Double Checked Locking

[Kalyan Anand]

I have to implement a singleton in Java 5+ environment. Is there more efficient implementation or some better practice other than DCL ?

[P. Re]

Initialize-On-Demand

[Rob Spoor]

1) Use a static holder class:

public class Singleton
{
    private Singleton() { ... }

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

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

The SingletonHolder class is only loaded and initialized when first needed, which is on the first call to the getInstance() method. Also, class loading is atomic by itself.

2) Use an enum:

public enum Singleton
{
    INSTANCE
}

This way the singleton is also automatically safe for serialization.

[Sam Yim]

Rob Prime wrote:1) Use a static holder class:

public class Singleton
{
    private Singleton() { ... }

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

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

The SingletonHolder class is only loaded and initialized when first needed, which is on the first call to the getInstance() method. Also, class loading is atomic by itself.

2) Use an enum:

public enum Singleton
{
    INSTANCE
}

This way the singleton is also automatically safe for serialization.

If the enum singleton contains methods that modify shared member variables, would those methods need to be synchronize?

[Steve Luke]

Sam Yim wrote:
If the enum singleton contains methods that modify shared member variables, would those methods need to be synchronize?

Yes, no matter what structures you use, if the object is used in multiple threads and the internal data is modified (in ways that will affect other threads' execution) then the code modifying and using the data should be synchronized. Nothing in how an enum works modifies that.

[Sam Yim]

Steve Luke wrote:
Yes, no matter what structures you use, if the object is used in multiple threads and the internal data is modified (in ways that will affect other threads' execution) then the code modifying and using the data should be synchronized. Nothing in how an enum works modifies that.

That was my reasoning but wasn't sure. Thanks for the confirm.