

You can download the same code from : https://github.com/SkilledMinds/easyNuts/tree/master/code-25-09-2015

Thread.sleep() can ditch your join()’s expected code execution , see it how !!

Gyanendra Basic concepts, Concurrency, Java 

public class JoinWithSleep
{

    public static void main(String[] args)
    {

        Thread t1 = new Thread(() -> {
            System.out.println(" In :: Thread t1");

        });

        Thread t2 = new Thread(() -> {
            System.out.println(" In :: Thread t2");
            try
            {
                t1.join();
            }
            catch (Exception e)
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println(" Out :: Thread t2");
        });

        try
        {
            t2.join();
        }
        catch (InterruptedException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        t2.start();
        
        // this single sleep can ruin your code !!
        try
        {
            Thread.sleep(4000);
        }
        catch (InterruptedException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        t1.start();

    }

}



/*
Without Sleep
In :: Thread t2
In :: Thread t1
Out :: Thread t2

With Sleep
In :: Thread t2
Out :: Thread t2
In :: Thread t1

*/

check your java class locking and classloader concept.

Gyanendra Java , ,

try below code to test your concept of class Locking and classLoader. I will post the
reason of output next week till then you do your finding !!

public class VerifyClassLoadingOnClassLock {
    public static void main(String[] args) {
        show();
    }

    private static void show() {
        synchronized (SharedQ.class) {
            System.out.println(" Method Show() executing from Main() .... ");
        }       
    }
} 



class SharedQ {  
    static {
        System.out.println(" Classloader is loading SharedQ ");
    }

    public static void writeStream() {
        // some multiThread code here
    }
}

How to avoid deadlock in Java Threads

Gyanendra Concurrency, Java ,

Today’s post is on breaking Deadlock of multiple Threads : on “November 5, 2014” I had posted a code of creating Deadlock.
Today I am modifying the same code for avoiding such Deadlock. There are many ways to avoid a deadlock but simple rule is: have all threads claim and release their locks in the same order. In this way, you never get into a situation where a deadlock can occur.

My updated code without Deadlock
========================================

public class CustomerUpdateDeadloackThread {

	public static void main(String[] args) {

		Customer cstmr = new Customer("Peter");
		Address adrs = new Address("B-232, Bangalore");

		for (int i = 0; i &lt; 10; i++) {
			new Thread(new TagObjectsToEachOther(cstmr, adrs)).start();
			new Thread(new TagObjectsToEachOther(adrs, cstmr)).start();
		}

	}
}

interface CustomerUpdater {

	public boolean update(Object obj);

}

class TagObjectsToEachOther implements Runnable {
	CustomerUpdater taskItem;
	Object objToUpdateWith;

	public TagObjectsToEachOther(CustomerUpdater cspdtr, Object obj2) {
		this.taskItem = cspdtr;
		this.objToUpdateWith = obj2;
	}

	@Override
	public void run() {
		taskItem.update(objToUpdateWith);
		System.out.println(&quot; Task done :&quot; + Thread.currentThread().getName());
	}

}

class Address implements CustomerUpdater {

	String address;
	Customer customer;

	public Address(String addrs) {
		this.address = addrs;
	}

	@Override
	public boolean update(Object cstmr) {

		synchronized ((Customer) cstmr) { // same order of locks will be used at other places !!!
			synchronized (this) {
				try {
					this.customer = (Customer) cstmr;
					Thread.sleep(2000); // or else do some other work here so that lock gets used for some more time
				} catch (CustomerUpdateFailureException e) {
					e.getCause();
					return false;
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				return true;
			}
		}
	}

}

class Customer implements CustomerUpdater {

	String name;
	Address address;

	public Customer(String nm) {
		this.name = nm;
	}

	@Override
	public boolean update(Object adrs) {
		synchronized (this) {
			synchronized ((Address) adrs) { // Maintaining order of accessing lock to avoid deadlock !!!
				try {
					this.address = (Address) adrs;
					Thread.sleep(2000); // or else do some other work here so that lock gets used for some more time
				} catch (CustomerUpdateFailureException e) {
					e.getCause();
					return false;
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				return true;
			}
		}
	}

}

class CustomerUpdateFailureException extends RuntimeException {

	private static final long serialVersionUID = 1L;

	@Override
	public String getMessage() {

		return &quot;Uncompitable update&quot;;
	}

}

===================  Output =========================

 Task done :Thread-0
 Task done :Thread-9
 Task done :Thread-8
 Task done :Thread-7
 Task done :Thread-6
 Task done :Thread-5
 Task done :Thread-4
 Task done :Thread-3
 Task done :Thread-2
 Task done :Thread-1

Code to produce deadlock between two threads

Gyanendra Concurrency, Java , ,

Deadlock describes a situation where two or more threads are blocked forever, waiting for each other.

Based on number of Threads ( tasks complexity ) and priority Thread Scheduler decides execution plan.  Here my intention is to generate DeadLock so I planned to execute both threads 10 times so that at sometime they will be blocked for locks !

Problem :  Many times we tag records/ Objects for better indexing. Though this will
not be real time problem but I am trying to simulate a similar scenario.
say for some requirement I am tagging a customer with an Address and again
Address with Customer in a social media Analytic Application. To do this
tasks I may need thousands of Threads.
Below is one code which may report deadlock for  such Threads.

===========================================================================================

public class CustomerUpdateDeadloackThread {

public static void main(String[] args) {

Customer cstmr = new Customer("Peter");
Address adrs = new Address("B-232, Bangalore");

for (int i=0; i&lt;10;i++){

// Lets tag a customer and address with each other for  indexing
new Thread(new TagObjectsToEachOther(cstmr, adrs)).start();
new Thread(new TagObjectsToEachOther(adrs, cstmr)).start();
}


}
}

interface CustomerUpdater {

public boolean update(Object obj);

}

class TagObjectsToEachOther implements Runnable {
CustomerUpdater taskItem;
Object objToUpdateWith;

public TagObjectsToEachOther(CustomerUpdater cspdtr, Object obj2) {
this.taskItem = cspdtr;
this.objToUpdateWith = obj2;
}

@Override
public void run() {
taskItem.update(objToUpdateWith);
System.out.println(" Task done :" + Thread.currentThread().getName());
}

}

class Address implements CustomerUpdater {

String address;
Customer customer;

public Address(String addrs) {
this.address = addrs;
}

@Override
public boolean update(Object cstmr) {
synchronized (this) {
synchronized ((Customer) cstmr) {
try {
this.customer = (Customer) cstmr;
Thread.sleep(2000); // or else do some other work here so that lock gets used for some more time
} catch (CustomerUpdateFailureException e) {
e.getCause();
return false;
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return true;
}
}
}

}

class Customer implements CustomerUpdater {

String name;
Address address;

public Customer(String nm) {
this.name = nm;
}

@Override
public boolean update(Object adrs) {
synchronized (this) {
synchronized ((Address) adrs) {
try {
this.address = (Address) adrs;
Thread.sleep(2000); // or else do some other work here so that lock gets used for some more time
} catch (CustomerUpdateFailureException e) {
e.getCause();
return false;
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return true;
}
}
}

}

class CustomerUpdateFailureException extends RuntimeException {

private static final long serialVersionUID = 1L;

@Override
public String getMessage() {

return "Uncompitable update";
}

}

who will dance with Nancy ? Find in below code !

Gyanendra Concurrency, Java 
public class DecideDancePair {

DecideDancePair ddp1;
StringBuffer ddp2 = new StringBuffer();

public static void main(String[] args) {

new Thread(new Runnable() {

@Override
public void run() {
synchronized (DecideDancePair.class) {
System.out.println("Its weekend .... Hey Guys !! this is Nancy , who will dance with me? ");
}

}
}).start();

new Thread(new Runnable() {

@Override
public void run() {

synchronized (new DecideDancePair().ddp1) {
System.out.println("Hey Nancy !! this is Peter , I will dance with you.");
}

}
}).start();

new Thread(new Runnable() {

@Override
public void run() {
synchronized (new DecideDancePair().ddp2) {
System.out.println("Hey Nancy !! this is Rozer , I will dance with you.");
}

}
}).start();

}

}

==============================

Answer :
My new post is going to take little more time so thought of to post solution of this problem.

Nancy will always go for dance because she having Lock over a object which is not NULL. same with Rozer , he is also having a non-NULL lock. while poor Peter is caught with NULL object Lock so that piece of code will throw exception below and Hence Nancy and Rozer will de in dance 🙂 !!!

Code execution will looks like :


Its weekend .... Hey Guys !! this is Nancy , who will dance with me? 
Exception in thread "Thread-1" Hey Nancy !! this is Rozer , I will dance with you.
java.lang.NullPointerException
	at BasicConcepts.DecideDancePair$2.run(DecideDancePair.java:26)
	at java.lang.Thread.run(Thread.java:745)

Sample code for Producer n Consumer Problem

Gyanendra Concurrency, Design pattern, Java , , ,

There are at least 5 direct ways to write a Producer n Consumer Problem. The basic is one which use wait() and notify().

// Queue
class CustomerQueue
{

    String sharedCustomerQueue[] = new String[5];
}

// Client
public class ProducerConsumerQ
{

    static int NEED_TO_SERVE_BATCH = 2;

    public static void main(String[] args)
    {

        System.out.println(" Producer will fill SharedQueue with 5 elements in each batch.");
        System.out.println(" Consumer will consume elements available in SharedQueue in each batch.");

        System.out.println(" Total " + NEED_TO_SERVE_BATCH + " will be served.");
        System.out.println(" \n ***************************************************************************** ");
        CustomerQueue cq = new CustomerQueue();
        Thread pro = new producer(cq);
        Thread con = new consumer(cq);
        pro.setName("Producer");
        con.setName("Consumer");
        pro.start();
        con.start();

    }

}

class producer extends Thread
{

    int count = 0;
    CustomerQueue qp;

    public producer(CustomerQueue q)
    {
        qp = q;
    }

    @Override
    public void run()
    {

        while (count < ProducerConsumerQ.NEED_TO_SERVE_BATCH)
        {
            System.out.println("\n");
            synchronized (qp.sharedCustomerQueue)
            {

                // everytime generate 5 random numbers and put in Queue
                for (int p = 0; p < 5; p++)
                {
                    qp.sharedCustomerQueue[p] = (int) ((Math.random() * 10) % 10) + "";
                    System.out.println("Producer : Genereated item is :" + qp.sharedCustomerQueue[p]);
                }
                qp.sharedCustomerQueue.notify();
                try
                {
                    qp.sharedCustomerQueue.wait();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
                count++;
            }
        }

    }
}

class consumer extends Thread
{

    int count = 0;
    CustomerQueue qc;

    public consumer(CustomerQueue q)
    {
        qc = q;
    }

    @Override
    public void run()
    {

        while (count < ProducerConsumerQ.NEED_TO_SERVE_BATCH)
        {
            if (qc.sharedCustomerQueue[0] != null) // consume only if queue is filled
            {
                synchronized (qc.sharedCustomerQueue)
                {

                    for (String qItem : qc.sharedCustomerQueue)
                    {
                        System.out.println("Consumer : Consuming item " + qItem);
                    }

                    System.out.println("\n");
                    qc.sharedCustomerQueue.notify();

                    try
                    {
                        qc.sharedCustomerQueue.wait();
                    }
                    catch (InterruptedException e)
                    {
                        e.printStackTrace();
                    }
                    count++;
                }
            }
        }

    }
}

/*
 Producer will fill SharedQueue with 5 elements in each batch.
 Consumer will consume elements available in SharedQueue in each batch.
 Total 2 will be served.
 
 ***************************************************************************** 
Producer : Genereated item is :3
Producer : Genereated item is :5
Producer : Genereated item is :5
Producer : Genereated item is :1
Producer : Genereated item is :1
Consumer : Consuming item 3
Consumer : Consuming item 5
Consumer : Consuming item 5
Consumer : Consuming item 1
Consumer : Consuming item 1




Producer : Genereated item is :9
Producer : Genereated item is :5
Producer : Genereated item is :8
Producer : Genereated item is :9
Producer : Genereated item is :8
Consumer : Consuming item 9
Consumer : Consuming item 5
Consumer : Consuming item 8
Consumer : Consuming item 9
Consumer : Consuming item 8



*/