81 lines
2.6 KiB
Markdown
81 lines
2.6 KiB
Markdown
---
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layout: pattern
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title: Singleton
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folder: singleton
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permalink: /patterns/singleton/
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categories: Creational
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tags:
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- Java
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- Gang Of Four
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- Difficulty-Beginner
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---
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## Intent
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Ensure a class only has one instance, and provide a global point of
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access to it.
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## Explanation
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Real world example
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> There can only be one ivory tower where the wizards study their magic. The same enchanted ivory tower is always used by the wizards. Ivory tower here is singleton.
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In plain words
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> Ensures that only one object of a particular class is ever created.
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Wikipedia says
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> In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.
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**Programmatic Example**
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Joshua Bloch, Effective Java 2nd Edition p.18
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> A single-element enum type is the best way to implement a singleton
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```java
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public enum EnumIvoryTower {
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INSTANCE;
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}
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```
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Then in order to use
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```java
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EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
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EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
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assertEquals(enumIvoryTower1, enumIvoryTower2); // true
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```
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## Applicability
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Use the Singleton pattern when
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* there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
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* when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code
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## Typical Use Case
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* the logging class
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* managing a connection to a database
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* file manager
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## Real world examples
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* [java.lang.Runtime#getRuntime()](http://docs.oracle.com/javase/8/docs/api/java/lang/Runtime.html#getRuntime%28%29)
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* [java.awt.Desktop#getDesktop()](http://docs.oracle.com/javase/8/docs/api/java/awt/Desktop.html#getDesktop--)
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* [java.lang.System#getSecurityManager()](http://docs.oracle.com/javase/8/docs/api/java/lang/System.html#getSecurityManager--)
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## Consequences
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* Violates Single Responsibility Principle (SRP) by controlling their own creation and lifecycle.
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* Encourages using a global shared instance which prevents an object and resources used by this object from being deallocated.
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* Creates tightly coupled code. The clients of the Singleton become difficult to test.
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* Makes it almost impossible to subclass a Singleton.
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## Credits
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* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
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* [Effective Java (2nd Edition)](http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp/0321356683)
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