If you search for the word Singleton on Google, you will probably find a bunch of rants about the fact that this design pattern is the absolute evil on the earth. Compared to the Singleton pattern, Darth Vader and Lord Voldemort are like little scout girls selling cookies door to door. Is it true? Or is the design pattern totally misunderstood by developers? In this series of articles I will try to analyze the reasons that stay behind those rants and I will try to give my point of view on the issue.
So, let’s start from the very beginning, the definition of the design pattern as it was given by the GoF’s book.
The Singleton design pattern is an object creational design pattern. It was formalized by the GoF inside their most famous book, Design Patterns: Elements of Reusable Object-Oriented Software. The main intent of the pattern is the following:
Ensure a class only has one instance, and provide a global point of access to it.
I know that some of you are beginning to stretch their nose. The word global sounds like a fart during a Mozart piano solo. But, let’s we continue.
In the book the authors give us some examples in which a Singleton should be used: for examples,
although there can be many printers in a system, there should be only one printer spooler. There should be only one file system and one window manager.
Obviously, GoF gives us also a possible solution to the problem, which is:
A global variable makes an object accessible, but it doesn’t keep you from instantiating multiple objects. A better solution is to make the class itself responsible for keeping track of its sole instance.
OMG: have you noticed the violation of the Single Responsibility Principle? Were they foolish? No, they were not, but let’s take one step at time.
Then, you will use a Singleton every time that:
- There must be exactly one instance of a class and it must be accessible to clients from a well-know access point.
- The sole instance should be extensible by sub-classing
The only structure that satisfies the previous constraints is the following:
Among the others, the virtues of a Singleton that are reported in GoF’s book are the following:
- Permits refinement of operations and representation: a Singleton may be subclassed and it’s easy to configure the application with an instance of this extended class.
- More flexible than class operations: has this sentence to be commented?
So far so good. You have to keep in mind that the book was published in 1994. Java, JVM and other related stuff are in an embryonal state, and at that time nobody knew of dependency injection*, not even the DI’s father, Martin Fowler. For sake of completeness, let’s look at Singleton implementation in Java and Scala.
Singletons in Java: not such a good friend
Ok, the first implementation we can try is the one that translates directly from the previous UML class diagram into code.
Clearly, this naive implementation suffers of a well known problem: This implementation is not thread safe, due
if condition on the variable
INSTANCE. Then, be aware of this and don’t use this type of Singleton! However,
this implementation has a positive feature, which is it lazily initialises the variable
Is it a good thing to have lazy initialisation in your production code? Well, the lazy initialisation violates the fail fast principle. It’s better to know as sooner as possible if there is a problem in initialisation code of a class.
Then, if you want to adhere to the fail fast principle, you can change the previous Singleton implementation to the following.
If you still want to have the lazy initialisation, you can use the Initialization On Demand Holder idiom.
With these approaches, we gain the thread safety feature (if you want to know why the IODH approach is thread safe, please read this SO thread). However, these types of Singleton cannot be sub-classed and they are not serializable.
If you want to get back the serializability, you can use the approach suggested by Joshua Block in item 3 of the second edition of the book Effetctive Java.
As of release 1.5, there is a third approach to implementing singletons. Simply make an enum type with one element:
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.
Is this enough? What about sub-classing? If you remember, the ability to be sub-classed is one of the main
features of Singleton reported in the GoF’s book. Sub-classing a Singleton is a very difficult topic, because you have
to guarantee that the
INSTANCE variable of the base class will be instantiated only once using an instance of one of its
sub-class. This topic is far beyond the focus on this article. An example of Singleton sub-classing can
be found in the Implementation section of the chapter related to Singleton in the GoF’s book.
Idiomatic Singleton in Scala
What about the Scala language? Scala became a mainstream programming language a decade after Java. So, it had the time to learn from the errors of others language. Some of you could say: “So, why the f***k is there a way to have an idiomatic implementation of an anti-pattern like the Singleton pattern?”. We will try to answer to this question in the next article.
In Scala the Singleton pattern is integrated into the language specification. The
object notation enforces that a type
will have one and only one instance.
As you know, if we have an
object type that has the same name as a
class type, we will call it companion object of
the class. But, that is another story ;)
In this article we have analyzed the Singleton pattern from an agnostic point of view. First, we have introduced it through the definition given in the GoF’s book. Then, we have discussed how to implement the pattern using the Java language and the Scala language.
We intentionally have not given any judgment from a design point of view. In the upcoming article, we will try to understand why the Singleton pattern is so hated by so many software architects and programmers. We will introduce the difference between “Singleton” and “singleton” and we will probably reach the conclusion that the best answer to the question “How will you implement a Singleton in a specific programming language?” is: “I will not implement it at all!”.