Dr. R R Manza @ 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 declarationsynchronized(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();}