ThreadsThreads

Eivind J. Nordby

University of Karlstad

Inst. for Information Technology

Dept. of Computer Science

1998-09-22 Computer Science, University of

Karlstad2

ThreadThread

Definition: A thread is a single sequential flow of

control within a program.

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Karlstad3

Introduction Introduction

A thread is an instantiation of a code running in a

separate time space is a convenient means for synchronization

of asynchronous tasks is running in the parent process’ address

space

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Karlstad4

Introduction Introduction

A thread is not a process, it does not have an address

space an OS task, it is part of its parent task

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Karlstad5

Multiple Threads can run in a Single Program

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Karlstad6

The Basic Idea, ExampleThe Basic Idea, Example

Create some need

get information

continue

Produce something

deliver

continue

transfer

wait

Producer Consumer

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Threads Threads runrunpublic class SimpleThread extends Thread { public SimpleThread(String name) { super(name); } public void run() { for (int i = 0; i < 10; i++) { System.out.println(i + " " + getName()); try { sleep((int)(Math.random() * 1000)); } catch (InterruptedException e) {} } System.out.println("DONE! " + getName()); }}

Called when the thread is start()-ed

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Karlstad8

Threads run in ParallelThreads run in Parallel

public class TwoThreadsTest { public static void main (String[] args) { new SimpleThread("Jamaica").start(); new SimpleThread("Fiji").start(); }}

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A Possible ResultA Possible Result

0 Jamaica0 Fiji1 Fiji1 Jamaica2 Jamaica2 Fiji3 Fiji3 Jamaica4 Jamaica4 Fiji5 Jamaica

5 Fiji6 Fiji6 Jamaica7 Jamaica7 Fiji8 Fiji9 Fiji8 JamaicaDONE! Fiji9 JamaicaDONE! Jamaica

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Karlstad10

The Life Cycle of a Thread

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Karlstad11

ThreadsThreadslang.Runnable

lang.Thread

+Thread+currentThread:Thread+yield:void+sleep:void+start:void+run:void

SimpleThread

+SimpleThread+run:void

{ super(name); // any other initialization}

{ // do the useful stuff. // When run finishes, // the thread dies}

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Karlstad12

Implementing the Runnable Interface

public class Clock extends Applet implements Runnable{ private Thread clockThread = null; public void start() { if (clockThread == null) { clockThread = new Thread(this, "Clock"); clockThread.start(); } }

Applet.start()

Thread.start()

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RunnablesRunnables

target

clockThread

lang.Thread

+Thread+currentThread:Thread+start:void+run:void

interfacelang.Runnable

+run:void

Clock

-clockThread:Thread

+Clock+run:void

{ clockThread = new Thread(this, "Clock"); // any other initialization}

{ // do the useful stuff}

{ if (target != null) target.run();}

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Karlstad14

Implementing the Runnable Interface

public void run() { Thread myThread = Thread.currentThread() while (myThread == clockThread) { repaint(); try { Thread.sleep(1000); } catch (InterruptedException e){} // the VM doesn't want us to sleep // anymore, so get back to work } // My creator has nulled out clockThread // to signal that I should stop working} // run

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Thread synchronization: exampleThread synchronization: example

Public class ThreadApplet

extends SequentialApplet {

public void start() {

new Thread(hello).start();

new Thread(goodbye).start();

}

}

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hello and goodbyehello and goodbye

are two Runnable objects display hello or goodbye in a common text

area txt use the Java awt function appendText for

the display code page 9

– run() {txt.appendText(msg);}

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Karlstad17

hello and goodbyehello and goodbye

txt

txt<<instantiate>>

<<instantiate>>

target

target

hello:Messenger[msg="Hello"]

goodbye:Messenger[msg="Goodbye"]

interfacelang.Runnable

+run:void

Messenger:TextArea

:Thread

:Thread

{ txt.appendText(msg);}

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Karlstad18

Interaction diagrams: Interaction diagrams: DescriptionDescription

Like the GoF patterns, interaction diagrams describe the flow of control

Unlike the GoF patterns the diagrams show discrete steps per thread– no synchronization is implied between the

threads

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Karlstad19

Interaction diagram: ExampleInteraction diagram: Examplegoodbye applet hello

start start/run

start/run

appendText

appendText

return

return

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Karlstad20

Java synchronization Java synchronization primitivesprimitives

to avoid interference when more than one concurrent thread can execute a function on a shared object

the keyword synchronized locks out other synchronized threads from that object

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Karlstad21

Synchronized wrapperSynchronized wrapper

assume appendText was not already synchronized– could produce

HeGoodlbyelo

GHoeoldlboye

– or other funny output instead of Hello

Goodbye (or the other way round)

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Synchronized wrapperSynchronized wrapper

class Appender { // page 17 private TextArea textArea; Appender(TextArea t) { textArea = t; } synchronized void append(String s) { textArea.appendText(s); }}

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What is synchronized?What is synchronized?

Always an object– the object executing a synchronized method– an explicitly mentioned object

Object syncher = new Object;void f1() {… synchronized(syncher) {…} … }void f2() {… synchronized(syncher) {…} … }

only one block is executed at a time

– a synchroinzed method synchronized on the current object (

synchronized(this) {…}

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Giving up a synchGiving up a synch

wait suspends the current thread and releases synchronization

another waiting thread synchronized on that object can proceed

the other thread can resume the waiting thread– normally using notifyAll();

one of the waiting threads starts when the lock is freed