04bf566dc1
* Grammatical fixes to command pattern * Update bridge pattern readme * Fixes to builder pattern grammar * Update chain of responsibility * Improvements to the composite example * Fixes to headings * Minor updates to decorator pattern * Update facade * Update factory example * Update factory method * Update flyweight * Interpreter explanation * Update iterator readme * Add explanation for mediator pattern * Grammatical fixes to memento * Grammar fixes for observer * Update explanation for the prototype pattern * Proxy pattern grammar fixes * Update singleton * Grammar fixes to state pattern * Grammar fixes for strategy * Grammar fixes, template method * Grammar fixes for visitor * Fix typo
98 lines
3.5 KiB
Markdown
98 lines
3.5 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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language: en
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tags:
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- Gang of Four
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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 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
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> tower is always used by the wizards. The ivory tower here is a 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
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> instantiation of a class to one object. This is useful when exactly one object is needed to
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> 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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var enumIvoryTower1 = EnumIvoryTower.INSTANCE;
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var enumIvoryTower2 = EnumIvoryTower.INSTANCE;
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LOGGER.info("enumIvoryTower1={}", enumIvoryTower1);
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LOGGER.info("enumIvoryTower2={}", enumIvoryTower2);
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```
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The console output
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```
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enumIvoryTower1=com.iluwatar.singleton.EnumIvoryTower@1221555852
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enumIvoryTower2=com.iluwatar.singleton.EnumIvoryTower@1221555852
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```
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## Class diagram
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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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Some typical use cases for the Singleton
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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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## Known uses
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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 creation and lifecycle.
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* Encourages using a globally 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](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Effective Java](https://www.amazon.com/gp/product/0134685997/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0134685997&linkCode=as2&tag=javadesignpat-20&linkId=4e349f4b3ff8c50123f8147c828e53eb)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
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