ICS11 Introduction to Computer Programming II

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Xavier University Computer Center Sun
Microsystems, Philippines
Java sa Eskwela (JsE) Program

• ICS11 Introduction to Computer Programming II
• Joselito dela Cruz Ty

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Chapter 15
Multithreading Performing Several Tasks
Concurrently
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Outline

• Multitasking Concepts
• Multithreading Concepts
• Thread Life Cycle
• Java Thread Commands

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Multitasking

• Refers to performing several tasks at the same
  time
• Common multitasking activities
• Studying while listening to music
• Doing homework while watching tv
• Texting while listening to the teacher
• Doing Java laboratory exercises while chatting
  and looking at Friendster

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Multitasking

• Computers normally perform several tasks at the
  same time
• Each computer user usually opens several windows
  at the same time
• When an active window finishes processing and
  while the user is still reading the output, the
  computer is expected not to be idle
• The computer is expected to be processing the
  other non-active windows

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Multitasking

• In reality, a person can not do several tasks at
  the same time
• A person doing multitasking switches among
  several tasks
• One has to leave behind a current task
  momentarily to attend to a new task

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Multitasking

• Similarly, a computer can not do several tasks at
  the same time
• A computers multitasking feature involves
  switching rapidly among several tasks
• But the computer performs switching so fast that
  it gives the illusion that it is performing all
  the tasks at the same time

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Multithreading

• In a particular program, several tasks can also
  happen at the same time
• In the same way, the computer switches among
  these tasks rapidly that it appears as if the
  computer is performing them at the same time
• What are the tasks that are happening at the same
  time in a PacMan program?

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Multithreading

• A Thread is a sequential task that happen
  concurrently with other tasks
• A program can have several threads running at the
  same time
• This feature is called Multithreading

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Multithreading

• A Java program has at least one Thread running
  the Thread that reads, interprets, and runs
  public static void main()
• Other Threads can be created dynamically to run
  concurrently with this main Thread
• When all Threads die in a program, the program
  ends

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Multithreading

• Threads compete for CPU time according to their
  level of priority
• The Thread with the currently highest priority is
  set in the foreground and is given a slice of
  time with the CPU
• The other Threads are set in the background and
  await for their turn when their priority number
  is called

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Multithreading

• User Threads
• Threads that affect how the program runs
• These threads actively compete for CPU time so
  that they can be executed
• When all User Threads die, the program ends
• Daemon Threads
• Low-priority Threads that perform administrative
  tasks
• e.g. The Java Garbage Collector

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Thread Dependence

• Sometimes, a Thread can depend on the results of
  another Thread to execute
• For example, several Threads can be used to
  download and play a media clip from the Internet
• So that the user does not have to wait long, a
  Thread can play the incomplete media clip even
  though the other Thread has not yet finished
  downloading

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Deadlocks

• A Thread that depends on the results of another
  Thread sleeps while the other Thread finishes
  execution
• What happens if Thread1 waits for the results of
  Thread2, while Thread2 waits for the results of
  Thread3, and Thread3 waits for the results of
  Thread1?
• This situation is called a Deadlock

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Multithreading Issues

• Thread issues and solutions will be covered in
  detail in your Operating Systems class in 4th
  Year
• Managing thread priorities
• Switching CPU time among threads
• Detecting and solving deadlocks, etc

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Java Thread Commands

• Creating a Thread
• Either extend class Thread or implement interface
  Runnable
• e.g.
• public class Ghost extends Thread
• public class Ghost implements Runnable

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Java Thread Commands

• Creating a Thread
• Thread classes should overwrite the method run( )
  which is invoked every time the Thread gets CPU
  time
• View Thread Demo samples

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Java Thread Commands

• Methods of class Thread
• start()
• Starts the Thread. Calls the run() method of the
  Thread.
• yield()
• Allows other Threads of lower priorities to take
  the CPU time.
• sleep(int milli)
• Stops the Thread for indicated number of
  milliseconds.

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Thread Life Cycle
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Thread Life Cycle

• Thread started
• A Thread starts its execution with the call to
  method start(). This calls the Thread run()
  method.
• myThread.start()
• Thread runnable
• A started Thread enters the running phase once
  method start() is invoked. It waits for the CPU
  to give it a slice of time.

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Thread Life Cycle

• Thread not runnable
• A Thread will enter the Not Runnable phase for
  any of the following reasons
• It has been called to sleep
• myThread.sleep(1000)
• It has been called to wait
• myThread.wait()
• Waiting Thread can be put in ready state
• myThread.notify()
• It issues an I/O request

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Thread Life Cycle

• Thread dies
• A Thread will die after it has completed the task
  in the run() method
• A dead Thread will no longer be brought back to
  life (resurrected), but the reference can be
  recycled into a new Thread (reincarnation)
• See Thread demo

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