Introduction to Threads and Concurrency

1
Introduction to Threads and Concurrency

• Introduction to Threads
• Multithreading Examples
• Life-cycle of a Thread
• Thread Priorities
• More Examples

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Introduction What is a Thread?

• Just like human being can walk, talk, see, hear
  etc at the same time, computers can also download
  documents, print a file, receive e-mail
  concurrently
• Computers normally achieve concurrency using
  threads
• A thread or thread of control is a section of
  code executed independently of other threads of
  control within a single program
• Each thread has its own stack, priority, and
  virtual set of registers
• Every program has at least one thread
• Every Java applet or application is multithreaded

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Single and Multithreaded Programs
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Introduction Where are Threads Used?

• Threads are used by virtually every computer user
  in the following instances
• In Internet browsers
• In databases
• In operating systems (for controlling access to
  shared resources etc)
• Benefits of threads
• More productivity to the end user (such as
  responsive user interface)
• More efficient use of the computer (such as
  using the CPU while performing input-output)
• Sometimes advantageous to the programmer (such
  as simplifying program logic)

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Threads in Java

• Java is one of the few (the only?) languages that
  supports threads at the language level
• Writing multithreaded programs can be tricky
  imagine reading three books by reading few words
  from each book for a few seconds repeatedly in
  succession
• Although Java is portable, its multithreading is
  platform dependent a multithreaded program could
  behave differently on different platforms
• Under Solaris implementation, an executing thread
  can run to completion or can be preempted by a
  higher priority thread
• On windows (NT and 98) threads are timesliced and
  preemption occurs with higher as well as with
  equal priority threads
• To create a thread, you extend java.lang.Thread
  or implement java.lang.Runnable

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Example 1 Extending java.lang.Thread
1 class OurClass2 extends Thread 2 public
void run() 3 for(int i0 ilt100 i) 4
System.out.print("Salam ") 5 6
7 public class OurClass2Tester 8 public
static void main(String args) 9
OurClass2 oc new OurClass2() 10
oc.start() 11 run() 12 13 static
void run() 14 for(int i0 ilt100 i) 15
System.out.print("Shabab.") 16 17
1 class OurClass1 2 public void run() 3
for(int i0 ilt100 i) 4
System.out.print("Salam ") 5 6 7
public class OurClass1Tester 8 public static
void main(String args) 9 OurClass1 oc
new OurClass1() 10 oc.run() 11
run() 12 13 static void run() 14
for(int i0 ilt100 i) 15
System.out.print("Shabab.") 16 17
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Example 2 Implementing java.lang.Runnable
1 class OurClass3 implements Runnable 2
public void run() 3 for(int i0 ilt100
i) 4 System.out.print("Salam ") 5
6 7 public class OurClass3Tester 8
public static void main(String args) 9
OurClass3 oc new OurClass3() 10 Thread
th new Thread(oc) 11 th.start() 12
run() 13 14 static void run() 15
for(int i0 ilt100 i) 16
System.out.print("Shabab.") 17 18
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Example 3 Creating Multiple Threads
1 public class SleepingThread extends Thread
2 private int countDown 5 3 private
static int threadCount 0 4 private int
threadNumber threadCount 5 public
SleepingThread() 6 System.out.println("Maki
ng " getName()) 7 8 public void run()
9 while(true) 10 try // dont
wake a sleeping thread before its sleep time
expires! 11 System.out.println(getName()
" Executing.") 12
sleep((long)(Math.random()5000)) 13
catch(InterruptedException ie) 14
System.out.println(getName() "
Interrupted.") 15 16
if(--countDown 0) return 17 18
public static void main(String args) 19
for(int i 0 i lt 5 i) 20 new
SleepingThread().start() 21
System.out.println("All Threads Started") 22
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Threads Pictorial View of a Lifetime.
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Example 4 A Timer Thread
1 import java.awt. 2 import
java.awt.event. 3 import javax.swing. 4
class TimerThread extends JFrame implements
Runnable 5 private JTextField jtf new
JTextField(10) 6 private JButton jb new
JButton("Start/Stop") 7 private int counter
0 8 private boolean startStop true 9
TimerThread() 10 Container cp
getContentPane() 11 cp.setLayout(new
FlowLayout()) 12 cp.add(jtf) 13
cp.add(jb) 14 setSize(200,200) 15
setTitle("A Timer Thread") 16 show() 17
jb.addActionListener(new ActionListener() 1
8 public void actionPerformed(ActionE
vent ae) 19 startStop
!startStop 20 )
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Example 4 A Timer Thread (contd)
21 public void run() 22 while
(true) 23 try 24
Thread.sleep(1000) 25 catch(Exception
e) 26 if (startStop) 27
jtf.setText(Integer.toString(counter)) 28
29 30 public static void
main(String args) 31 new Thread(new
TimerThread()).start() 32 33
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Example 5 A Visitor Thread
1 import java.util. 2 public class
VisitorThread extends Thread 3 private
Object array 4 public VisitorThread(Object
array) 5 this.array array 6 7
public void run() 8 Integer temp 9
this.setName("The Visitor") 10 for(int i
0 i lt array.length i) 11 temp
(Integer)arrayi 12 arrayi new
Integer (temp.intValue() i) 13 if(i3
0) Thread.yield() 14 15
System.out.println(getName() " done!") 16
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Example 5 A Sorter Thread
1 import java.util. 2 public class
SorterThread extends Thread 3 private Object
array 4 public SorterThread(Object
array) 5 this.array array 6 7
public void run() 8 int minPos 0
Object temp 9 for(int i 0 i lt
array.length - 1 i) 10 minPos
i 11 if(i3 0) Thread.yield() 12
for(int k i 1 k lt array.length
k) 13 if(((Integer)arrayk).compar
eTo(arrayminPos) lt 0) 14
minPos k 15 16 temp
arrayminPos 17 arrayminPos
arrayi 18 arrayi temp 19
20 System.out.println(getName() "
done!") 21 22
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Example 5 A Tester Thread
1 import java.util. 2 public class
TesterThread extends Thread 3 private static
Object array 4 public TesterThread(int
size) 5 array new Objectsize 6
7 public static void main(String args) 8
Random r new Random() 9 new
TesterThread(100) 10 for(int i 0 i lt
array.length i) 11 arrayi new
Integer(r.nextInt(100)) 12 13
SorterThread sorter new SorterThread(array) 14
VisitorThread visitor new
VisitorThread(array) 15 sorter.start() 16
visitor.start() 17 for(int i 0 i lt
array.length i) 18
System.out.println(arrayi) 19 20
System.out.println(Thread.currentThread().getName(
) " done!") 21 22
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Threads Priorities

• As wehave seen, the run( ) methods of the threads
  in a program will be executed simultaneously
• The programmer can influence the order of
  threads executions using threads priorities
• Every Java thread has a priority in the range
  Thread.MIN_PRIORITY (a constant value, 1) and
  Thread.MAX_PRIORITY (a constant value, 10).
• The default priority is Thread.NORMAL_PRIORITY (a
  constant value, 5)
• Each new thread inherits the priority of the
  thread that creates it
• Every thread has a name (including anonymous
  threads) for identification purposes
• More than one thread may have the same name
• If a name is not specified when a thread is
  created, a new name is generated for it

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Example 6 Prioritizing the Visitor Thread
1 import java.util. 2 public class
TesterThread extends Thread 3 private static
Object array 4 public TesterThread(int
size) 5 array new Objectsize 6
7 public static void main(String args) 8
Random r new Random() new
TesterThread(100) 9 for(int i 0 i lt
array.length i) 10 arrayi new
Integer(r.nextInt(100)) 11 12
SorterThread sorter new SorterThread(array) 13
VisitorThread visitor new
VisitorThread(array) 14 sorter.start()
visitor.start() 15 visitor.setPriority(Threa
d.MAX_PRIORITY) 16 Thread.currentThread().se
tPriority(Thread.MIN_PRIORITY) 17 for(int i
0 i lt array.length i) 18
System.out.println(arrayi) 19 20
System.out.println(Thread.currentThread().getName(
) " done!") 21 22