Chapter6 threads

Java2

An Introduction to Threads

Objectives

 Compare Multitasking and Multithreading
 Define a thread
 Discuss Benefits of multithreading
 Explain how to create threads
 Describe Thread states
 Discuss Thread priorities
 Discuss Daemon thread
eACCP 2002/Sem 1/ JPL/
Session 6/ 2 of 26

Multitasking Vs Multithreading

 Multitasking is the ability to run one or more
programs concurrently
 Operating system controls the way in which
these programs run by scheduling them
 Time between switching of programs is
minute
 Multithreading is the ability to execute
different parts of a program called threads,
simultaneously eACCP 2002/Sem 1/ JPL/
Session 6/ 3 of 26

Thread
 Smallest unit of executable code that
performs a task
 An application can be divided into multiple
tasks and each task can be assigned to a
thread
 Many threads executing simultaneously is
termed as Multithreading
 Although it may appear that the processes
are running concurrently, it is not so 1/ JPL/
eACCP 2002/Sem
Session 6/ 4 of 26

Benefits of multithreading

 Multithreading requires less overhead than
multitasking
 In multitasking, processes run in their own
different address space
 Tasks involved in multithreading can share the
same address space
 Inter-process calling involves more overhead
than inter-thread communication
Session 6/ 5 of 26

Applications of thread

 Playing sound and displaying images
simultaneously

 Displaying multiple images on the screen

 Displaying scrolling text patterns or images
on the screen
Session 6/ 6 of 26

Creating threads (1)

 When Java programs execute, there is
always one thread running and that is the
main thread
 It is this thread from which child threads are
created
 Program is terminated when main thread stops
execution
 Main thread can be controlled through ‘Thread’
objects
Session 6/ 7 of 26

 Thread objects can be created in two ways:
 Declare a class that is a sub-class of the class
Thread defined in java.lang package
 class mythread extends Thread
 Declare a class that implements the Runnable
interface
 class mythread implements Runnable

 While using applets, Thread class cannot be
extended. Therefore one has to implement the
Runnable interface eACCP 2002/Sem 1/ JPL/
Session 6/ 8 of 26


 To initiate a new thread, we use the start()
method
Mythread t = new Mythread();
t.start();

 When start() method is invoked, a new thread
of control is created
 It then calls the run() method
Session 6/ 9 of 26

Example 1 –
Creating a thread by
extending the Thread
class

Output

Session 6/ 10 of 26

Example2 –
Creating a thread by
implementing the
Runnable interface

Output

Session 6/ 11 of 26

Thread states (1)

 Born – a newly created thread is in a born
state

 Ready – after a thread is created, it is in its
ready state waiting for start() method to be
called

Session 6/ 12 of 26

Thread states (2)

 Running – thread enters the running state
when it starts executing

 Sleeping – execution of a thread can be
halted temporarily by using sleep() method.
The thread becomes ready after sleep time
expires
Session 6/ 13 of 26

Thread states (3)

 Waiting – thread is in waiting state if wait()
method has been invoked

 Blocked – when the thread waits for an
Input/Output operation

 Dead – after the run() method has finished or
the threads stop() method is2002/Sem 1/ JPL/
eACCP called
Session 6/ 14 of 26

Different thread states
New Thread
(BORN)

READY

SLEEPING SUSPENDED

RUNNING

WAITING BLOCKED

DEADeACCP 2002/Sem 1/ JPL/
Session 6/ 15 of 26

Some methods
of thread class (1)

 isAlive() – returns true if the thread is alive

 getName() – returns the name of the thread

 start() – used to start a thread by calling the
method run()
Session 6/ 16 of 26

Some methods
of thread class (2)
 resume() – to resume the execution of a
suspended thread

 yield() – causes the currently executing
thread to temporarily pause and allow other
threads to execute

 setName(String name) – sets the name of
the thread to the name that is passed as
argument Session 6/ 17 of 26

Some methods
of thread class (3)
 join() – waits for the thread to die

 isDaemon() – checks if the thread is a
Daemon thread

 activeCount() – returns the number of active
threads

 sleep() – used to suspend a thread for a
certain period of time eACCP 2002/Sem 1/ JPL/
Session 6/ 18 of 26

Conditions preventing
thread execution

 Thread is:
 Not of highest priority
 Put to sleep using sleep() method
 Suspended using suspend() method
 Thread is waiting because wait() method was
called
 Explicitly yielded using yield() method
 Blocked for file I/O
Session 6/ 19 of 26

Managing threads (1)
 Priorities for carrying out activities changes at
times
 eg :Planned to visit museum in the afternoon but
due to toothache, had to go to doctor
 Similarly while programming, we may have to
run a thread of higher importance without
stopping or suspending the current running
thread
 Thread priorities play an important role in
such a situation eACCP 2002/Sem 1/ JPL/
Session 6/ 20 of 26

Managing threads (2)

 Thread priorities in Java are constants
defined in the Thread class
 NORM_PRIORITY – value is
 MAX_PRIORITY – value is
 MIN_PRIORITY – value is
 The default priority is NORM_PRIORITY
 Two methods used to change priority:

setPriority() - changes the thread’s current priority

getPriority() - returns the thread’s priority
Session 6/ 21 of 26

Daemon threads (1)
 Two types of threads in Java:
 User threads – created by the user
 Daemon threads – threads that work in the
background providing service to other threads

e.g. – the garbage collector thread
 When user thread exits, JVM checks to find
out if any other thread is running
 If the only executing threads are Daemon
threads, it exits eACCP 2002/Sem 1/ JPL/
Session 6/ 22 of 26

Daemon threads (2)

 We can set a thread to be a Daemon if we do
not want the main program to wait until a
thread ends
 Thread class has two methods to deal with
Daemon threads:
 public void setDaemon(boolean value) : sets a
thread to be a daemon thread
 public void isDaemon() : checks if the given thread
is a daemon thread eACCP 2002/Sem 1/ JPL/
Session 6/ 23 of 26

Daemon threads (3)
An example

Output

Session 6/ 24 of 26

Session summary (1)

 When Java programs are executed, there is
always one thread that is running and that is
the main thread
 Threads can be used in two ways:
 Declare the class to be a sub-class of the Thread
class
 Declare a class that implements the Runnable
interface
Session 6/ 25 of 26

Session summary (2)

 Each thread in a Java program is assigned a
priority
 The default priority of a thread that is created
is 5
 The methods Thread.suspend(),
Thread.resume() and Thread.stop() have
been deprecated since Java2
 The threads providing service to other
threads are referred to as daemon threads
Session 6/ 26 of 26

Chapter6 threads

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