1. G. Pullaiah College of Engineering and Technology
Object Oriented Programming through Java
Department of Computer Science & Engineering

2. UNIT IV
Multithreaded Programming
I/O basics
Applets
Generics

3. Multitasking and Multithreading
Multitasking refers to a computer's ability to perform multiple jobs concurrently
more than one program are running concurrently, e.g., UNIX
A thread is a single sequence of execution within a program
Multithreading refers to multiple threads of control within a single program
each program can run multiple threads of control within it, e.g., Web Browser

4. Concurrency vs. Parallelism
CPU
CPU1
CPU2

5. Threads and Processes CPU Process 1 Process 2 Process 3 Process 4 main
run GC

6. What are Threads Good For?
To maintain responsiveness of an application during a long running task.
To enable cancellation of separable tasks.
Some problems are intrinsically parallel.
To monitor status of some resource (DB).
Some APIs and systems demand it: Swing.

7. Application Thread When we execute an application:
The JVM creates a Thread object whose task is defined by the main() method
It starts the thread
The thread executes the statements of the program one by one until the method returns and the thread dies

8. Multiple Threads in an Application
Each thread has its private run-time stack
If two threads execute the same method, each will have its own copy of the local variables the methods uses
However, all threads see the same dynamic memory (heap)
Two different threads can act on the same object and same static fields concurrently

9. Creating Threads There are two ways to create our own Thread object
Subclassing the Thread class and instantiating a new object of that class
Implementing the Runnable interface
In both cases the run() method should be implemented

10. Extending Thread public class ThreadExample extends Thread {
public void run () {
for (int i = 1; i <= 100; i++) {
System.out.println(“Thread: ” + i); }

11. Thread Methods void start() void run() void stop() (deprecated)
Creates a new thread and makes it runnable
This method can be called only once
void run()
The new thread begins its life inside this method
void stop() (deprecated)
The thread is being terminated

12. Thread Methods yield() sleep(int m)/sleep(int m,int n)
Causes the currently executing thread object to temporarily pause and allow other threads to execute
Allow only threads of the same priority to run
sleep(int m)/sleep(int m,int n)  
The thread sleeps for m milliseconds, plus n nanoseconds

13. Implementing Runnable
public class RunnableExample implements Runnable {
public void run () {
for (int i = 1; i <= 100; i++) {
System.out.println (“Runnable: ” + i); }

14. A Runnable Object
The Thread object’s run() method calls the Runnable object’s run() method
Allows threads to run inside any object, regardless of inheritance
Example – an applet that is
also a thread

15. Starting the Threads public class ThreadsStartExample {
public static void main (String argv[]) {
new ThreadExample ().start ();
new Thread(new RunnableExample ()).start (); }
RESULT

16. Scheduling Threads start() Ready queue Newly created threads
Currently executed
thread
Waiting for I/O operation to be completed
Waiting to be notified
Sleeping
Waiting to enter a synchronized section
I/O operation completes
What happens when
a program with a
ServerSocket calls
accept()?

17. Thread State Diagram Alive Running new ThreadExample();
while (…) { … }
New Thread
Runnable
Dead Thread
thread.start();
run() method returns
Blocked
Object.wait()
Thread.sleep()
blocking IO call
waiting on a monitor

18. Example public class PrintThread1 extends Thread { String name;
public PrintThread1(String name) {
this.name = name; }
public void run() {
for (int i=1; i<500 ; i++) {
try {
sleep((long)(Math.random() * 100));
} catch (InterruptedException ie) { }
System.out.print(name);
} }

19. Example (cont) RESULT public static void main(String args[]) {
PrintThread1 a = new PrintThread1("*");
PrintThread1 b = new PrintThread1("-");
PrintThread1 c = new PrintThread1("=");
a.start();
b.start();
c.start(); }
RESULT

20. Scheduling
Thread scheduling is the mechanism used to determine how runnable threads are allocated CPU time
A thread-scheduling mechanism is either preemptive or nonpreemptive

21. Preemptive Scheduling
Preemptive scheduling – the thread scheduler preempts (pauses) a running thread to allow different threads to execute
Nonpreemptive scheduling – the scheduler never interrupts a running thread
The nonpreemptive scheduler relies on the running thread to yield control of the CPU so that other threads may execute

22. Starvation
A nonpreemptive scheduler may cause starvation (runnable threads, ready to be executed, wait to be executed in the CPU a lot of time, maybe even forever)
Sometimes, starvation is also called a livelock

23. Time-Sliced Scheduling
Time-sliced scheduling – the scheduler allocates a period of time that each thread can use the CPU
when that amount of time has elapsed, the scheduler preempts the thread and switches to a different thread
Nontime-sliced scheduler – the scheduler does not use elapsed time to determine when to preempt a thread
it uses other criteria such as priority or I/O status

24. Java Scheduling
Scheduler is preemptive and based on priority of threads
Uses fixed-priority scheduling:
Threads are scheduled according to their priority w.r.t. other threads in the ready queue

25. Java Scheduling What is the danger of such scheduler?
The highest priority runnable thread is always selected for execution above lower priority threads
When multiple threads have equally high priorities, only one of those threads is guaranteed to be executing
Java threads are guaranteed to be preemptive-but not time sliced
Q: Why can’t we guarantee time-sliced scheduling?
What is the danger of such scheduler?

26. Thread Priority Every thread has a priority
When a thread is created, it inherits the priority of the thread that created it
The priority values range from 1 to 10, in increasing priority

27. Thread Priority (cont.)
The priority can be adjusted subsequently using the setPriority() method
The priority of a thread may be obtained using getPriority()
Priority constants are defined:
MIN_PRIORITY=1
MAX_PRIORITY=10
NORM_PRIORITY=5

28. Some Notes
Thread implementation in Java is actually based on operating system support
Some Windows operating systems support only 7 priority levels, so different levels in Java may actually be mapped to the same operating system level
What should we do about this?

29. Daemon Threads
Daemon threads are “background” threads, that provide services to other threads, e.g., the garbage collection thread
The Java VM will not exit if non-Daemon threads are executing
The Java VM will exit if only Daemon threads are executing
Daemon threads die when the Java VM exits

30. ThreadGroup
The ThreadGroup class is used to create groups of similar threads. Why is this needed?
“Thread groups are best viewed as an unsuccessful experiment, and you may simply ignore their existence.”
Joshua Bloch, software architect at Sun

31. Server import java.net.*;import java.io.*; class HelloServer {
public static void main(String[] args) {
int port = Integer.parseInt(args[0]);
try {
ServerSocket server =
new ServerSocket(port);
} catch (IOException ioe) {
System.err.println(“Couldn't run “ +
“server on port “ + port);
return; }

32. while(true) {
try {
Socket connection = server.accept();
ConnectionHandler handler =
new ConnectionHandler(connection);
new Thread(handler).start();
} catch (IOException ioe1) { }

33. Connection Handler
// Handles a connection of a client to an HelloServer.
// Talks with the client in the 'hello' protocol
class ConnectionHandler implements Runnable {
// The connection with the client
private Socket connection;
public ConnectionHandler(Socket connection) {
this.connection = connection; }

34. public void run() {
try {
BufferedReader reader =
new BufferedReader(
new InputStreamReader(
connection.getInputStream()));
PrintWriter writer =
new PrintWriter(
new OutputStreamWriter(
connection.getOutputStream()));
String clientName = reader.readLine();
writer.println(“Hello “ + clientName);
writer.flush();
} catch (IOException ioe) {} }

35. Client side import java.net.*; import java.io.*;
// A client of an HelloServer
class HelloClient {
public static void main(String[] args) {
String hostname = args[0];
int port = Integer.parseInt(args[1]);
Socket connection = null;
try {
connection = new Socket(hostname, port);
} catch (IOException ioe) {
System.err.println("Connection failed");
return; }

36. Note that the Client has not changed from last week
try {
BufferedReader reader =
new BufferedReader(
new InputStreamReader(
connection.getInputStream()));
PrintWriter writer =
new PrintWriter(
new OutputStreamWriter(
connection.getOutputStream()));
writer.println(args[2]); // client name
String reply = reader.readLine();
System.out.println("Server reply: "+reply);
writer.flush();
} catch (IOException ioe1) { }
Note that the Client has not changed from last week

37. Concurrency
An object in a program can be changed by more than one thread
Q: Is the order of changes that were preformed on the object important?

38. Race Condition
A race condition – the outcome of a program is affected by the order in which the program's threads are allocated CPU time
Two threads are simultaneously modifying a single object
Both threads “race” to store their value

39. Race Condition Example
How can we have
alternating colors?
Put red pieces
Put green pieces

40. Monitors
Each object has a “monitor” that is a token used to determine which application thread has control of a particular object instance
In execution of a synchronized method (or block), access to the object monitor must be gained before the execution
Access to the object monitor is queued

41. Monitor (cont.)
Entering a monitor is also referred to as locking the monitor, or acquiring ownership of the monitor
If a thread A tries to acquire ownership of a monitor and a different thread has already entered the monitor, the current thread (A) must wait until the other thread leaves the monitor

42. Critical Section The synchronized methods define critical sections
Execution of critical sections is mutually exclusive. Why?

43. Example public class BankAccount { private float balance;
public synchronized void deposit(float amount) {
balance += amount; }
public synchronized void withdraw(float amount) {
balance -= amount;

44. Critical Sections t3 t2 t1
deposit()
Bank Account

45. Static Synchronized Methods
Marking a static method as synchronized, associates a monitor with the class itself
The execution of synchronized static methods of the same class is mutually exclusive. Why?

46. Example public class PrintThread2 extends Thread { String name;
public PrintThread2(String name) {
this.name = name; }
public static synchronized void print(String name) {
for (int i=1; i<500 ; i++) {
try {
Thread.sleep((long)(Math.random() * 100));
} catch (InterruptedException ie) { }
System.out.print(str);

47. Example (cont) RESULT public void run() { print(name); }
public static void main(String args[]) {
PrintThread2 a = new PrintThread2("*“);
PrintThread2 b = new PrintThread2("-“);
PrintThread2 c = new PrintThread2("=“);
a.start();
b.start();
c.start();
RESULT

48. Deadlock Example public class BankAccount { private float balance;
public synchronized void deposit(float amount) {
balance += amount; }
public synchronized void withdraw(float amount) {
balance -= amount;
public synchronized void transfer(float amount,
BankAccount target) {
withdraw(amount);
target.deposit(amount);

49. public class MoneyTransfer implements Runnable {
private BankAccount from, to;
private float amount;
public MoneyTransfer(
BankAccount from, BankAccount to, float amount) {
this.from = from;
this.to = to;
this.amount = amount; }
public void run() {
source.transfer(amount, target);

50. BankAccount aliceAccount = new BankAccount();
BankAccount bobAccount = new BankAccount();
...
// At one place
Runnable transaction1 =
new MoneyTransfer(aliceAccount, bobAccount, 1200);
Thread t1 = new Thread(transaction1);
t1.start();
// At another place
Runnable transaction2 =
new MoneyTransfer(bobAccount, aliceAccount, 700);
Thread t2 = new Thread(transaction2);
t2.start();

51. ? Deadlocks aliceAccount bobAccount t1 t2 transfer() transfer()
withdraw()
withdraw()
deposit()
deposit()

52. Java Locks are Reentrant
Is there a problem with the following code?
public class Test {
public synchronized void a() {
b();
System.out.println(“I am at a”); }
public synchronized void b() {
System.out.println(“I am at b”);

53. Synchronized Statements
A monitor can be assigned to a block
It can be used to monitor access to a data element that is not an object, e.g., array
Example:
void arrayShift(byte[] array, int count) {
synchronized(array) {
System.arraycopy (array, count,array, , array.size - count); }

54. Thread Synchronization
We need to synchronized between transactions, for example, the consumer-producer scenario

55. Wait and Notify Allows two threads to cooperate
Based on a single shared lock object
Marge put a cookie wait and notify Homer
Homer eat a cookie wait and notify Marge

56. The wait() Method
The wait() method is part of the java.lang.Object interface
It requires a lock on the object’s monitor to execute
It must be called from a synchronized method, or from a synchronized segment of code. Why?

57. The wait() Method
wait() causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object
Upon call for wait(), the thread releases ownership of this monitor and waits until another thread notifies the waiting threads of the object

58. The wait() Method
wait() is also similar to yield()
Both take the current thread off the execution stack and force it to be rescheduled
However, wait() is not automatically put back into the scheduler queue
notify() must be called in order to get a thread back into the scheduler’s queue

59. Consumer synchronized (lock) { while (!resourceAvailable()) {
lock.wait(); }
consumeResource();

60. Producer
produceResource();
synchronized (lock) {
lock.notifyAll(); }

61. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

62. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

63. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

64. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

65. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

66. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

67. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

68. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

69. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

70. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

71. Wait/Notify Sequence Lock Object 7. Reacquire lock
1. synchronized(lock){
3. produceResource()
4. synchronized(lock) {
2. lock.wait();
5. lock.notify();
9. consumeResource();
6.}
10. }
7. Reacquire lock
8. Return from wait()
Consumer
Thread
Producer
Thread

72. The Simpsons Scenario: SimpsonsTest
public class SimpsonsTest {
public static void main(String[] args) {
CookyJar jar = new CookyJar();
Homer homer = new Homer(jar);
Marge marge = new Marge(jar);
new Thread(homer).start();
new Thread(marge).start(); }
Change

73. The Simpsons Scenario: Homer
public class Homer implements Runnable {
CookyJar jar;
public Homer(CookyJar jar) {
this.jar = jar; }
public void eat() {
jar.getCooky("Homer");
try {
Thread.sleep((int)Math.random() * 1000);
} catch (InterruptedException ie) {}
public void run() {
for (int i = 1 ; i <= 10 ; i++) eat();
Change

74. The Simpsons Scenario: Marge
public class Marge implements Runnable {
CookyJar jar;
public Marge(CookyJar jar) {
this.jar = jar; }
public void bake(int cookyNumber) {
jar.putCooky("Marge", cookyNumber);
try {
Thread.sleep((int)Math.random() * 500);
} catch (InterruptedException ie) {}
public void run() {
for (int i = 0 ; i < 10 ; i++) bake(i);
Change

75. The Simpsons Scenario: CookieJar
public class CookyJar {
private int contents;
private boolean available = false;
public synchronized void getCooky(String who) {
while (!available) {
try {
wait();
} catch (InterruptedException e) { } }
available = false;
notifyAll();
System.out.println( who + " ate cooky " + contents);
Change

76. The Simpsons Scenario: CookieJar
public synchronized void putCooky(String who, int value) {
while (available) {
try {
wait();
} catch (InterruptedException e) { } }
contents = value;
available = true;
System.out.println(who + " put cooky " + contents +
" in the jar");
notifyAll();
} }
Change

77. Timers and TimerTask
The classes Timer and TimerTask are part of the java.util package
Useful for
performing a task after a specified delay
performing a sequence of tasks at constant time intervals

78. Scheduling Timers
The schedule method of a timer can get as parameters:
Task, time
Task, time, period
Task, delay
Task, delay, period
When to start
What to do
At which rate

79. Timer Example import java.util.*;
public class MinchaTask extends TimerTask {
public void run() {
System.out.println(“Time for Mincha!!!!”); }
public static void main(String args[]) {
Timer timer = new Timer();
long day = 1000 * 60 * 60 * 24;
timer.scheduleAtFixedRate(new MinchaTask(),
new Date(), day);

80. Stopping Timers A Timer thread can be stopped in the following ways:
Apply cancel() on the timer
Make the thread a daemon
Remove all references to the timer after all the TimerTask tasks have finished
Call System.exit()

81. Applets
An applet is a Panel that allows interaction with a Java program.
A applet is typically embedded in a Web page and can be run from a browser.
You need special HTML in the Web page to tell the browser about the applet.
Applets run in a sandbox: they have no access to the client’s file system.

82. Applet Support
Netscape claims to support Java 1.1, but has serious omissions.
MS Internet Explorer supports most of 1.1.
The best support isn't a browser, but the standalone program appletviewer.
In general you want to write applets that can be run with any browser

83. What an applet is You write an applet by extending the class Applet.
Applet is just a class like any other; you can even use it in applications if you want.
When you write an applet, you are only writing part of a program.
The browser supplies the main program.

84. The genealogy of Applet
java.lang.Object |
+----java.awt.Component
+----java.awt.Container
+----java.awt.Panel
+----java.applet.Applet

85. The simplest possible applet
TrivialApplet.java
import java.applet.Applet;
public class TrivialApplet extends Applet { }
TrivialApplet.html
<applet code="TrivialApplet.class” width=150 height=100>
</applet>

86. The simplest reasonable applet
import java.awt.*;
import java.applet.Applet;
public class HelloWorld extends Applet { public void paint( Graphics g ) {
g.drawString( "Hello World!", 30, 30 ); }

87. Applet methods public void init () public void start ()
public void stop ()
public void destroy ()
public void paint (Graphics g)

88. Why an applet works You write an applet by extending the class Applet.
Applet defines methods init( ), start( ), stop( ), paint(Graphics), destroy( )
These methods do nothing--they are stubs.
You make the applet do something by overriding these methods.

89. public void init ( ) This is the first method to execute
It is an ideal place to initialize variables
It is the best place to define and use buttons, text fields, sliders, layouts, etc.
Almost every applet you ever write will have an init( ) method

90. public void start ( ) Not always needed Called after init( )
Called each time the page is loaded and restarted
Used mostly in conjunction with stop( )

91. public void stop( ) Not always needed
Called when the browser leaves the page
Called just before destroy( )
Use stop( ) if the applet is doing heavy computation that you don’t want to continue when the browser is on some other page
Used mostly in conjunction with start()

92. public void destroy( ) Seldom needed Called after stop( )
Use to explicitly release system resources (like threads)
System resources are usually released automatically

93. Applet flow of control

94. public void paint(Graphics g)
Almost always needed
Any painting you want to do should be done here, or in a method you call from here
Painting that you do in other methods may or may not happen
Don’t call this method. It’s called automatically.
Call repaint( ) instead.

95. Sample Graphics methods
A Graphics is something you can paint on.
g.drawString(“Hello, World”, 20, 20);
g.drawRect(x, y, width, height);
g.fillRect(x, y, width, height);
g.drawOval(x, y, width, height);
g.fillOval(x, y, width, height); g.setColor(Color.red);

96. repaint( )
Call repaint( ) when you have changed something and want your changes to show up on the screen
repaint( ) is a request--it might not happen.
When you call repaint( ), Java schedules a call to update(Graphics g).

97. update( )
When you call repaint( ), Java schedules a call to update(Graphics g)
Here's what update does:
public void update(Graphics g) { // Fill applet with background color paint(g); }

98. Other useful Applet methods
System.out.println(String s) still works.
Automatically opens an output window.
showStatus(String) displays the String in the applet’s status line.
Each call overwrites the previous call.
You have to allow time to read the line!

99. Applets are not magic!
Anything you can do in an applet, you can do in an application.
You can do some things in an application that you can’t do in an applet.
If you want to access files from an applet, it must be a “trusted” applet.
Trusted applets are beyond the scope of this course.

100. Structure of an HTML page
Most HTML tags are containers.
A container is <tag> to </tag>
HTML
TITLE
BODY
HEAD
(content)

101. HTML <html> <head>
<title> Hi World Applet </title>
</head>
<body>
<applet code="HiWorld.class” width=300 height=200>
<param name=arraysize value=10> </applet>
</body>
</html>

102. <param name=arraysize value=10>
public String getParameter(String name)
String s = getParameter(“arraysize”);
try { size = Integer.parseInt (s) } catch (NumberFormatException) {…}