A singleton is simply a class that is instantiated exactly once.
Singletons typically represent a system component that is intrinsically unique, such as the window manager or file system.

Making a class a singleton can make it difficult to test its clients, as it’s impossible to substitute a mock imple- mentation for a singleton unless it implements an interface that serves as its type.
Before release 1.5, there were two ways to implement singletons. Both are based on keeping the constructor private and exporting a public static member to provide access to the sole instance. In one approach, the member is a final field:
public static final Elvis INSTANCE = new Elvis();
private Elvis() { … } public void leaveTheBuilding() { … }
The private constructor is called only once, to initialize the public static final field Elvis.INSTANCE. The lack of a public or protected constructor guarantees a “monoelvistic” universe: exactly one Elvis instance will exist once the Elvis class is initialized—no more, no less. Nothing that a client does can change this, with one caveat: a privileged client can invoke the private constructor reflectively (Item 53) with the aid of the AccessibleObject.setAccessible method. If you need to defend against this attack, modify the constructor to make it throw an exception if it’s asked to create a second instance.
In the second approach to implementing singletons, the public member is a static factory method:
// Singleton with static factory
public class Elvis { private static final Elvis INSTANCE = new Elvis(); private Elvis() { … } public static Elvis getInstance() { return INSTANCE; }
public void leaveTheBuilding() { … }
All calls to Elvis.getInstance return the same object reference, and no other Elvis instance will ever be created (with the same caveat mentioned above).

The main advantage of the public field approach is that the declarations make it clear that the class is a singleton: the public static field is final, so it will always contain the same object reference. There is no longer any performance advantage to the public field approach: modern Java virtual machine (JVM) implementations are almost certain to inline the call to the static factory method.
One advantage of the factory-method approach is that it gives you the flexibil- ity to change your mind about whether the class should be a singleton without changing its API. The factory method returns the sole instance but could easily be modified to return, say, a unique instance for each thread that invokes it. A second advantage, concerning generic types, is discussed in Item 27. Often neither of these advantages is relevant, and the final-field approach is simpler.
To make a singleton class that is implemented using either of the previous approaches serializable (Chapter 11), it is not sufficient merely to add imple- ments Serializable to its declaration. To maintain the singleton guarantee, you have to declare all instance fields transient and provide a readResolve method (Item 77). Otherwise, each time a serialized instance is deserialized, a new instance will be created, leading, in the case of our example, to spurious Elvis sightings. To prevent this, add this readResolve method to the Elvis class:
// readResolve method to preserve singleton property
private Object readResolve() { // Return the one true Elvis and let the garbage collector // take care of the Elvis impersonator.
return INSTANCE;
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.

Sorce: http://java.sun.com/docs/books/effective/