Thread vs Process, Multithreading, Concurrency, Thread Monitors

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Concurrency/Threads

2. Mother’s morning
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Concurrency & Threads
1:
2:
3:
4:
6:
single threading
multi threading
1

3. Threading
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Single Threading Multithreading
Thread
Thread
Thread
Thread
Thread
OS
OS

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OS
CPU Cycle
Thread
Thread
Thread
Thread
Thread
Thread
Thread

5. HelloNoThread
1. public class HelloNoThread {
2. String name = null;
3. public HelloNoThread(String n) {
4. name = n;
5. }
6. public void run() {
7. for (int i = 0; i < 50; i++) {
8. System.out.println(i + name);
9. }
10. }
11.}
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6. TestHelloNoThread
1. public class TestHelloNoThread {
2. public static void main(String[] args) {
3. HelloNoThread t1 = new HelloNoThread("Ram");
4. HelloNoThread t2 = new HelloNoThread("Shyam");
5. t1.run();
6. t2.run();
7. }
8. }
 0 Ram
 1 Ram
 2 Ram
 …..
 48 Ram
 49 Ram
 0 Shyam
 1 Shyam
 2 Shyam
 ……..
 48 Shyam
 49 Shyam
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t1.run() t2.run()

7. Create Thread
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Thread
+run()
+start()
+sleep()
MyThread HelloThread Other
+run() +run() +run()
extends

8. HelloThread
1. public class HelloThread extends Thread{
2. String name = null;
3. public HelloThread(String n) {
4. name = n;
5. }
6. public void run() {
7. for (int i = 0; i < 50; i++) {
8. System.out.println(i + name);
9. }
10. }
11.}
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9. TestHelloThread
1. public class TestHelloThread{
2. public static void main(String[] args) {
3. HelloThread t1 = new HelloThread("Ram");
4. HelloThread t2 = new HelloThread("Shyam");
5. t1.start();
6. t2.start();
7. for (int i = 0; i < 50; i++) {
8. System.out.println(“main");
9. }
10. }
11.}
 0 Ram
 1 Ram
 2 Ram
 0 Shyam
 1 Shyam
 1 main
 2 main
 3 main
 3 Ram
 4 Ram
 2 Shyam
 3 Shyam
 4 main
 ……..
 48 Ram
 49 Ram
 48 Shyam
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10. CPU Scheduling
CPU
Main
Ram
Shyam
T1 T2 T3

11. Pani puri –Thread scheduling
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OS
CPU Cycle
Ready Thread Queue
Thread
Thread Thread Thread
Thread

12. Thread Priority
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13. Priority range from 1-10
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CPU Cycle
Priority

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Set and get priority
 Thread t = new Thread(“Ram”);
o t.setPriority(9);
o int i = t.getPriority();
 Default priority is 5
 Priority constants are defined in Thread class:
o Thread.MIN_PRIORITY=1
o Thread.MAX_PRIORITY=10
o Thread.NORM_PRIORITY=5

15. Runnable interface
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Runnable
+run()
Implements
ParentClass
extends
ChildClass
+run()

16. Runnable interface
1. public class HelloRunnable implements Runnable {
2. String name = null;
3. public HelloRunnable(String n) {
4. name = n;
5. }
6. public void run() {
7. for (int i = 0; i < 50; i++) {
8. System.out.println(i + name);
9. }
10. }
11.}
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17. Test Runnable
 public class TestHelloRunnable {
 public static void main(String[] args) {
 Thread t1 = new Thread(new HelloRunnable("Ram"));
 Thread t2 = new Thread(new HelloRunnable("Shyam"));
 t1.start();
 t2.start();
 for (int i = 0; i < 500; i++) {
 System.out.println("Main");
 }
 }
 } www.SunilOS.com 17

18. www.SunilOS.com 18
Threads and Processes

19. Thread creation
A java program executed as thread inside JVM
A thread can create another thread.
Threads work concurrently.
If one thread dies, other will remain active in
memory
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20. www.SunilOS.com 20
Thread Lifecycle - States
2: start()
finish run()
1: new
Born Blocked
Runnable
Dead
1. I/O available
2. notify()
3. wake up()
1. block on I/O
2. wait()
3. sleep()

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Thread Methods
 start()
o Starts the thread and put into runnable queue
 run()
o Performs operation
 Thread.sleep(int m)/sleep(int m,int n)
o Sleeps the thread for given time in milliseconds and nanoseconds
 Thread.yield()
o Pauses currently executing thread and put at last in the runnable queue

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Daemon Threads
 Daemon threads are supporting and background threads
 It provides services to normal threads
 Garbage Collector is a Daemon thread.
 t.setDaemon(true)
Daemon
Thread
Normal
Thread

23. Usage | multi user system
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Web Application
Database

24. Race condition
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Thread 1
Thread 2
Object

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Race Condition
 Threads of a process can share common memory area
 When two threads simultaneously try to access and modify an
object, it is called race-condition
 The result of a program is affected by the order in which threads
are allocated CPU time and executed

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Deposit in the Account
Account
Balance = 1000
Balance = 2000
getBalance():1000
setBalance(2000)
setBalance(2000)
getBalance():1000
getBalance()
CPU
getBalance()
setBalance()
setBalance()
User1 User 2
Deposit
Deposit

27. www.SunilOS.com 27
Synchronized access
Account
Balance = 1000
Balance = 3000
getBalance():1000
setBalance(2000)
setBalance(3000)
getBalance():2000
User1 User 2
Deposit Deposit
getBalance()
CPU
locked
setBalance()
setBalance()
getBalance()

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Account
1. public class Account {
2. private int balance = 0;
3. public int getBalance() {
4. try {
5. Thread.sleep(200);
6. } catch (InterruptedException e) {}
7. return balance;
8. }
9. public void setBalance(int balance) {
10. try {
11. Thread.sleep(200);
12. } catch (InterruptedException e) {}
13. this.balance = balance;
14. }
15.

29. www.SunilOS.com 29
Account
1. public synchronized void deposit(String msg, int amt) {
2. int bal = getBalance();
3. bal = bal + amt;
4. setBalance(bal);
5. System.out.println(msg + " new balance " + bal);
6. }
7. }

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RacingCondThread
public class RacingCondThread extends Thread {
public static Account data = new Account();
String name = null;
public RacingCondThread(String name) {
this.name = name;
}
public void run() {
for(int i=0;i<5;i++){
data.deposit(name, 1000);
}
}

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TestRacingCondThread
public static void main(String[] args) {
RacingCondThread t1 = new RacingCondThread(“Ram");
RacingCondThread t2 = new RacingCondThread(“Shyam");
t1.start();
t2.start();
}

32. Output
 void deposit(..)
o Ram balance 1000
o Shyam balance 1000
o Ram balance 2000
o Shyam balance 2000
o Shyam balance 3000
o Ram balance 3000
o Shyam balance 4000
o Ram balance 4000
o Shyam balance 5000
o Ram balance 5000
 synchronized void deposit(..)
o Ram balance 1000
o Ram balance 2000
o Ram balance 3000
o Ram balance 4000
o Ram balance 5000
o Shyam balance 6000
o Shyam balance 7000
o Shyam balance 8000
o Shyam balance 9000
o Shyam balance 10000
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33. www.SunilOS.com 33
Synchronization types
Method
public synchronized void deposit(String message, int amount)
{….}
Block
public void deposit(String message, int amount) {
synchronized (this){
int bal = getBalance() + amount;
setBalance(bal);
}
System.out.println(message + " Now Bal is " + bal);
}

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Monitors / Locks
 It is a lock or token of an object
 The thread has lock, can access object
 Other threads will be in waiting queue
 One object has two monitors or tokens
o One for instance methods, called Object Monitor
o Second for static methods, called Class Monitor

35. www.SunilOS.com 35
Monitor Keys
Static Sync Methods
x
z
y
Instance Sync Methods
a
c
b
Thread1
Thread2
x() Class
Monitor
y()
a() Object
Monitor
b()
CLASS

36. Disclaimer
This is an educational presentation to enhance the skill
of computer science students.
This presentation is available for free to computer
science students.
Some internet images from different URLs are used in
this presentation to simplify technical examples and
correlate examples with the real world.
We are grateful to owners of these URLs and pictures.
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37. Thank You!
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