Singletons and lazy loading

Probably the first design pattern that every software developer learns is Singleton and lazy loading of Singleton classes.

The usual example, goes something like this:

public class Singleton {

 static Singleton instance;

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

}

The problem with this solution is that synchronized method getInstance() is called every time, while synchronization is actually needed only for the first call of the method (which introduces performance overhead in your application). There were attempts to tackle this problem by using Double-checked locking pattern, which although great in theory didn’t work in practice.

Today, thanks to Bob Lee, I found out that there is a solution to this problem that is both simple and fast: Initialization on Demand Holder (IODH) idiom. The appropriate example follows:

public class Singleton {

  static class SingletonHolder {
    static Singleton instance = new Singleton();
  }

  public static Singleton getInstance() {
    return SingletonHolder.instance;
  }

}

Basicaly, Java Language Specification (JLS) guarantees that instance would not be initialized until someone calls getInstance() method (more information could be found in articles that I’ve linked to before). Elegant and fast, just as it should be.