MUTHAYAMMAL COLLEGE OF ARTS & SCIENCE, RASIPURAM.
DEPARTMENT OF BCA
Staff Name: S.KARTHIK
Class: III BCA „A‟
Paper Name: PROGRAMMING IN JAVA
Unit: IV
Multithreaded Programming
 A multithreaded program contains two or more task that can run at the same
time .
 For example, one part can display an animation on the screen while another
may build the next animation to be displayed.
 Figure illustrates a Java program with four threads, one main and three
others.
 The main thread is actually the main method module, which is designed to
create and start other three threads, namely A, B and C.
Main
Thread
Module
Thread A
Switching
Thread B
Thread C
Switching
 Once initiated by the main thread, the threads A,B and C run concurrently
and share the resources jointly.
ADVANTAGES OF MULTITHREADING
Multithreading is used to write very efficient programs.
Maximum use of CPU time. The idle time of CPU is reduced.
Interprocess communication is faster
Multithreading requires less overheads
The time required to perform a context switch from one thread to another is
less
Multithreading reduces the complexity of the large program
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The resources required for a thread is less than the resources required by a
process
Creating Threads
 Threads are implemented in the form of objects that contain a method called
run( ). The run( ) method is the heart and soul of any thread.
 A typical run( ) would appear as follows:
public void run( )
{
---- ( Statements for implementing thread )
---- }
 The run( ) method should be invoked by an object of the concerned thread.
A new thread can be created in two ways.
1. By creating a thread class: Define a class that extends thread class and
override its run ( ) method .
2. By converting a class to a thread: Define a class that implements Runnable
interface.
1)Extending the thread class
 Extending the class by java.lang.Thread.
 It includes the following steps:
1. Declare the class as extending the Thread class
2. Implement the run( ) method that is responsible for executing the
sequence of code that the thread will execute.
3. Create a thread object and call the start( ) method to initiate the thread
execution.
Program:
class A extends Thread
{
public void run( )
{
for ( int i=1;i<=3;i++)
{
System.out.println(“\t From thread A:i=”+i);
}}
}
class B extends Thread{
public void run( ){
for ( int i=1;i<=3;i++)
{
System.out.println(“\t From thread B:i=”+i);
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}}
}
class ThreadTest
{
public static void main(String[] args)
{
new A( ).start( );
new B( ).start( );
}}
Output
From thread A:i=1
From thread A:i=2
From thread B:i=1
From thread B:i=2
From thread A:i=3
Exit from A
From thread B:i=3
Exit from B
2)Implementing the Runnable Interface
 To create thread using the Runnable interface, we must perform the steps
listed below:
1. Declare the class as implementing the Runnable interface.
2. Implement the run( ) method.
3. Create a thread by defining an object that is instantiated from this
“runnable” class as the target of the thread.
4. Call the thread‟s start( ) method to run the thread.
 If the direct reference to the thread threadX is not required, then we may use
a shortcut as shown below:
new Thread( new X( ) ).start( );
Example
class X implements Runnable
{
public void run( )
{
for(int i=1;i<3;i++)
System.out.println(“\t ThreadX:”+ i);
}
}
class RunnableTest
{
public static void main(String[] args)
3
{
X runnable = new X( );
Thread threadX = new Thread( runnable );
threadX.start( );
}
}
Output
End of main Thread
ThreadX:1
ThreadX:2
End of ThreadX
Thread Priority
 In Java, each thread is assigned a priority for running.
 The threads having same priority are given equal treat by first-come,firstserve basis.
 Java permits us to set the priority of a thread using the setPriority( ) method
as follows:
ThreadName.setPriority( int number );
 The number is an integer value to which the thread‟s priority is set.
MIN_PRIORITY = 1
NORM_PRIORITY=5
MAX_PRIORITY = 10
 Whenever multiple threads are ready for execution, the Java system chooses
the highest priority thread and executes it.
Example
class A extends Thread
{
public void run( )
{
System.out.println(“Thread A Started”);
for(int i=1;i<=3;i++)
System.out.println(i);
}
}
class B extends Thread
{
public void run( )
{
System.out.println(“Thread B Started”);
for(int i=1;i<=3;i++)
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System.out.println(j);
}}}
class ThreadPriority
{
public static void main(String args[])
{
A threadA = new A( );
B threadB = new B( );
threadB.setPriority( Thread.MAX_PRIORITY);
threadA.setPriority(Thread.MIN_PRIORITY);
threadA.start( );
threadB.start( );
}}
Synchronization
 When the thread try to use the own data and methods outside themselves on
that time one thread may try to send a record from a file while another is
still writing to the same file.
 Depending on the situation, we may get wrong results.
 Java enables us to overcome this problem using a technique known as
synchronization by the keyword synchronized .
 For example, the method that will read information from a file and the
method that will update the same file may be declared as synchronized.
Example
synchronized void update( )
{
---- // code here is synchronized
---}
----}
 Whenever a thread has completed its work by using synchronized method, it
will hand over the monitor to the next thread that is ready to use the same
resource.
 An interesting situation may occur when two or more threads are waiting to
gain control of a resource.
 This result in what is known as deadlock.
LIFE CYCLE OF THE THREAD
The following states can occur during the life time of a thread. Each thread
is always in one of the five states.
1. Newborn state
2. Runnable state
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3. Running state
4. Blocked state
5. Dead state
wait ( )
Blocked
suspend ( )
sleep( )
notify ( )
resume ( )
Newborn
Runnable
Running
start( )
yield ( )
stop ( )
stop( )
stop ( )
Dead
NEWBORN STATE
A thread is in newborn state immediately after it is created. The thread
object has created the new state . It requires the start( ) method to start it.
Newborn
next
stop
Runnable
State
Dead
State
RUNNABLE STATE
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The runnable state means that a thread is ready to run and is awaiting for the
control of the processor. That is, the thread has joined the queue of threads that are
waiting for execution.
yield ( )
Runnable Threads
Running Thread
RUNNING STATE
A thread is said to be in running state, when it is executing a set of
instructions. The run( ) method contains the set of instructionThe run ( ) method is
called automatically after start ( ) method. A running state continues until any one
of the following occurs
Suspend ( )
running
runnable
suspended
wait
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running
runnable
suspended
sleep (t)
running
runnable
suspended
After the completion of the execution
When the method sleep ( ) is called.
When the method wait ( ) is called.
When the method suspend ( ) is called.
BLOCKED STATE
A thread is in blocked state, if it is being prevented from the runnable state
or running state. This happens when thread is suspended. Sleeping, or waiting in
order to satisfy certain requirements.
A blocked state is considered as “not runnable” but not dead
DEAD STATE
A thread is Dead when it finishs its execution (natural death) or is stopped (killed)
by another thread (premature death)
Example
class A extends Thread
{
public void run( )
{
for(int i=1; i< 3;i++)
{
if (i == 1 ) yield( );
System.out.println(i);}
}}
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class B extends Thread
{
public void run( )
{
for(int j=1 ;j<=5;j++)
{
System.out.println(j);
if (j==3 )
stop( );
}}}
class C extends Thread
{
public void run( )
{
for(int k=1; k<=3;k++)
{
System.out.println(k);
if (k = = 1 )
try{
sleep(1000);
}
catch( Exception e)
{}
}}
}class ThreadMethods
{
public static void main(String[] args)
{
A threadA =new threadA( );
B threadB =new threadB( );
C threadC =new threadC( );
ThreadA.start( );
ThreadB.start( );
ThreadC.start( );
}
}
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