Question

Can anyone provide an example of a singleton pattern and explain why they are necessary?

Answer 1

Before going the singleton route, reconsider. Do you really need a singleton? If you’re asking for scenarios when you need to implement singletons, it’s because the need for them didn’t really express. You’re better off not introducing singletons in your code base just because it feels cool to follow design patterns.

Clean Code Talks – Global State and Singletons

Once Is Not Enough

Performant Singletons

However, what’s really worth knowing is Dependency Injection.

Now if you really want to implement singletons in Java, I would recommend Joshua Bloch’s “Effective Java” way of implementing them:

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

  private Singleton() {}

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

The JLS guarantees the JVM will not initialize instance until someone calls getInstance();

Final note, the Double Checked Locking Pattern is broken in Java up to Java 5. The Java 5 memory model makes the DCL pattern thread safe but it makes it slower than the SingletonHolder class method while the original intent was performance optimization.

EDIT: As @Luno pointed out, since the second edition of the book, the preferred way is:

As of release 1.5, there is a third approach to implementing singletons. Simply make an enum type with one element:

// Enum singleton - the preferred approach
public enum Elvis {
    INSTANCE;

    public void leaveTheBuilding() { ... }
}

This approach is functionally equivalent to the public field approach, except that it is more concise, provides the serialization machinery for free, and provides an ironclad guarantee against multiple instantiation, even in the face of sophisticated serialization or reflection attacks. While this approach has yet to be widely adopted, a single-element enum type is the best way to implement a singleton.