Singleton object- In static block or in getInstance(); which should be used

Question

Below are two ways to implement a singleton. What are the advantages and disadvantages of each?

Static initialization:

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

Lazy initialization is:

class Singleton {
    private Singleton instance;
    public Singleton getInstance(){
        if (instance == null) instance = new Singleton();
        return instance;
    }
}

Answer 1

1. Synchronized Accessor

   public class Singleton {
       private static Singleton instance;
   
       public static synchronized Singleton getInstance() {
           if (instance == null) {
               instance = new Singleton();
           }
           return instance;
       }
   }
   

   • lazy load
   • low perfomance

2. Double Checked Locking & volatile

   public class Singleton {
       private static volatile Singleton instance;
       public static Singleton getInstance() {
           Singleton localInstance = instance;
           if (localInstance == null) {
               synchronized (Singleton.class) {
                   localInstance = instance;
                   if (localInstance == null) {
                       instance = localInstance = new Singleton();
                   }
               }
           }
           return localInstance;
       }
   }
   

   • lazy load
   • high perfomance
   • JDK should be 1,5 ++

3. On Demand Holder idiom

   public class Singleton {
   
       public static class SingletonHolder {
           public static final Singleton HOLDER_INSTANCE = new Singleton();
       }
   
       public static Singleton getInstance() {
           return SingletonHolder.HOLDER_INSTANCE;
       }
   }
   

   • lazy load
   • high performance
   • cant be used for non static class fields

Answer 2

First one is eager initialisation. eager initialization creates the instance even before it’s being used and that is not the best practice to use.

Second one is Lazy Initialization. Lazy implementation works fine incase of single threaded environment but when it comes to multithreaded systems, it can cause issues if multiple threads are inside the if loop at the same time. It will destroy the singleton pattern and both threads will get the different instances of singleton class.

Please visit: http://www.journaldev.com/1377/java-singleton-design-pattern-best-practices-with-examples#static-block-initialization for more information

Answer 3

NOTE: static blocks can access only static variables defined outside static block directly. The object you want to restrict to only one instance creation should be static. Your first methods fails during compilation, second method requires you to create the object first
Follow this approach:

class Singleton
{
private static Singleton instance;
private Singleton()
{
}
public static Singleton getInstance()
{
    if(instance == null)
         instance = new Singleton();
    return instance;
}
}

Ensure no creation of the object elsewhere, so change the constructor to private