Synchronization in java is the capability to control the access of multiple threads to any shared resource.
Java Synchronization is better option where we want to allow only one thread to access the shared resource.

Table of Contents

Why use Synchronization

The synchronization is mainly used to
1. To prevent thread interference.
2. To prevent consistency problem.

Types of Synchronization

There are two types of synchronization
1. Process Synchronization
2. Thread Synchronization
Here, we will discuss only thread synchronization.

Thread Synchronization

There are two types of thread synchronization mutual exclusive and inter-thread communication.
1. Mutual Exclusive
1. Synchronized method.
2. Synchronized block.
3. static synchronization.
2. Cooperation (Inter-thread communication in java)

Mutual Exclusive

Mutual Exclusive helps keep threads from interfering with one another while sharing data. This can be done by three ways in java:
1. by synchronized method
2. by synchronized block
3. by static synchronization

Concept of Lock in Java

Synchronization is built around an internal entity known as the lock or monitor. Every object has an lock associated with it. By convention, a thread that needs consistent access to an object’s fields has to acquire the object’s lock before accessing them, and then release the lock when it’s done with them.
From Java 5 the package java.util.concurrent.locks contains several lock implementations.

Understanding the problem without Synchronization

In this example, there is no synchronization, so output is inconsistent. Let’s see the example:

1. class Table{ 
2. void printTable(int n){//method not synchronized 
3. for(int i=1;i<=5;i++){ 
4. System.out.println(n*i); 
5. try{ 
6. Thread.sleep(400); 
7. }catch(Exception e){System.out.println(e);} 
8. } 
9. 
10. } 
11. } 
12. 
13. class MyThread1 extends Thread{ 
14. Table t; 
15. MyThread1(Table t){ 
16. this.t=t; 
17. } 
18. public void run(){ 
19. t.printTable(5); 
20. } 
21. 
22. } 
23. class MyThread2 extends Thread{ 
24. Table t; 
25. MyThread2(Table t){ 
26. this.t=t; 
27. } 
28. public void run(){ 
29. t.printTable(100); 
30. } 
31. } 
32. 
33. class TestSynchronization1{ 
34. public static void main(String args[]){ 
35. Table obj = new Table();//only one object 
36. MyThread1 t1=new MyThread1(obj); 
37. MyThread2 t2=new MyThread2(obj); 
38. t1.start(); 
39. t2.start(); 
40. } 
41. }

Output: 5
100
10
200
15
300
20
400
25
500

Java synchronized method

If you declare any method as synchronized, it is known as synchronized method.
Synchronized method is used to lock an object for any shared resource.
When a thread invokes a synchronized method, it automatically acquires the lock for that object and releases it when the thread completes its task.

1. //example of java synchronized method 
2. class Table{ 
3. synchronized void printTable(int n){//synchronized method 
4. for(int i=1;i<=5;i++){ 
5. System.out.println(n*i); 
6. try{ 
7. Thread.sleep(400); 
8. }catch(Exception e){System.out.println(e);} 
9. } 
10. 
11. } 
12. } 
13. 
14. class MyThread1 extends Thread{ 
15. Table t; 
16. MyThread1(Table t){ 
17. this.t=t; 
18. } 
19. public void run(){ 
20. t.printTable(5); 
21. } 
22. 
23. } 
24. class MyThread2 extends Thread{ 
25. Table t; 
26. MyThread2(Table t){ 
27. this.t=t; 
28. } 
29. public void run(){ 
30. t.printTable(100); 
31. } 
32. } 
33. 
34. public class TestSynchronization2{ 
35. public static void main(String args[]){ 
36. Table obj = new Table();//only one object 
37. MyThread1 t1=new MyThread1(obj); 
38. MyThread2 t2=new MyThread2(obj); 
39. t1.start(); 
40. t2.start(); 
41. } 
42. }

Output: 5
10
15
20
25
100
200
300
400
500

Example of synchronized method by using annonymous class

In this program, we have created the two threads by annonymous class, so less coding is required.

1. //Program of synchronized method by using annonymous class 
2. class Table{ 
3. synchronized void printTable(int n){//synchronized method 
4. for(int i=1;i<=5;i++){ 
5. System.out.println(n*i); 
6. try{ 
7. Thread.sleep(400); 
8. }catch(Exception e){System.out.println(e);} 
9. } 
10. 
11. } 
12. } 
13. 
14. public class TestSynchronization3{ 
15. public static void main(String args[]){ 
16. final Table obj = new Table();//only one object 
17. 
18. Thread t1=new Thread(){ 
19. public void run(){ 
20. obj.printTable(5); 
21. } 
22. }; 
23. Thread t2=new Thread(){ 
24. public void run(){ 
25. obj.printTable(100); 
26. } 
27. }; 
28. 
29. t1.start(); 
30. t2.start(); 
31. } 
32. }

Output: 5
10
15
20
25
100
200
300
400
500

Synchronized Block in Java

Synchronized block can be used to perform synchronization on any specific resource of the method.
Suppose you have 50 lines of code in your method, but you want to synchronize only 5 lines, you can use synchronized block.
If you put all the codes of the method in the synchronized block, it will work same as the synchronized method.

Points to remember for Synchronized block

• Synchronized block is used to lock an object for any shared resource.
• Scope of synchronized block is smaller than the method.

Syntax to use synchronized block

1. synchronized (object reference expression) { 
2. //code block 
3. }

Example of synchronized block

Let’s see the simple example of synchronized block.

Program of synchronized block

1. class Table{ 
2. 
3. void printTable(int n){ 
4. synchronized(this){//synchronized block 
5. for(int i=1;i<=5;i++){ 
6. System.out.println(n*i); 
7. try{ 
8. Thread.sleep(400); 
9. }catch(Exception e){System.out.println(e);} 
10. } 
11. } 
12. }//end of the method 
13. } 
14. 
15. class MyThread1 extends Thread{ 
16. Table t; 
17. MyThread1(Table t){ 
18. this.t=t; 
19. } 
20. public void run(){ 
21. t.printTable(5); 
22. } 
23. 
24. } 
25. class MyThread2 extends Thread{ 
26. Table t; 
27. MyThread2(Table t){ 
28. this.t=t; 
29. } 
30. public void run(){ 
31. t.printTable(100); 
32. } 
33. } 
34. 
35. public class TestSynchronizedBlock1{ 
36. public static void main(String args[]){ 
37. Table obj = new Table();//only one object 
38. MyThread1 t1=new MyThread1(obj); 
39. MyThread2 t2=new MyThread2(obj); 
40. t1.start(); 
41. t2.start(); 
42. } 
43. }

Output:5
10
15
20
25
100
200
300
400
500

Same Example of synchronized block by using annonymous class:

//Program of synchronized block by using annonymous class

1. class Table{ 
2. 
3. void printTable(int n){ 
4. synchronized(this){//synchronized block 
5. for(int i=1;i<=5;i++){ 
6. System.out.println(n*i); 
7. try{ 
8. Thread.sleep(400); 
9. }catch(Exception e){System.out.println(e);} 
10. } 
11. } 
12. }//end of the method 
13. } 
14. 
15. public class TestSynchronizedBlock2{ 
16. public static void main(String args[]){ 
17. final Table obj = new Table();//only one object 
18. 
19. Thread t1=new Thread(){ 
20. public void run(){ 
21. obj.printTable(5); 
22. } 
23. }; 
24. Thread t2=new Thread(){ 
25. public void run(){ 
26. obj.printTable(100); 
27. } 
28. }; 
29. 
30. t1.start(); 
31. t2.start(); 
32. } 
33. }

Output:5
10
15
20
25
100
200
300
400
500

Static Synchronization

If you make any static method as synchronized, the lock will be on the class not on object.

java 125

Problem without static synchronization

Suppose there are two objects of a shared class(e.g. Table) named object1 and object2.In case of synchronized method and synchronized block there cannot be interference between t1 and t2 or t3 and t4 because t1 and t2 both refers to a common object that have a single lock.But there can be interference between t1 and t3 or t2 and t4 because t1 acquires another lock and t3 acquires another lock.I want no interference between t1 and t3 or t2 and t4.Static synchronization solves this problem.

Example of static synchronization

In this example we are applying synchronized keyword on the static method to perform static synchronization.

1. class Table{ 
2. 
3. synchronized static void printTable(int n){ 
4. for(int i=1;i<=10;i++){ 
5. System.out.println(n*i); 
6. try{ 
7. Thread.sleep(400); 
8. }catch(Exception e){} 
9. } 
10. } 
11. } 
12. 
13. class MyThread1 extends Thread{ 
14. public void run(){ 
15. Table.printTable(1); 
16. } 
17. } 
18. 
19. class MyThread2 extends Thread{ 
20. public void run(){ 
21. Table.printTable(10); 
22. } 
23. } 
24. 
25. class MyThread3 extends Thread{ 
26. public void run(){ 
27. Table.printTable(100); 
28. } 
29. } 
30. 
31. 
32. 
33. 
34. class MyThread4 extends Thread{ 
35. public void run(){ 
36. Table.printTable(1000); 
37. } 
38. } 
39. 
40. public class TestSynchronization4{ 
41. public static void main(String t[]){ 
42. MyThread1 t1=new MyThread1(); 
43. MyThread2 t2=new MyThread2(); 
44. MyThread3 t3=new MyThread3(); 
45. MyThread4 t4=new MyThread4(); 
46. t1.start(); 
47. t2.start(); 
48. t3.start(); 
49. t4.start(); 
50. } 
51. }

Output: 1
2
3
4
5
6
7
8
9
10
10
20
30
40
50
60
70
80
90
100
100
200
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800
900
1000
1000
2000
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4000
5000
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7000
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9000
10000

Same example of static synchronization by annonymous class

In this example, we are using annonymous class to create the threads.

1. class Table{ 
2. 
3. synchronized static void printTable(int n){ 
4. for(int i=1;i<=10;i++){ 
5. System.out.println(n*i); 
6. try{ 
7. Thread.sleep(400); 
8. }catch(Exception e){} 
9. } 
10. } 
11. } 
12. 
13. public class TestSynchronization5 { 
14. public static void main(String[] args) { 
15. 
16. Thread t1=new Thread(){ 
17. public void run(){ 
18. Table.printTable(1); 
19. } 
20. }; 
21. 
22. Thread t2=new Thread(){ 
23. public void run(){ 
24. Table.printTable(10); 
25. } 
26. }; 
27. 
28. Thread t3=new Thread(){ 
29. public void run(){ 
30. Table.printTable(100); 
31. } 
32. }; 
33. 
34. Thread t4=new Thread(){ 
35. public void run(){ 
36. Table.printTable(1000); 
37. } 
38. }; 
39. t1.start(); 
40. t2.start(); 
41. t3.start(); 
42. t4.start(); 
43. 
44. } 
45. }

Output: 1
2
3
4
5
6
7
8
9
10
10
20
30
40
50
60
70
80
90
100
100
200
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7000
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9000
10000

Synchronized block on a class lock:

The block synchronizes on the lock of the object denoted by the reference .class name .class. A static synchronized method printTable(int n) in class Table is equivalent to the following declaration:

1. static void printTable(int n) { 
2. synchronized (Table.class) { // Synchronized block on class A 
3. // ... 
4. } 
}

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