Singleton class + Lazy loading

[kri shan]

How can i load Singleton class using lazy loading?

[Rob Spoor]

There are several ways.

The easiest way is to make the entire getInstance() method synchronized. A bit harder is double-checked locking:

class Singleton
{
    private static volatile Singleton instance; // volatile is required!

    private Singleton() {}

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

Then there is the holder class:

class Singleton
{
    private Singleton() {}

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

    private static class SingletonHolder
    {
        private static final Singleton INSTANCE = new Singleton();
    }
}

This nested class will only be initialized when needed; that is, when Singleton.getInstance() is called. Only then will it create the Singleton object. Since class loading is atomic, this approach is thread safe as well.

[Jesper de Jong]

Note that double-checked locking is tricky, and it will not work correctly if you use an older Java version than Java 5: The "Double-Checked Locking is Broken" Declaration.

As far as I know, the memory model was changed in Java 5 so that it's no longer broken (I don't remember all the details right now), but it's still tricky.