http://java.sun.com/developer/JDCTechTips/2006/tt0113.html#1

   public class MySingleton {
     private static final MySingleton INSTANCE = 
       new MySingleton();

     private MySingleton() {
     }

     public static final MySingleton getInstance() {
       return INSTANCE;
     }
   }

Theoretically, you don't need the getInstance() method because the
INSTANCE variable could be public. However, the getInstance()
method does provide flexibility in case of future system
changes. Good virtual machine implementations should inline the
call to the static getInstance() method.

That's not quite all there is to creating a Singleton. If you need
to make your Singleton class Serializable, you must provide a
readResolve() method:

  /**
   * Ensure Singleton class
   */
  private Object readResolve() throws ObjectStreamException {
    return INSTANCE;
  }

With the readResolve() method in place, deserialization results in
the one (and only one) object -- the same object as produced by
calls to the getInstance() method. If you don't provide a
readResolve() method, an instance of the object is created each
time you deserialize the object.

Note that Singletons are only guaranteed to be unique within a
given class loader. If you use the same class across multiple
distinct enterprise containers, you'll get one instance for each
container.