Concurrency in Java: Understanding Threading and Synchronization - Prof. Travis Doom, Study notes of Computer Science

Download Concurrency in Java: Understanding Threading and Synchronization - Prof. Travis Doom and more Study notes Computer Science in PDF only on Docsity!

ConcurrencyConcurrency^ Multiprocessing

p^

g

Threads/Lightweight objectsThread methodsConcurrency Issues

What is program “execution”?What is program “execution”?

z^ In order to run, a program must be

loaded

into memory and given an

initial^ state

–^ Code space, static variables, heap, stack (with main pushed on)–^ This is a

process, job,

or^ task.

z^ The

Operating System

allows multiple programs/processes to run

z^ The

Operating

System

allows^

multiple programs/processes to run

simultaneously! How?!?–^ multi-tasking (multi-processing) z How can we take advantage of this in our applications?z How^ can we take advantage of this in our applications?–^ Consider Event-driven programming, for starters… z Most corporations have several departments (say management,

i^

f^ i^

d^ l^ )

accounting, manufacturing, and sales).–^ For efficiency, each of these operations should be able to work ontheir tasks on their own, interacting only as necessary

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

–^ Multitasking

with multiple programs

communicating

with each other

Making Threads in JavaMaking Threads in Java

z^ Threads are built into Java (in java.lang.Thread) – no imports necessary! z^ There are two ways to make classes run in separate threads in Java

y^

p

–^ (1) Extend the

Thread

class [includes abstract method:

run ()]

z^ Provides direct access to thread methods, but no other inheritance.– (2) Implement the

Runnable

interface [defines interface method:

run ()]

(2) Implement the

Runnable

interface [defines interface method:

run ()]

z^ Most common mechanism

z^ A new thread can only be created by someone/somewhere calling^ –^

(1) new subThread();

//subThread is a subclass of Thread

-^ (2) new Thread(myRunnable); // myRunnable’s class implements Runnablepublic class MyRunnable implements Runnable {public void run() {doStuff();} // end method run… MyRunnable myRunnable = new MyRunnable();Thread t = new Thread(myRunnable); // define the method on the stackt.start();

// sets thread runnable, and

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

t.start();

// sets thread runnable, and

…^

// pushes run() on stack

Example: Thread RaceExample: Thread Race

public^ class

Threadrace

… public^ void^ go^

()^ {

JFrame^ frame

ne^ JFrame("Thread

Racer")

JFrame^ frame

=^ new^ JFrame("Thread

Racer");

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);frame.setSize

frame.setVisible

(true); frame.setLayout

(new^ GridLayout(numberOfRacers,1)); int^ numberOfNonRacerThreads

=^ Thread.activeCount();

for^ (int^

i^ =^ 0;^ i <^ numberOfRacers;

i++)^ {

Racer^ racer

=^ new^ Racer("Racer

"^ +^ i);

frame.add(racer); Thread^ thread

=^ new^ Thread(racer

thread.start

} … } //^ end^ method

go }^ //^ end class^ ThreadRace public^ class

Main^ { public^ static

void^ main

(String[]

args)^ {

ThreadRace

theRace =^ new^ ThreadRace(3); theRace.go();} //^ end^ method

main

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

}^ // } //^ end^ class

main

Example: Thread RaceExample: Thread Race

import^ java.awt.;import^ javax.swing.;public^ class

Racer^ extends

JPanel implements^ Runnable

public { void^ paintComponent

(Graphics

g)^ {

g.setColor

(Color.BLACK);

set^ background

g.fillRect

(0,0,getWidth(),getHeight

g setColor

(Color^

BLUE)^

//^ dra^

line

implements

Runnable

private^ String^ myName; private^ int^ myPosition

=^ 0;

private^ final^ int

numberofSteps

=^ 600;

private^ final^ int

racerHeight

=^ 20;

private^ final^ int

racerWidth

=^ 10;

g.setColor

(Color.BLUE);

//^ draw^ line

g.drawLine

(0,getHeight()/2,getWidth(),getHeight

g.^ setColor

(Color.RED);

//^ draw^

racer

g.fillRect

(myPosition*getWidth()/numberofSteps),

private^ final^ int

racerWidth

private^ int^ speed

=^ 1;

public^ Racer

(String name)^ { myName^ = new^ String

(name); }^ //^ end constructor

Racer

(myPosition

getWidth()/numberofSteps), (getHeight

()^ -^ racerHeight)/2, racerWidth,

racerHeight); }^ //^ end method^ paintComponent public^ void

run^ ()^

private^ void^ pause

(int^ millisec)

//for^ (int

i^ =^ 0; //^ i

<^1000000

*^ millisec;

i++)^ {

//^ busy^ wait^ -^ do

nothing //}

while^ (myPosition

+^ racerWidth

<^ numberofSteps

)^ {

myPosition

+=^ speed; repaint^

pause^ (5);} ThreadRace

setWinner

(myName);

//}try^ { Thread.sleep

(millisec); }^ catch^ (Exception

e)^ { e.printStackTrace

ThreadRace.setWinner

(myName); System.out.println

("Racer "^ +^ myName

"^ has^ finished

the^ race");

}^ //^ end method^ run }^ //^ end class^ Racer

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

} } //^ end^ method

pause

}^ //

Thread swappingThread swapping

z^ How does a computer perform threading?–^

It can do multiple things at one time (if multi-processor)

p^

g^

(^

p^

–^ It can

appear

to do multiple things at one time

z^ The computer has a clock that counts down a quantum for eachexecutable process (and thread)executable process (and thread).–^

When process’s quantum expires, it is taking off the CPU^ z^ Its state is saved for later…The^ scheduler

decides which process gets the next quantum of time

–^ The^

scheduler

decides which process gets the next quantum of time

z^ How is this done fairly?

z^ The programmer can have impact on thread processing by setting priority

int getPriority()int^ getPriority()void setPriority(int)

z^ In Java, the thread with the highest priority

always

get the processor

when it is not

blocked

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

Execution with threads of unequal priorityExecution with threads of unequal priority public^ void

go^ ()^ { JFrame^ frame

=^ new^ JFrame("Thread

Racer");

frame.setDefaultCloseOperation

(JFrame.EXIT_ON_CLOSE);

_^ _

frame.setSize

frame.setVisible

(true); frame.setLayout

(new^ GridLayout(numberOfRacers,1)); int^ numberOfNonRacerThreads

=^ Thread.activeCount();

f^ (i t^

i^0 i <^ b^

OfR^

i++)^ {

for^ (int^

i^ =^ 0;^ i <^ numberOfRacers;

i++)^ {

Racer^ racer

=^ new^ Racer("Racer

"^ +^ i);

frame.add(racer);Thread^ thread

=^ new^ Thread(racer); thread.setPriority(thread.getPriority

()^ +^ (2i)-4);*

System.out.println(

"Racer^ " +^ i^ +^ " now^ has priority

"^ +

System.out.println(

Racer^

+^ i^ +^ now^ has^ priority

thread.getPriority()

thread.start

} while^ (Thread.activeCount()

^ numberOfNonRacerThreads)

pause(10);} System.out.println("The

Race^ is over!^ " +^ winner

"^ wins!"); }^ //^ end method^ go Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

Example: a concern with race conditionsExample: a concern with race conditions public^ class

Main^ { public^ static

void^ main(String[]

args)^ {

public^ static

void^ main(String[]

args)^ {

Thread^ t; CentralBank

bank^ =^ new^ CentralBank(); System.out.printf("Creating

a^ threads

(%d^ active)\n",

Thread.activeCount()

//^ Create

Thread^ One BranchBank

branch =^ new^ BranchBank

("Branch

Alpha",bank);

t^ =^ new^ Thread^ (branch1); t.start();S^

i^ f("^ l h^ !^ (%d^

i^ )^ "^

h^ d^

i^ C^

System.out.printf("Alpha

away!^ (%d

active)\n",

Thread.activeCount());

//^ Create

Thread^ Two BranchBank

branch =^ new^ BranchBank

("Branch

Beta",^ bank);

t^ =^ new^ Thread^ (branch2); t start();t.start();System.out.printf("Beta

away!^ (%d

active)\n",

Thread.activeCount());

}^ //^ end method^ main }^ //^ end class^ Main Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

Concurrency IssuesConcurrency Issues

run:Creating a threads (1 active)

run:Creating a threads (1 active)

Creating a threads (1 active)Alpha away! (2 active)Beta away! (3 active)Branch Beta adds $1.00 to $0.00 New total: $1.00Branch Alpha adds $1.00 to $0.00 New total: $1.00Branch Alpha adds $1.00 to $1.00 New total: $2.00Branch Beta adds $1.00 to $1.00 New total: $2.

Creating a threads (1 active)Alpha away! (2 active)Beta away! (3 active)Branch Alpha adds $1.00 to $0.00 New total: $1.00Branch Beta adds $1.00 to $0.00 New total: $1.00Branch Alpha adds $1.00 to $1.00 New total: $2.00Branch Beta adds $1.00 to $1.00 New total: $2.

Branch Alpha adds $1.00 to $2.00 New total: $3.00Branch Beta adds $1.00 to $2.00 New total: $3.00Branch Alpha adds $1.00 to $3.00 New total: $4.00Branch Beta adds $1.00 to $3.00 New total: $4.00Branch Alpha adds $1.00 to $4.00 New total: $5.00Branch Beta adds $1.00 to $4.00 New total: $5.00Branch Alpha adds $1 00 to $5 00 New total: $6 00

Branch Alpha adds $1.00 to $2.00 New total: $3.00Branch Beta adds $1.00 to $2.00 New total: $3.00Branch Alpha adds $1.00 to $3.00 New total: $4.00Branch Beta adds $1.00 to $3.00 New total: $4.00Branch Beta adds $1.00 to $5.00 New total: $6.00Branch Beta adds $1.00 to $6.00 New total: $7.00Branch Beta adds $1 00 to $7 00 New total: $8 00

Branch Alpha adds $1.00 to $5.00 New total: $6.00Branch Beta adds $1.00 to $5.00 New total: $6.00Branch Alpha adds $1.00 to $6.00 New total: $7.00Branch Beta adds $1.00 to $6.00 New total: $7.00Branch Beta adds $1.00 to $7.00 New total: $8.00Branch Alpha adds $1.00 to $7.00 New total: $8.00Branch Alpha adds $1.00 to $8.00 New total: $9.

Branch Beta adds $1.00 to $7.00 New total: $8.00Branch Beta adds $1.00 to $8.00 New total: $9.00Branch Alpha adds $1.00 to $4.00 New total: $5.00Branch Alpha adds $1.00 to $9.00 New total: $10.00Branch Alpha adds $1.00 to $10.00 New total: $11.00Branch Beta adds $1.00 to $9.00 New total: $10.00Branch Alpha adds $1.00 to $11.00 New total: $12.

p Branch Beta adds $1.00 to $8.00 New total: $9.00Branch Alpha adds $1.00 to $9.00 New total: $10.00Branch Beta adds $1.00 to $10.00 New total: $11.00BUILD SUCCESSFUL (total time: 1 second)

p Branch Beta adds $1.00 to $12.00 New total: $13.00Branch Alpha adds $1.00 to $12.00 New total: $13.00Branch Alpha adds $1.00 to $13.00 New total: $14.

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

SynchronizationSynchronization

z^ What we need is a mechanism to identify operations which should NOT beinterrupted by another thread z^ Such operations/methods are called

atomic

–^ That is, they cannot be “divided” and must be done to completionbefore anyone can start the method

y

z^ In Java, you can designate a method as being atomic by using the keyword^ syncronized

in the method header

z^ How does Mutual Exclusion (Mutex) work?z^ How does Mutual Exclusion (Mutex) work?–^

Java provides a “lock” on each object.– An object becomes locked when a thread calls any of its synchronizedmethods

That thread gets the “key”

methods. That thread gets the

key.

–^ It requires a key to access any synchronized methods of a locked object.–^ The key is returned (and the object unlocked) when the thread executionl^

h^

h^ i^

d^ h d

Wright State University, College of EngineeringDr. T. Doom, Computer Science & Engineering

CS 241Computer Programming II

leaves the synchronized method.