This document discusses synchronization in multi-threaded programs. It covers monitors, which are used as mutually exclusive locks to synchronize access to shared resources. The synchronized keyword in Java can be used in two ways - by prefixing it to a method header, or by synchronizing an object within a synchronized statement. Examples are provided to demonstrate synchronization issues without locking, and how to resolve them by using the synchronized keyword in methods or on objects.

1. Concept of Synchronization
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2. Objectives
On completion of this period, you would be
able to learn
• Synchronization
• Monitors
• Unsynchronized Example
• Locking Object
• Synchronized Examples
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3. Recap
In this previous class, you have learnt
• Concept of thread priority
• Relevant methods and constants
• Relevant programs
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4. Synchronization
• We learnt in operating systems the concept of
critical section
• Two or more processes shares a resource like
variables, files etc
• They need to be synchronized, so that, the state
of the shared resource is consistent
• The same concept can be applied to the treads as
well
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5. Synchronization
Contd . . .
• What is synchronization?
• Concurrently running threads may require outside
resources (shared resources) or methods
• Need to communicate with other concurrently running
threads to know their status and activities
• Example: Producer-Consumer problem
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6. Monitor
• An object which is used as a mutually
exclusive lock
• Only one thread can acquire a monitor at a
time
• All other threads attempting to enter the
monitor will be suspended
• Every java object has its own implicit monitor
• Such monitors can be used for
synchronization
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7. Unsynchronized Example
class TwoStrings {
static void print(String str1, String
str2) {
System.out.print(str1);
try {
Thread.sleep(500);
} catch (InterruptedException ie) {
}
System.out.println(str2);
}
}
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8. Unsynchronized Example Contd . . .
class PrintStringsThread implements Runnable {
Thread thread;
A Thread implementation
String str1, str2; class
PrintStringsThread(String str1, String str2) {
this.str1 = str1;
this.str2 = str2;
thread = new Thread(this);
thread.start();
}
public void run() {
TwoStrings.print(str1, str2); Shared Resource
}
}
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9. Unsynchronized Example
Contd . . .
class TestThread {
public static void main(String args[]) {
new PrintStringsThread("Hello ", "there.");
new PrintStringsThread("How are ", "you?");
new PrintStringsThread("Thank you ", "very much!");
}
Output
}
Expected output is
Hello there
How are you?
Thank you very much!
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10. Locking of an object
• Assures that only one thread gets to access a
particular method
• Java allows you to lock objects with the use of
monitors
• All Java objects have their own implicit monitor
• Object enters the implicit monitor when the object's
‘synchronized’ method is invoked
• Once an object is in the monitor, the monitor makes
sure that no other thread accesses the same object
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11. Locking of an object
Contd . . .
• Monitor is implemented in Java through
‘synchronized’ keyword
• The ‘synchronized’ keyword can be used in two
ways:
1. Prefixed to the header of the method definition
2. Can synchronize the object of which the method is a
member of
• synchronized (<object>) {
//statements to be synchronized
}
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12. First Approach : synchronized
Use of synchronized keyword
class TwoStrings {
synchronized static void
print(String str1, String str2) {
System.out.print(str1);
try {
Thread.sleep(500);
}catch(InterruptedException ie) {
}
System.out.println(str2);
}
}
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13. Contd . . .
First Approach : synchronized
class PrintStringsThread
implements Runnable {
Thread thread;
String str1, str2;
PrintStringsThread(String str1,String str2) {
this.str1 = str1;
this.str2 = str2;
thread = new Thread(this);
thread.start();
}
public void run() {
TwoStrings.print(str1, str2);
}
} Shared Resource
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14. Contd . . .
First Approach : synchronized
class TestThread {
public static void main(String args[]) {
new PrintStringsThread("Hello ", "there.");
new PrintStringsThread("How are ", "you?");
new PrintStringsThread("Thank you ", "very much!");
}
Output
}
Got the output as
expected
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15. Second Approach : synchronized
class TwoStrings {
static void print(String str1, String str2) {
System.out.print(str1);
try {
Thread.sleep(500);
}catch(InterruptedException ie){ }
System.out.println(str2);
}
}
class PrintStringsThread implements Runnable {
Thread thread;
String str1, str2;
TwoStrings ts;
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16. Second Approach : synchronized Contd . . .
PrintStringsThread(String str1, String str2, TwoStrings ts)
{
this.str1 = str1;
this.str2 = str2;
this.ts = ts;
thread = new Thread(this);
thread.start();
}
Use of synchronized statement
public void run() {
synchronized (ts) {
ts.print(str1, str2);
}
}
}
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17. Contd . . .
Second Approach : synchronized
class TestThread {
public static void main(String args[]) {
new PrintStringsThread("Hello ", "there.");
new PrintStringsThread("How are ", "you?");
new PrintStringsThread("Thank you ", "very much!");
}
}
Output
Got the output as
expected
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18. Summary
• In this class we have discussed
• Problem of synchronization
• Monitors and locking objects
• Two approaches of using synchronized keyword
• Relevant example programs
• In the next lesson we look at Inter-Thread
Communication
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19. Quiz
1. Which concept is used by Java in inter-thread-
synchronization
A. Process
B. Monitor
C. Multitasking
D. None
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20. Quiz Contd..
2. Which keyword is used for synchronization in
Java ?
A. synchronize
B. synchronised
C. synchronized
D. synchronizing
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21. Frequently Asked Questions
1. Explain the concept of synchronization
2. What is monitor?
3. How monitor is implemented in Java?
4. What are the different ways of using
‘synchronized’ keyword ?
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