Synchronization

Synchronization

•  When two or more threads need to access shared memory, then there is some way to ensure that the access to the resource will be by only one thread at a time. The process of ensuring one access at a time by one thread is called synchronization. The synchronization is the concept which is based on monitor. Monitor is used as mutually exclusive lock or mutex. When a thread owns this monitor at a time then the other threads can not access the resources. Other threads have to be there in waiting state.

• In Java every object has implicit monitor associated with it. For entering in object's monitor, the method is associated with a keyword synchronized. When a particular method is in synchronized state then all other threads have to be there in waiting state.

• There are two ways to achieve the synchronization -

1. Using Synchronized Methods 2. Using Synchronized Blocks (Statements).

Let us make the method synchronized to achieve the synchronization by using following Java program -

1. Using Synchronized Method

class Test

{

synchronized void display(int num)

System.out.println("\nTable for "+num);

{

System.out.print(" "+num*i);

}

System.out.print("\nEnd of Table");

try

{

Thread.sleep(1000);

}catch(Exception e){}

}

}

class A extends Thread

{

Test th1;

A(Test t)

{

th1=t;

}

public void run()

{

th1.display(2);

}

}

class B extends Thread

{

Test th2;

B(Test t)

{

th2=t;

}

public void run()

{

th2.display(100);

}

}

class MySynThread

{

public static void main(String args[])

{

Test obj=new Test();

A t1=new A(obj);

B t2=new B(obj);

t1.start();

t2.start();

}

}

Output

Program Explanation:

• In above program we have written one class named Test. Inside this class the synchronized method named display is written. This method displays the table of numbers.

• We have written two more classes named A and B for executing the thread. The constructors for class A and Class B are written and to initialize the instance variables of class Test as th1 (as a variable for class A)and th2(as a variable for class B)

• Inside the run methods of these classes we have passed number 2 and 10 respectively and display method is invoked.

• The display method executes firstly for thread t1 and then for t2 in synchronized manner.

2. Using Synchronized Block

• When we want to achieve synchronization using the synchronized block then create a block of code and mark it as synchronized.

• Synchronized statements must specify the object that provides the intrinsic lock.

Syntax

The syntax for using the synchronized block is

synchronized(object reference)

{

statement;

statement; //block of code to be synchronized

.

.

.

}

Java Program

class Test

{

void display(int num)

{

Synchronized(this)

{

System.out.println(“\n Table for”+num);

for( int i=1;i<=10;i++)

{

System.out.println(“ ”+num*i);

}

System.out.println(“\n End of Table”);

try

{

Thread.sleep (1000);

} catch(Exception e){}

}

}

class A extends Thread

{

Test th1;

A(Test t)

{

th1=t;

}

public void run()

{

th1.display(2);

}

}

class B extends Thread

{

Test th2;

B(Test t)

{

th2=t;

}

public void run()

{

th2.display(100);

}

}

class MySynThreadBlock

{

public static void main(String args[])

{

Test obj=new Test();

A t1=new A(obj);

B t2=new B(obj);

t1.start();

t2.start();

}

}

Output

Points to Remember about Synchronization

1. Only methods can be synchronized but the variables and classes can not be synchronized.

2. Each object has one lock.

3. A class contains several methods and all methods need not be synchronized.

4. If two threads in a class wants to execute synchronized methods and both the methods are using the same instance of a class then only one thread can access the synchronized method at a time.

5. We can not synchronize the constructors.

6. A thread can acquire more than one lock.

Review Questions

1.What is thread synchronization ? Discuss with an example.

2. What is synchronization? Explain the different types of synchronization in java.