Java Technology: Threads

Java Technology Threads

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Session Plan 

Life cycle of threads



Using Thread class and its methods



Synchronization



Deadlock



Inter-Thread communication

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Threads 

Thread is an object it has variables and methods that lives and dies on the heap

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But a thread of execution is an individual process (‘light weight process’) that has its own call stack.

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In java there is one thread per call stack even if you don’t create thread there will be a default thread running called main thread ie.,main method.

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Motivation for Threads in Java 

Threads have become prominent due to trends in 

Software design 



More naturally expresses inherently parallel tasks

Performance 

Scales better to multiprocessor systems

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Creating Threads 

Two approaches 

Thread class public class Thread extends Object { … }

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Runnable interface public interface Runnable { public void run(); // work ⇒ thread }

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A Runnable Object 

The Thread object’s run() method calls the Runnable object’s run() method

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

Allows threads to run inside any object, regardless of inheritance Example – an applet that is also a thread

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Execution of Thread 

Threads are scheduled according to their priority w.r.t. other threads in the ready queue

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The highest priority runnable thread is always selected for execution above lower priority threads

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When multiple threads have equally high priorities, only one of those threads is guaranteed to be executing

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Priority in Thread 

Every thread has a priority

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When a thread is created, it inherits the priority of the thread that created it (normal-5, maximum-10,minimum-1)

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The priority values range from 1 to 10, in increasing priority

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The priority can be adjusted and may be obtained using setPriority(), getPriority() method

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Life cycle methods 

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void start() 

Creates a new thread and makes it runnable

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This method can be called only once

void run() 

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The new thread begins its life inside this method

void stop() (deprecated) 

The thread is being terminated

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Thread States-Life cycle 

Thread states 

Born state

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Ready state (runnable state)

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Running state

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Dead state

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Blocked state

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Waiting state

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Sleeping state

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Thread Lifecycle sleep(500) Active wake up

JV M Born

suspend()

start() Runnable

stop()

resume()

Blocked wait

stop() notify

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block on I/O

Methods in Java 

yield() 

Causes the currently executing thread object to temporarily pause and allow other threads to execute

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

Allow only threads of the same priority to run

sleep(int m)/sleep(int m,int n) 

The thread sleeps for m milliseconds, plus n nanoseconds

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Multi Thread Example class A extends Thread { public void run(){ for(int i=1;i<=5;i++){ System.out.println("\t From ThreadA: i= "+i); } System.out.println("Exit from A"); } } class B extends Thread { public void run(){ for(int j=1;j<=5;j++){ System.out.println("\t From ThreadB: j= "+j); } System.out.println("Exit from B"); Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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}}

Multi Thread Example class C extends Thread { public void run() { for(int k=1;k<=5;k++) { System.out.println("\t From ThreadC: k= "+k); } System.out.println("Exit from C"); }}

class ThreadTest { public static void main(String args[]) { new A().start(); new B().start(); new C().start(); }} Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Output: From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 Exit from A From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5 Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5 Exit from B

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Online Bank: Serving Many Customers and Operations PC client

Internet Bank Server Local Area Network

Bank Database Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Shared Resources 

If one thread tries to read the data and other thread tries to update the same date, it leads to inconsistent state.

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This can be prevented by synchronising access to data.

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Synchronization 

It allows only one thread to execute at a time. Synchronized keyword applies for method level or block level

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Takes out a monitor lock on an object 

Exclusive lock for that thread

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If lock is currently unavailable, thread will block

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For any object, only one thread may execute inside any of that object’s synchronized methods

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Synchronization Example synchronized void call(String msg) { System.out.println(“Synchronized”); try{ Thread.sleep(1000); } catch(InterruptedException e){ System.out.println("Interrupted"); } System.out.print("] \n"); } Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Thread Deadlock 

If two threads are competing for more than one lock

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Example: bank transfer between two accounts 

Thread A has a lock on account 1 and wants to lock account 2

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Thread B has a lock on account 2 and wants to lock account 1

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Avoiding Deadlock 

No universal solution

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Ordered lock acquisition

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Encapsulation (“forcing directionality”)

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Spawn new threads

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Check and back off

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Timeout

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Minimize or remove synchronization

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Inter Thread Communication 

Sometimes one thread may be interested in the activities of another. Or, one could have a functional dependency on another. 

Reading from a file or over a network?

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Waiting for a given thread to return a result.

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Polling (Busy Waiting) vs. Notification

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BadConsumer Example

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Waiting for notification 

As defined in object, every object has a wait(), notify(), and notifyAll() method. 

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These should never be overridden

They can only be called from inside synchronized blocks, and they only effect other threads in synchronized blocks which are synchronized on the same object.

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wait() (cont’d) 

When a thread enters a wait state, it does nothing until it is notified by another thread.

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It also gives up it’s lock on the object when wait is called.

public synchronized blog() { wait(); … // do something } Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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notify() 

To awaken a thread, a different thread which has a lock on the same object must call notify.

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When notify is called, the block that had the lock on the object continues to have it’s lock it releases it. 

Then a thread is awakened from its wait() and can grab the lock and continue processing.

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notifyAll() 

There are two versions - notify() and notifyAll().

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Notify is safe only under 2 conditions: 

When only 1 thread is waiting, and thus guaranteed to be awakened.

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When multiple threads are waiting on the same condition, and it doesn’t matter which one awakens.

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In general, use notifyAll()

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Summary 

Introduction to Threads



Life cycle and its Methods



Synchronization



Deadlock



Avoiding Deadlock



Inter Thread Communication

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