CODEWITHSUNDEEP
java
Neera
Neera

Reputation: 41

Regarding Notify()

I was going through threads and I read that the notify() method is used to send a signal to one and only one of the threads that are waiting in that same object's waiting pool. The method notifyAll() works in the same way as notify(), only it sends the signal to all of the threads waiting on the Object.

Now my query is that if let's say I have 5 threads and one main thread, so initially the main thread starts and then five other threads start. Now I want to send notification to third thread only. How could it be possible with the use of notify(), since here I am sending notification to third thread only? Please advise.

Upvotes: 1

Views: 163

Answers (2)

ulmangt
ulmangt

Reputation: 5423

The ReentrantLock class gives you more fined grained control than the synchronized keyword by allowing multiple wait sets per object.

You can get multiple Conditions from a ReentrantLock using ReentrantLock.newCondition(). A thread may then call Condition.await() (similar in function to Object.wait()) and will block until another thread calls Condition.signal() (similar in function to Object.notify()).

The difference is you can create multiple Conditions for a single ReentrantLock. So you could create one Condition for each Thread and call signal() on the condition corresponding to the Thread which you wanted to wake up.

Here's a simple example code which demonstrates the above. It creates 5 Threads which all wait on different Conditions of the same ReentrantLock. The main thread then calls signal() to wake up a specific one of the Threads.

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class ReentranLockTest
{
    public static void main( String[] args )
    {
        final ReentrantLock lock = new ReentrantLock( );

        int numThreads = 5;

        Condition[] conditions = new Condition[numThreads];

        // start five threads, storing their associated Conditions
        for ( int i = 0; i < numThreads; i++ )
        {
            final int threadNumber = i;

            System.out.printf( "Started thread number %d%n", threadNumber );

            // to create a Condition we must lock the associated ReentrantLock
            lock.lock( );
            try
            {
                final Condition condition = lock.newCondition( );
                conditions[i] = condition;

                // start the worker Thread
                ( new Thread( )
                {
                    @Override
                    public void run( )
                    {
                        // wait on the Condition
                        // to do so we must be holding the
                        // associated ReentrantLock
                        lock.lock( );
                        try
                        {
                            condition.await( );
                        }
                        catch ( InterruptedException e )
                        {
                            e.printStackTrace( );
                        }
                        finally
                        {
                            lock.unlock( );
                        }

                        // print something when signal()
                        // is called on our Condition
                        System.out.printf( "Thread number %d woke up!%n", threadNumber );
                    }
                } ).start( );
            }
            finally
            {
                lock.unlock( );
            }
        }

        // acquire the ReentrantLock and call
        // Condition.signal() to wake up Thread number 3    
        lock.lock( );
        try
        {
            System.out.printf( "Waking up Thead number %d%n", 3 );

            conditions[3].signal( );
        }
        finally
        {
            lock.unlock( );
        }
    }
}

This should print the following:

Started thread number 0

Started thread number 1

Started thread number 2

Started thread number 3

Started thread number 4

Waking up Thead number 3

Thread number 3 woke up!

Upvotes: 3

MByD
MByD

Reputation: 137432

If you want a specific thread to be notified, let it wait() on a different object, and call notify() on this object.

Upvotes: 3

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