Dr. R R DOCSIT, Dr BAMU

Basic Java :Multi Threading Cont. 2 Objectives of This Session Explain Synchronization in threads Demonstrate use of synchronization using Bank Account example Explain & demonstrate wait (), notify () & notifyAll(). Identify the need for thread Groups.

Basic Java :Multi Threading Cont. 3 Synchronization Many times, 2 or more threads need to share access to the same objects. When this happens, they need some way to ensure that the object will be modified by only one thread at a time. Else, the threads enter into a “race condition”

Basic Java :Multi Threading Cont. 4 Synchronization consider a deposit operation: public void deposit(Account a, float amount) { float temp; temp = a.bal; temp+=amount; a.bal = temp; }

Basic Java :Multi Threading Cont. 5 Thread Synchronization Key to synchronization is the concept of the monitor. A monitor is an object that is used as a mutually exclusive lock. Only one thread can own a monitor at a given time. When a thread acquires a lock, it is said to have entered the monitor. All other threads attempting to enter the locked monitor will be suspended until the first thread exits the monitor.

Basic Java :Multi Threading Cont. 6 Thread Synchronization Thread synchronization can be achieved in two ways -  Synchronized methods  Synchronized block

Basic Java :Multi Threading Cont. 7 Synchronization A thread becomes the owner of the object's monitor in one of three ways:  By executing a synchronized instance method of that object.  By executing the body of a synchronized statement that synchronizes on the object. Only one thread at a time can own an object's monitor.

Basic Java :Multi Threading Cont. 8 Synchronized Method public synchronized void deposit(Account a, float amount) { float temp; temp = a.bal; temp+=amount; a.bal = temp; }

Basic Java :Multi Threading Cont. 9 Synchronized Blocks synchronized blocks allow for “activity- centered” Synchronization. Lock of an object is acquired prior to entry into block & released upon exit from block. public void printValue(Point p){ // variable declaration synchronized(p){ // code } // more code of the printValue method }

Basic Java :Multi Threading Cont. 10 Thread Synchronization Periodically the thread scheduler activates the threads that are waiting for the key. When one of the threads waiting to use the object runs again, it checks if object is still locked. All threads are still free to call unsynchronized methods on the locked object. If a thread exits a synchronized method by throwing an exception, it still relinquishes the object lock.

Basic Java :Multi Threading Cont. 11 wait() Method of the Object super class (not class Thread). Allows thread to wait inside a synchronized method. When invoked, the current thread is blocked & gives up the object lock. Waits to be notified by another thread of a change in this object. This lets another thread entry into the monitor.

Basic Java :Multi Threading Cont. 12 Necessity for notify(), notifyAll() When a thread enters wait(), it has no way of unblocking itself. If all threads wait, it leads to DEADLOCKS. notify() :Wakes up a single thread that is waiting. This method should only be called by a thread that is the owner of the object's monitor. notifyAll () - Wakes up all threads that are waiting on this object.

Basic Java :Multi Threading Cont. 13 Thread Groups Some programs contain a number of threads. It would be useful to categorize them by functionality. This lets you work simultaneously with a number of threads. Class ThreadGroup contains methods for creating & manipulating groups of threads.

Basic Java :Multi Threading Cont. 14 Thread Groups Example ThreadGroup tg = new ThreadGroup(“TG1”); Thread t1 = new Thread(tg,”Thread1”); Thread t2 = new Thread(tg,”Thread2”); if (tg.activeCount() != 0) { tg.interrupt(); }