static Synchronized methods

synchronized methods acquires a lock on object where as static synchronized methods acquire a lock on class. Only one object of particular class can enter into the static synchronized method at a time. One class has one class lock with it.

Syntax

   static synchronized returnType methodName(parameters){

   }

Example: without synchronization for static method    

class ClassLockEx implements Runnable{
 static void printMethod(){
  for(int i=0; i < 5; i++){
   try{
    Thread.currentThread().sleep(1000);
   }
   catch(Exception e){
    System.out.println(e);
   }
   System.out.println(Thread.currentThread().getName() + " " + i);
  }
 }

 public void run(){
  printMethod();
 }

 public static void main(String args[])throws Exception{
  ClassLockEx obj1 = new ClassLockEx();
  ClassLockEx obj2 = new ClassLockEx();
  Thread t1 = new Thread(obj1);
  Thread t2 = new Thread(obj2);
  t1.start();
  t2.start();
 }
}

    

Sample Output    

Thread-0 0
Thread-1 0
Thread-0 1
Thread-1 1
Thread-0 2
Thread-1 2
Thread-0 3
Thread-1 3
Thread-0 4
Thread-1 4

As you see run method calling the“ printMethod”, which is static, so both the objects obj1, obj2 has access to the static method. So both are running parallelly. By using static synchronized method, Only one object can run the method at a time.

Use of static synchronized method

class ClassLockEx implements Runnable{
 static synchronized void printMethod(){
  for(int i=0; i < 5; i++){
   try{
    Thread.currentThread().sleep(1000);
   }
   catch(Exception e){
    System.out.println(e);
   }
   System.out.println(Thread.currentThread().getName() + " " + i);
  }
 }

 public void run(){
  printMethod();
 }

 public static void main(String args[])throws Exception{
  ClassLockEx obj1 = new ClassLockEx();
  ClassLockEx obj2 = new ClassLockEx();
  Thread t1 = new Thread(obj1);
  Thread t2 = new Thread(obj2);
  t1.start();
  t2.start();
 }
} 

Output    

Thread-0 0
Thread-0 1
Thread-0 2
Thread-0 3
Thread-0 4
Thread-1 0
Thread-1 1
Thread-1 2
Thread-1 3
Thread-1 4

   

With the help of static synchronization, until one object finishes it's execution with the static synchronized method, other objects must wait to run the same or other static synchronization methods of the same class.

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