multi threading by kamalakar dandu

8/2/2019 Multi Threading by Kamalakar Dandu

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Multithreading


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Threads

What are threads?

Virtual CPU


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Three Parts of a Thread

Code Data


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Creating a Thread

Multithreaded programming

Multiple threads from same Runnable

instance

Threads share same data and code


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Starting a Thread

Using the start() method

Placing the thread in runnable state


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Basic Control of Threads

Testing a thread

isAlive()

sleep()

join()


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New

Runnable

Running

Dead

otherwise

Blocked

Blocked in

ObjectsLock Pool

Blocked in

Objects

Wait Pool

Scheduler

notify()

interrupt()

run() completes

sleep() /join()

synchronizedLock available

join() or interrupt()

wait()

start()


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Which to Use?

Implementing Runnable

Better object-oriented design Single inheritance

Consistency

Extending Thread

Simpler code


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The Object Lock Flag

Every object has a flag which can be thought of as alock flag

synchronized allows interaction with the lock flag


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Releasing the Lock Flag

Released when the thread passes the end of the

synchronized () code block

Automatically released when a break or exception isthrown by the synchronized() code block


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synchronized -Putting It Together

Allaccess to delicate data should besynchronized.

Delicate data protected by synchronizedshould be private.


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Deadlock

Two threads waiting for a lock from the other

It is not detected or avoided It can be avoided by

Deciding on the order to obtain locks

Adhering to this order throughout

Releasing locks in reverse order


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Thread Interaction - wait() and notify() Scenario

Consider yourself and a cab driver as threads

The problem How to determine when you are at your destination

The solution

You notify the cabbie of your destination and relax

Cabbie drivers and notifies you upon arrival at yourdestination


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Thread Interaction

wait() and notify()

The pools

Wait pool

Lock pool


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Monitor Model forSynchronization

Leave shared data in a consistent state

Ensure programs cannot deadlock

Do not put threads expecting different notifications

in the same wait pool


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The suspend()and resume()Methods

Have been depredated in JDK 1.2 Should be replaced with wait() andnotify()


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The stop() Method

Releases the lock before it terminates

Can leave shared data in an inconsistent state

Should be replaced with wait() andnotify()


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Lab Session

Write a program for generating odd and even numbers

between 1 to 20 using two threads. One thread for odd

numbers and another for even numbers. Create two classes

extending thread class.

You can use the below example as a reference.

Continued


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class NormalThr extends Thread {public void run() {

for(int i =0;i


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Exercise (contd..)

13. A simulation of multiple usersaccessing the StudentArray class is to beprogrammed


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Exercise (Contd..)

14. When the program is accessingthe Students class it has to be ensuredthat the data is not directly accessed by

two simulated users at the same time.While data is being updated all otherthreads should be blocked (Write lock) and

while reading data, all threads trying toupdate the thread should be blocked(Read lock)

multi threading by kamalakar dandu

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