Singleton Pattern In Java

Singleton pattern is probably one of most popular widely used patterns that I have seen. It’s implemented to create an access restriction to a specific resource like: printer, network, pooling, memory, logging.

Here is a simple and correct implementation:

public class Singleton implements Serializable{
	private static Singleton _INSTANCE = null;

	static{
  		_INSTANCE = new Singleton();
 	}

	private Singleton() {
  	}

	public static Singleton getInstance() {
		return _INSTANCE;
	}
 }

Lets look at one by one.

private static Singleton  _INSTANCE keeps reference of the instance
static block this will prevent multi-thread initialization
readResolve() to make sure a new instance is not created by deserialization process

Beware that double locking solution will not work:

class Singleton {
  private static  Singleton singleton = null;
  public static Singleton getSingleton() {
    if (singleton == null){
        synchronized(this) {
           if (singleton == null)
              singleton = new Singleton ();
        }
   }
    return singleton ;
    }
  }

Please read the article for further information.