Threads and Multithreading Multiprocessing Modern operating systems are multiprocessing Appear to do more than one thing at a time Three general approaches Cooperative multiprocessing Preemptive multiprocessing Really having multiple processors Multithreading Multithreading programs appear to do more than one thing at a time Same ideas as multiprocessing but within a single program More efficient than multiprocessing Java tries to hide the underlying multiprocessing implementation Why multithreading Allows you to do more than one thing at once Play music on your computer s CD player Download several files in the background while you are writing a letter Multithreading is essential for animation One thread does the animation Another thread responds to user inputs Threads A Thread is a single flow of control When you step through a program you are following a Thread Your previous programs all had one Thread A Thread is an Object you can create and control Sleeping Every program uses at least one Thread Thread sleep int milliseconds try Thread sleep 1000 catch InterruptedException e sleep only works for the current Thread States of a Thread A Thread can be in one of four states Ready all set to run Running actually doing something Waiting or blocked needs something Dead will never do anything again State names vary across textbooks You have some control but the Java scheduler has more State transitions waiting start ready running dead Two ways of creating Threads You can extend the Thread class class Animation extends Thread Limiting since you can only extend one class Or you can implement the Runnable interface class Animation implements Runnable requires public void run I recommend the second for most programs Extending Thread class 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 state To start the anim Thread running call anim start start is a request to the scheduler to run the Thread it may not happen right away The Thread should eventually enter the Running state Implementing Runnable class 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 Java As always start is a request to the scheduler to run the Thread it may not happen right away Starting a Thread Every Thread has a start method Do not write or override start You call start to request a Thread to run The scheduler then eventually calls run You must supply public void run This is where you put the code that the Thread is going to run Extending Thread summary class Animation extends Thread public void run while okToRun Animation anim new Animation anim start Implementing Runnable summary class Animation extends Applet implements Runnable public void run while okToRun Animation anim new Animation Thread myThread new Thread anim myThread start Things a Thread can do Thread sleep milliseconds yield Thread me currentThread int myPriority me getPriority me setPriority NORM PRIORITY if otherThread isAlive join otherThread Animation requires two Threads Suppose 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 animation sleep ms doesn t help How to animate Create your buttons and attach listeners in your first original Thread Create a second Thread to run the animation Start the animation The original Thread is free to listen to the buttons Things a Thread should NOT do The Thread controls its own destiny Deprecated methods myThread stop myThread suspend myThread resume Outside control turned out to be a Bad Idea Don t do this How to control another Thread Don t use the deprecated methods Instead put a request where the other Thread can find it boolean okToRun true animation start public void run while controller okToRun A problem int k 0 Thread 1 k k 1 Thread 2 System out print k What gets printed as the value of k This is a trivial example of what is in general a very difficult problem Tools for a solution You can synchronize an object synchronized obj code that uses modifies obj No other code can use or modify this object at the same time You 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 problem The End
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