MultiThreading

1
Multi-Threading

• Threads, operating Systems and processors.
• Using Java to create and start new threads
• Controlling Threads
• Controlling Threads in groups
• Synchronising
• Threads in Animation.

2
Threads, Operating Systems processors

• Computers appear to be able to do several things
  at once. Mulitasking.
• Each sequence of tasks performed by the CPU is
  called a thread.
• The priority and execution of threads is
  controlled by the operating system. Windows and
  Unix both support multithreading.
• When the computer has more than one CPU then it
  can truly perform several tasks at once.
  Otherwise it is an illusion.

3
MultiTasking

• One CPU can only perform one task at a
  time.Nevertheless it can appear to be performing
  many by sharing small portions of its time
  between each. This is controlled by the operating
  system which starts processes, monitors and
  assigns priorities to them.
• The CPU has a lot of idle time waiting for other
  events. Multithreading uses this time to perform
  other tasks without over taxing the CPU.A
  restaurant chef can cook several meals at once,
  eg. cutting up vegetables while waiting for the
  rice to cook.

4
Java and Multithreading

• Java allows us to start and control new threads.
  Javas multithreading commands work through the
  operating system.
• All programs create a new thread when they are
  started but this usually occurs from the
  operating system. Java allows us to start new
  Java threads from within a Java program.
• When a Java program begins the main or init
  methods are called from the JVM.
• Java classes that are to be run as threads do not
  have a main or init method, they have a run
  method that is the first method to be executed.

5
Java Threads

• Thread classes can be made in two ways
• By extending the Thread class.
• By implementing the Runnable interface.As Java
  programs can only be extended from one class,
  this frees the class to be extended from a
  different class such as JFrame or JAplett.

6
Threads by extending Thread

• The threaded class inherits a number of methods
  from Thread.

Must catch InterruptedException
7
Threads by extending Thread(2)
class TestThread p. s. v. main(String
args) //-- Create thread objects Thrd1 t1 new
Thrd1() Thrd2 t2 new Thrd2() //-- Start
Threads t1.start() t2.start()
class Thrd1 extends Thread public void run()
for (int n0nlt100n) System.out.println(
Port)
class Thrd2 extends Thread public void run()
for (int n0nlt100n) System.out.println(
Crows)
8
Threads by implementing Runnable

• By extending Thread we use up the single
  inheritance option available to our class.
• By implementing an Interface we can use the
  extends option for another class.
• The Runnable interface has only one abstract
  member, run().
• Access to the other methods, start(), wait()
  notify() etc is obtained by wrapping an object of
  the class in a Thread class.

9
Threads by implementing Runnable (2)
class TestRunnable p. s. v. main(String
args) //-- Create thread objects Thread t1 new
Thread(new Thrd1()) Thread t2 new Thread(new
Thrd2()) //-- Start Threads t1.start()
t2.start()
Class wrapped in Thread Object
class Thrd1 implements Runnable public void
run() for (int n0nlt100n)
System.out.println(Port)
class Thrd2 implements Runnable public void
run() for (int n0nlt100n)
System.out.println(Crows)
10
Thread State Diagram
Thread Created
Ready
finished
Start
Yield
run
Stop or finish
stop
New
Running
paused
Suspend Sleep or Wait
11
Thread Groups

• Often there will be a number of threads combining
  to serve a particular purpose. These can be
  controlled as a group.
• Create a ThreadGroup object and allocate a
  name.
• ThreadGroup tg new ThreadGroup(Thread
  Group)
• Add runnable objects to the group when you
  create the Thread object via its
  constructor.Thread t new Thread (tg, new
  Thrd1())

12
Thread Groups (2)

• A Thread group is itself a runnable object with
  all the methods of normal threads.
• You can start, wait, sleep, notify etc with the
  whole group via the group object name.
• As it is runnable it can be added to other Thread
  groups so building up a tree or hierarchy of
  groups that can be controlled at different levels.

13
Synchronization

• Where many processes are using and updating the
  same data errors can arise.
• One program reads data, and takes some time
  processing it before updating it. Meanwhile
  another process has opened the same data before
  the first process is complete. When the data is
  updated it will soon be overwritten by the other
  process.
• Synchronization prevents a method from being
  called on the same data before other processes
  are finished.

14
Synchronization (2)

• The method header of the method that updates the
  data includes the keyword synchronized. This
  prevents this method being run till the data is
  released from another process.
• public static synchronized void updateData()
• The static keyword ensures that the method is
  locked in all instances of the class.

15
The Timer class

• Belongs to the javax.swing package.
• Creates ActionListener events at a set period
  which are caught by the actionPerformed method.
• Register with the ActionListener in the
  constructor.Timer t
• t new Timer(1000,this)