c150871a94
* Updated saga to JUnit 5 * Update fix for CI job in trampoline module * Updated update-method module to JUnit 5 * Upgraded to latest JUnit Jupiter JUnit 4 is not needed when using JUnit-Vintage * Reverted change to access modifier on Trampoline * Cleanup to resolve code smells * Formatting * Formatting * Migrating to JUnit5 and updating some Mockito patterns * Migrating to JUnit5 * Migrating to JUnit5 * Migrating to JUnit 5 * Formatting cleanup * Added missing scope for junit * Fixed tests that were not running previously. Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>
layout, title, folder, permalink, categories, tags
| layout | title | folder | permalink | categories | tags | |
|---|---|---|---|---|---|---|
| pattern | Singleton | singleton | /patterns/singleton/ | Creational |
|
Intent
Ensure a class only has one instance, and provide a global point of access to it.
Explanation
Real world example
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.
In plain words
Ensures that only one object of a particular class is ever created.
Wikipedia says
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.
Programmatic Example
Joshua Bloch, Effective Java 2nd Edition p.18
A single-element enum type is the best way to implement a singleton
public enum EnumIvoryTower {
INSTANCE
}
Then in order to use:
var enumIvoryTower1 = EnumIvoryTower.INSTANCE;
var enumIvoryTower2 = EnumIvoryTower.INSTANCE;
assertEquals(enumIvoryTower1, enumIvoryTower2); // true
Class diagram
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. The clients of the Singleton become difficult to test.
- Makes it almost impossible to subclass a Singleton.