Question

This may be a simple question,but still it is a problem for me.I am having a class that have method called task and i want to ensure only one instance of this class can run this method at a time.I have implemented it like this and am i doing it correct way?

class A{

 public void task(){
   synchronized(A.this){ 
   //method stuff 
   }
 }

}

class B{
  public static void main(String a[]){
  new A().task();
  }
}

class C{
  public static void main(String a[]){
  new A().task();
  }
}

If B started the task and before it completes C also came to task i want to C to wait until B completes.
so far it seems works.But i want to know is this the correct way,

thank you.

Answer 1

Not quite. As you have it, you guarantee that only one thread will execute task() on a given instance at a time — but separate instances can still run task() at the same time. That is, if you did this (in pseudo-code):

Thread t1 = new A().task();
Thread t2 = new A().task();
t1.start();
t2.start();

Then since each thread creates a separate instance of A, they’ll each be able to run task() at the same time.

You need to synchronize on a static field (or A.class), or make task() a static, synchronized method (in which case synchronized locks based off of the Class object).