Threads and MultithreadingMultiprocessingMultithreadingThreadsSleepingStates of a ThreadState transitionsTwo ways of creating ThreadsExtending ThreadImplementing RunnableStarting a ThreadExtending Thread: summaryImplementing Runnable: summaryThings a Thread can doAnimation requires two ThreadsHow to animateThings a Thread should NOT doHow to control another ThreadA problemTools for a solutionThe EndJan 13, 2019Threads and Multithreading2MultiprocessingModern operating systems are multiprocessingAppear to do more than one thing at a timeThree general approaches:Cooperative multiprocessingPreemptive multiprocessingReally having multiple processors3MultithreadingMultithreading programs appear to do more than one thing at a timeSame ideas as multiprocessing, but within a single programMore efficient than multiprocessingJava tries to hide the underlying multiprocessing implementation4ThreadsA Thread is a single flow of controlWhen you step through a program, you are following a ThreadYour previous programs all had one ThreadA Thread is an Object you can create and control5SleepingEvery program uses at least one ThreadThread.sleep(int milliseconds);try { Thread.sleep(1000); }catch (InterruptedException e) { }sleep only works for the current Thread6States of a ThreadA Thread can be in one of four states:Ready: all set to runRunning: actually doing somethingWai ting, or blocked: needs somethingDead: will never do anything againState names vary across textbooksYou have some control, but the Java scheduler has more7State transitionsreadywaitingrunning deadstart8Two ways of creating ThreadsYou can extend the Thread class:class Animation extends Thread {…}Limiting, since you can only extend one classOr you can implement the Runnable interface:class Animation implements Runnable {…}requires public void run( )I recommend the second for most programs9Extending Threadclass Animation extends Thread { public void run( ) { code for this thread } Anything else you want in this class}Animation anim = new Animation( );A newly created Thread is in the Ready stateTo start the anim Thread running, call anim.start( );start( ) is a request to the scheduler to run the Thread --it may not happen right awayThe Thread should eventually enter the Running state10Implementing Runnableclass Animation implements Runnable {…}The Runnable interface requires run( )This is the “main” method of your new Thread Animation anim = new Animation( ); Thread myThread = new Thread(anim);To start the Thread running, call myThread.start( );You do not write the start() method—it’s provided by JavaAs always, start( ) is a request to the scheduler to run the Thread--it may not happen right away11Starting a ThreadEvery Thread has a start( ) methodDo not write or override start( ) You call start( ) to request a Thread to runThe scheduler then (eventually) calls run( )You must supply public void run( )This is where you put the code that the Thread is going to run12Extending Thread: summaryclass Animation extends Thread { public void run( ) { while (okToRun) { ... } }}Animation anim = new Animation( );anim.start( );13Implementing Runnable: summaryclass Animation extends Applet implements Runnable { public void run( ) { while (okToRun) { ... } }}Animation anim = new Animation( );Thread myThread = new Thread(anim);myThread.start( );14Things a Thread can doThread.sleep(milliseconds)yield( )Thread me = currentThread( );int myPriority = me.getPriority( );me.setPriority(NORM_PRIORITY);if (otherThread.isAlive( )) { … }join(otherThread);15Animation requires two ThreadsSuppose you set up Buttons and attach Listeners to those buttons...…then your code goes into a loop doing the animation……who’s listening?Not this code; it’s busy doing the animationsleep(ms) doesn’t help!16How to animateCreate your buttons and attach listeners in your first (original) ThreadCreate a second Thread to run the animationStart the animationThe original Thread is free to listen to the buttonsHowever,Whenever you have a GUI, Java automatically creates a second Thread for youYou only have to do this yourself for more complex programs17Things a Thread should NOT doThe Thread controls its own destinyDeprecated methods:myThread.stop( )myThread.suspend( )myThread.resume( )Outside control turned out to be a Bad IdeaDon’t do this!18How to control another ThreadDon’t use the deprecated methods!Instead, put a request where the other Thread can find itboolean okToRun = true;animation.start( );public void run( ) { while (controller.okToRun) {…}19A problemWhat gets printed as the value of k?This is a trivial example of what is, in general, a very difficult problemint k = 0;Thread #1: k = k + 1;Thread #2: System.out.print(k);20Tools for a solutionYou can synchronize an object:synchronized (obj) { code that uses/modifies obj }No other code can use or modify this object at the same timeYou can synchronize a method:synchronized void addOne(arg1, arg2, ...) { code }Only one synchronized method in a class can be used at a time (other methods can be used simultaneously)Synchronization is a tool, not a solution—multithreading is in general a very hard problem21The
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