gaspersic/java-design-patterns
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity
java-design-patterns/singleton
History
daniel-bryla 0a427710bb Merge branch 'master' of https://github.com/iluwatar/java-design-patterns 
* 'master' of https://github.com/iluwatar/java-design-patterns: (27 commits)
  Remove use of coveralls-maven-plugin (sonarqube.com covers this)
  Add SonarQube.com badge
  Fix environment variable
  Add Travis instructions for SonarQube.com analysis
  Adds more criticism to Singleton pattern.
  Event Based Asynchronous pattern: Add missing license header and puml diagram
  Changed config to non-interactive
  Moved config into a separate dir
  Unused import removed.
  End process logic clause has been corrected.
  Caching pattern: Documentation and diagram
  Fixes #437. Adds criticism to Singleton pattern.
  Alter JUnit tests to run in lesser time.
  Updated version snapshot to 1.14.0
  Changes based on review feedback.
  Closes #436. Adds criticism to service locator pattern.
  Caching pattern: Implementation of Cache-Aside pattern
  Caching pattern: Style fix for null check
  Caching pattern: Refactor LRU cache to avoid NPE and unnecessary cache lookup
  Caching pattern: Refactor shutdown hook to use method reference
  ...
2016-11-04 11:51:45 +01:00
..
etc
UML generation: Mark the urm-maven-plugin execution to be ignored in Eclipse and recreate all .puml files
2016-09-18 17:51:09 +03:00
src
#502 Replaced usages of System.out with logger.
2016-10-23 19:59:03 +02:00
pom.xml
Set version number for next development iteration
2016-09-30 19:40:28 +03:00
README.md
Adds more criticism to Singleton pattern.
2016-10-18 14:18:47 +02:00

README.md

layout, title, folder, permalink, pumlid, categories, tags
layout title folder permalink pumlid categories tags
pattern Singleton singleton /patterns/singleton/ HSV14SCm20J0Lk82BFxf1ikCh0n26ZZizfDVVhjRjwfvIhg-Bc35cyZvAQtZoYD3l4w364gTWxhcms2d3z-ydnAzsRuO4BUWmV43HRUcWcaagF-Lz55M3lq2 Creational
Java
Gang Of Four
Difficulty-Beginner

Intent

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

alt text

Applicability

Use the Singleton pattern when

  • there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
  • when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code

Typical Use Case

  • the logging class
  • managing a connection to a database
  • file manager

Real world examples

Consequences

  • Violates Single Responsibility Principle (SRP) by controlling their own creation and lifecycle.
  • Encourages using a global shared instance which prevents an object and resources used by this object from being deallocated.
  • Creates tightly coupled code that is difficult to test.
  • Makes it almost impossible to subclass a Singleton.

Credits