Singleton Design Pattern

Simply Singleton

Navigate the deceptively simple Singleton pattern

By David Geary, JavaWorld.com, 04/25/03

Sometimes it's appropriate to have exactly one instance of a class: window managers, print spoolers, and filesystems are prototypical examples. Typically, those types of objects—known as singletons—are accessed by disparate objects throughout a software system, and therefore require a global point of access. Of course, just when you're certain you will never need more than one instance, it's a good bet you'll change your mind.

The Singleton design pattern addresses all of the previous paragraph's concerns. With the Singleton design pattern you can:

• Ensure that only one instance of a class is created
• Provide a global point of access to the object
• Allow multiple instances in the future without affecting a singleton class's clients

Although the Singleton design pattern—as evidenced below by the figure below—is one of the simplest design patterns, it presents a number of pitfalls for the unwary Java developer. This article discusses the Singleton design pattern and addresses those pitfalls.

Note: You can download this article's source code from Resources.

The Singleton pattern

In Design Patterns, the authors describe the Singleton pattern like this:

Ensure a class has only one instance, and provide a global point of access to it.

The figure below illustrates the Singleton design pattern class diagram.

Singleton class diagram

As you can see from the figure above, there's not a whole lot to the Singleton design pattern. Singletons maintain a static reference to the sole singleton instance and return a reference to that instance from a static instance() method.

Example 1 shows a classic Singleton design pattern implementation:

Example 1. The classic singleton

public class ClassicSingleton {
  private static ClassicSingleton instance = null;
  protected ClassicSingleton() {
     // Exists only to defeat instantiation.
  }
  public static ClassicSingleton getInstance() {
     if(instance == null) {
        instance = new ClassicSingleton();
     }
     return instance;
  }
}

The singleton implemented in Example 1 is easy to understand. The ClassicSingleton class maintains a static reference to the lone singleton instance and returns that reference from the static getInstance() method.

There are several interesting points concerning the ClassicSingleton class. First, ClassicSingleton employs a technique known as lazy instantiation to create the singleton; as a result, the singleton instance is not created until the getInstance() method is called for the first time. This technique ensures that singleton instances are created only when needed.

Second, notice that ClassicSingleton implements a protected constructor so clients cannot instantiate ClassicSingleton instances; however, you may be surprised to discover that the following code is perfectly legal:

public class SingletonInstantiator {
 public SingletonInstantiator() {
  ClassicSingleton instance = ClassicSingleton.getInstance();
ClassicSingleton anotherInstance =
new ClassicSingleton();
      ...
 }
}

How can the class in the preceding code fragment—which does not extend ClassicSingleton—create a ClassicSingleton instance if the ClassicSingleton constructor is protected? The answer is that protected constructors can be called by subclasses and by other classes in the same package. Because ClassicSingleton and SingletonInstantiator are in the same package (the default package), SingletonInstantiator() methods can create ClassicSingleton instances. This dilemma has two solutions: You can make the ClassicSingleton constructor private so that only ClassicSingleton() methods call it; however, that means ClassicSingleton cannot be subclassed. Sometimes, that is a desirable solution; if so, it's a good idea to declare your singleton class final, which makes that intention explicit and allows the compiler to apply performance optimizations. The other solution is to put your singleton class in an explicit package, so classes in other packages (including the default package) cannot instantiate singleton instances.

A third interesting point about ClassicSingleton: it's possible to have multiple singleton instances if classes loaded by different classloaders access a singleton. That scenario is not so far-fetched; for example, some servlet containers use distinct classloaders for each servlet, so if two servlets access a singleton, they will each have their own instance.

Fourth, if ClassicSingleton implements the java.io.Serializable interface, the class's instances can be serialized and deserialized. However, if you serialize a singleton object and subsequently deserialize that object more than once, you will have multiple singleton instances.

Finally, and perhaps most important, Example 1's ClassicSingleton class is not thread-safe. If two threads—we'll call them Thread 1 and Thread 2—call ClassicSingleton.getInstance() at the same time, two ClassicSingleton instances can be created if Thread 1 is preempted just after it enters the if block and control is subsequently given to Thread 2.

As you can see from the preceding discussion, although the Singleton pattern is one of the simplest design patterns, implementing it in Java is anything but simple. The rest of this article addresses Java-specific considerations for the Singleton pattern, but first let's take a short detour to see how you can test your singleton classes.