1. Unit 151 Introduction to Threads and Concurrency Introduction to Threads Multithreading Examples Life-cycle of a Thread Thread Priorities More Examples

2. Unit 152 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

3. Unit 153 Single and Multithreaded Programs

4. Unit 154 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)

5. Unit 155 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

6. Unit 156 Example 1: Extending java.lang.Thread 1 class OurClass1{ 2 public void run(){ 3 for(int i=0; i<100; 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 i=0; i<100; i++) 15 System.out.print("Shabab."); 16 } 17 } 1 class OurClass2 extends Thread{ 2 public void run(){ 3 for(int i=0; i<100; 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 i=0; i<100; i++) 15 System.out.print("Shabab."); 16 } 17 }

7. Unit 157 Example 2: Implementing java.lang.Runnable 1 class OurClass3 implements Runnable{ 2 public void run(){ 3 for(int i=0; i<100; 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 i=0; i<100; i++) 16 System.out.print("Shabab."); 17 } 18 }

8. Unit 158 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("Making " + getName()); 7 } 8 public void run() { 9 while(true) { 10 try { // don’t 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 < 5; i++) 20 new SleepingThread().start(); 21 System.out.println("All Threads Started"); 22 }}

9. Unit 159 Threads: Pictorial View of a Lifetime. Thread created new ready running finished blocked start() run() yield(), or time expired stop(), or complete stop() suspend(), sleep(), or wait() stop() resume(), notify(), or notifyAll()

10. Unit 1510 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(){ 18 public void actionPerformed(ActionEvent ae){ 19 startStop = !startStop; 20 } });}

11. Unit 1511 Example 4: A Timer Thread (cont’d) 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 }

12. Unit 1512 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 < array.length; i++){ 11 temp = (Integer)array[i]; 12 array[i] = new Integer (temp.intValue() + i); 13 if(i%3 == 0) Thread.yield(); 14 } 15 System.out.println(getName() + " done!"); 16 } 17 }

13. Unit 1513 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 < array.length - 1; i++){ 10 minPos = i; 11 if(i%3 == 0) Thread.yield(); 12 for(int k = i + 1; k < array.length; k++){ 13 if(((Integer)array[k]).compareTo(array[minPos]) < 0) 14 minPos = k; 15 } 16 temp = array[minPos]; 17 array[minPos] = array[i]; 18 array[i] = temp; 19 } 20 System.out.println(getName() + " done!"); 21 } 22 }

14. Unit 1514 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 Object[size]; 6 } 7 public static void main(String args[]) { 8 Random r = new Random(); 9 new TesterThread(100); 10 for(int i = 0; i < array.length; i++){ 11 array[i] = 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 < array.length; i++){ 18 System.out.println(array[i]); 19 } 20 System.out.println(Thread.currentThread().getName() + " done!"); 21 } 22 }

15. Unit 1515 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

16. Unit 1516 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 Object[size]; 6 } 7 public static void main(String args[]) { 8 Random r = new Random(); new TesterThread(100); 9 for(int i = 0; i < array.length; i++){ 10 array[i] = 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(Thread.MAX_PRIORITY); 16 Thread.currentThread().setPriority(Thread.MIN_PRIORITY); 17 for(int i = 0; i < array.length; i++){ 18 System.out.println(array[i]); 19 } 20 System.out.println(Thread.currentThread().getName() + " done!"); 21 } 22 }