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A class diagram exemplifying the singleton pattern. In object-oriented programming, the singleton pattern is a software design pattern that restricts the instantiation of a class to a singular instance. It is one of the well-known "Gang of Four" design patterns, which describe how to solve recurring problems in object-oriented software. [1]
In software engineering, the initialization-on-demand holder (design pattern) idiom is a lazy-loaded singleton. In all versions of Java, the idiom enables a safe, highly concurrent lazy initialization of static fields with good performance. [1] [2]
This pattern can be implemented in several ways depending on the host programming language, such as the singleton design pattern, object-oriented static members in a class and procedural global functions. In Python, the pattern is built into the language, and each .py file is automatically a module.
Software design patterns offer the finest granularity compared to software architecture patterns and software architecture styles, as design patterns focus on solving detailed, low-level design problems within individual components or subsystems. Examples include Singleton, Factory Method, and Observer. [2] [3] [4]
The original form of the pattern, appearing in Pattern Languages of Program Design 3, [2] has data races, depending on the memory model in use, and it is hard to get right. Some consider it to be an anti-pattern. [3] There are valid forms of the pattern, including the use of the volatile keyword in Java and explicit memory barriers in C++. [4]
The object pool design pattern is used in several places in the standard classes of the .NET Framework. One example is the .NET Framework Data Provider for SQL Server. As SQL Server database connections can be slow to create, a pool of connections is maintained. Closing a connection does not actually relinquish the link to SQL Server.
For example, several GoF patterns, like the Factory method pattern, the Builder or even the Singleton are implementations of this concept. The Abstract factory pattern instead is a method to build collections of factories. In some design patterns, a factory object has a method for every kind of object it can create
In Java, the multiton pattern can be implemented using an enumerated type, with the values of the type corresponding to the instances. In the case of an enumerated type with a single value, this gives the singleton pattern. In C#, we can also use enums, as the following example shows: