Synchronization

     When two or more threads need access to a shared resource, they need some way to ensure that the resource will be used by only one thread at a time. The process by which this is achieved is called synchronization.

    Java uses the keyword synchronizedfor this purpose. While a thread is inside a synchronized method, all other threads that try to call it on the same instance have to wait.

     For example,

synchronized void update()

{

  ……..

  ……..

}

When a method is declared synchronized, Java creates a ‘monitor’ and hands it over to the thread that calls the method first time.

Ø It is also possible to mark a block of code as synchronized as shown below:

synchronized(lock-object)

{

    ……..

    ……..

}

Ø This may lead to the problem of deadlock.

Ø Example:

class Show

{

synchronized public void display(String s)

 {

System.out.print("Hello "+s);

try{

Thread.sleep(100);

   }

catch(Exception e){}

System.out.println(" Welcome");

 }

}

class A extends Thread

{

 Show s;

 String msg;

public A(String m,Showobj)

 {

  s=obj;

msg=m;

 }

public void run()

 {

s.display(msg);

 }

}

classThreadsyn

{

public static void main(String args[])

 {

  Show s=new Show();

  A a1= new A("Sayan",s);

  A a2= new A("Rohit",s);

  A a3= new A("Raju",s); 

a1.start();

a2.start();

a3.start();

 }

}

Output:

Hello Sayan Welcome

Hello Rohit Welcome

Hello Raju Welcome