如何实现@Singleton 注解？

Question

Possibly a repeated question. But I need to implement something like

@Singleton
public class Person {
}

That will ensure only single instance of Person object.

One way is to make constructor private. But that makes the Singleton annotation redundant.

I could not really understand if I can really restrict object creation to single object without making the constructor private.

Is that even possible?

Answer 1

No annotation can prevent a class from being instantiated. However, if you plan to implement something like a Dependency Injection framework, or just a simple object factory, then you can use reflection to read the annotation and prevent the class from being instantiated more than once, but I understand this is not the answer you were looking for.

You can actually think about dropping the singleton pattern and moving to some more modern solution like a proper DI framework, which can give you the same result - with more flexibility.

Answer 2

public class ASingletonClass
{
    private static ASingletonClass instance = null;
    private ASingletonClass()
    {

    }

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

This is the right way to implement singleton. The annotation cannot stop people from calling the public constructor.

Answer 3

You can also use lazy initialization of the class. Example code as below.

class LazyInitialization implements PrintStatement {
   private static LazyInitialization instance;

   private LazyInitialization() {

   }

   static LazyInitialization getInstance() {
     if (instance == null)
        instance = new LazyInitialization();

     return instance;
   }

   @Override
   public void print() {
      System.out.println("I am inside");
   }
}

Answer 4

It is arguably best to have a stateless singleton:

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();
    
    private Singleton() {}
    
    public static Singleton getInstance() {
        return INSTANCE;
    }
}