Will be the below code stop breaking singleton when two threads access at same time

Question

public class javaprac1 {

    private javaprac1() {

    }

   static private javaprac1 instance = new javaprac1();

   public static javaprac1 getInstance(){

       return instance; 
   }

}

Answer 1

Yes. That's a non-lazy singleton which is the simplest, and is thread-safe without any additional trickery. The only "downside" is that it's not lazily initialized, but often it seems the laziness is overemphasized. Lazy-init is preferrable when the singleton performs some potentially heavy initialization, and you want to defer it to the time it's first used.

If you need a lazily inited singleton, use the Enum singleton pattern, which is also very simple.

Answer 2

If you mean '..stop being singleton..' - answer is NO. But if you mean 'is this code thread safe' (which I doubt you mean) - answer is Yes.

Better / latest approach will be ujsing single value enum singleton. Other answers are really good.

Thank you @kayaman, I learned something today.

Answer 3

Yes. But as to your approach (first approach), you can make your private field final.

So all calls to javaprac1.getInstance return the same object reference, and no other javaprac1 instance will ever be created

eg.

public class javaprac1 {

    private javaprac1() {

    }

   static private final javaprac1 instance = new javaprac1();

   public static javaprac1 getInstance(){

       return instance; 
   }

}

For second approach, make also your final field public and call it. You don't need to create getInstance method.

eg.

public class javaprac1 {

    private javaprac1() {

    }

   static public final javaprac1 instance = new javaprac1();

}

For third approach, Enum singleton - the preferred approach (see Effective Java- Item3).

eg.

public enum javaprac1 {
   INSTANCE;  

}

Answer 4

I am asking whether two thread accessing the code at the same time creates two objects. Answer is no i believe from the answers given by the people.

Thanks

Answer 5

If you are asking whether your implementations is thread-safe or not, I would say that it is actually thread-safe

Anyway, I use to do this way to combine it with lazy initialization:

public class javaprac1 {

    private javaprac1() {}

    static private javaprac1 instance = null;

    public static synchronized javaprac1 getInstance(){
       if(instance == null){
          instance = new javaprac1();
       }
       return instance; 
    }
}