This document discusses threads in Java. It defines a thread as the smallest unit of executable code that performs a task. Threads allow an application to be divided into multiple simultaneous tasks. The document covers how to create threads, thread states like ready and running, setting thread priorities, and daemon threads which provide services to other threads.

1. Java2
An Introduction to Threads

2. 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
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3. 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/
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4. 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/
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5. 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
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6. Applications of thread
 Playing sound and displaying images
simultaneously
 Displaying multiple images on the screen
 Displaying scrolling text patterns or images
on the screen
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7. 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
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8. Creating threads (2)
 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/
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9. Creating threads (3)
 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
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10. Creating threads (4)
Example 1 –
Creating a thread by
extending the Thread
class
Output
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11. Creating threads (5)
Example2 –
Creating a thread by
implementing the
Runnable interface
Output
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12. 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
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13. 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
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14. 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
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15. Different thread states
New Thread
(BORN)
READY
SLEEPING SUSPENDED
RUNNING
WAITING BLOCKED
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16. 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()
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17. 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
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18. 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/
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19. 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
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20. 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/
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21. 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
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22. 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/
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23. 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/
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24. Daemon threads (3)
An example
Output
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25. 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
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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
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