Understanding Multithreading in Java: A Programmer's Guide

1. Threads Fall 2005 IST 311

2. Threads • A thread is a single sequence of executable statements within a program • Java applications, flow begins with first statement in main( ) • Java applets, flow control begins with first statement in init( ) • CPU executes one instruction at a time • Multiple threads executing concurrently and rapidly gives the appears that the CPU is running more than one task at a time

3. Threads • Multitasking – technique of concurrently executing several tasks within a program • Downloading an image from the internet while running a speadsheet calculation • Multitasking requires use of separate thread for each task

4. Threads • CPU uses fetch-execute cycle to retrieve next instruction from memory and execute it • Multithreaded programs make it possible to divide up the CPU’s time and share it among the threads • CPU’s schedule is managed by a scheduling algorithm • Controlled by the operating system and JVM

5. Threads • Thread scheduling is handled differently on Unix, Windows, and Macintosh systems • One common scheduling technique is known as time slicing • Each thread alternatively gets a slice of CPU time • System gives each thread a quantum of CPU time to execute instructions • When quantum expires, thread would be preempted and another thread given a chance to run • On a 300-megahertz CPU, a thread could execute 300,000 machine instructions during a millisecond

6. Threads • Priority scheduling – higher priority threads are allowed to run to completion before lower-priority threads • High-priority thread might be keyboard input from the user so the user doesn’t experience noticeable delays

7. Threads

8. Threads • How to create a multithreaded program: • Implement the Runnable interface for an existing class by implementing the void run( ) method • Create several Thread instances by first creating instances of the Runnable class and passing each as an argument to the Thread( ) constructor • For each thread instance, start it by invoking the start( ) method on it.

9. Threads • NumberPrinter and Number program

10. Threads • start( ) and stop( ) methods used to control thread’s execution • Sometimes called automatically • An applet is treated as a thread by a browser, which is responsible for starting and stopping it • setPriority( int) lets you set a thread’s priority setPriority(Thread.MAX_PRIORITY);

11. Threads • Thread implementation in Java is platform dependent • Test to ensure program will perform correctly on a given platform • Coordinate behavior of 2 threads by giving one a higher priority • High-priority thread that never gives up CPU time can starve lower-priority threads

12. Threads • Thread.sleep( ) method causes the thread to yield and not be scheduled until a certain amount of real time has passed • Halt a running thread for a given number of milliseconds • Throws an InterruptedException which is a checked exception • sleep( ) must be embedded within a try/ catch block or the method it is in must throw an InterruptedException

13. Threads • Thread.yield( ) method causes the thread to yield CPU, allowing the thread scheduler to choose another thread

14. Threads • Threads are asynchronous • The order of execution is sporadic and unpredictable from programmer’s point of view • Impossible for a programmer to predict when and for how long an individual thread will run

15. Thread States and Life Cycle

16. Part 2

17. Applets • start( ) is called once when the applet is started. • Calls init( ) and paint( ) automatically and then calls run( ) to run the applet • run( ) starts the applet running. • Called by start( ) • stop( ) is called by the browser or appletviewer when the applet is quit

18. Applets • init( ) is called automatically when the applet starts to perform initializations • paint( ) paints the graphics on the applet. Called once at start and then can be called by repaint( ) when applet’s appearance is changed.

19. Calendar • The class Calendar contains methods and constants to represent the date and time • Found in java.util.* • Constructors in Calendar are protected, so you must use getInstance( ) method to return an instance of a Calendar subclass • May retrieve hour, minutes, seconds: int hours = time.get(Calendar.HOUR); int mins = time.get(Calendar.MINUTE); int secs = time.get(Calendar.SECOND);

20. Threads • Applet to run a clock

21. Threads • We override methods init( ),start( ), and stop( ) • Web browser or appletviewer invokes these overridden methods • Applet responds to controlling program • With program like Clock, browser or viewer cannot regain control – stuck in an infinite loop

22. Threads • Java is an implicitly threaded language • Several threads are started automatically every time you run a Java applet or application • For example, a separate thread will start for each • Events generated by users (pressing buttons in a GUI) • Automatic garbage collection • Creation of a programmer’s own threads for running sections of a program independently

23. Threads • To solve the Clock problem, we need to allow the applet to run in parallel with the browser • Give it a thread of its own • Will react to the mouse button • Get into the habit of running an applet in its own thread