42eb7950ae
* Fix some broken links * Remove extra space * Update filename * Fix some links in localization folders * Fix link Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>
107 lines
3.4 KiB
Markdown
107 lines
3.4 KiB
Markdown
---
|
|
layout: pattern
|
|
title: Dependency Injection
|
|
folder: dependency-injection
|
|
permalink: /patterns/dependency-injection/
|
|
categories: Creational
|
|
language: en
|
|
tags:
|
|
- Decoupling
|
|
---
|
|
|
|
## Intent
|
|
|
|
Dependency Injection is a software design pattern in which one or more dependencies (or services)
|
|
are injected, or passed by reference, into a dependent object (or client) and are made part of the
|
|
client's state. The pattern separates the creation of a client's dependencies from its own behavior,
|
|
which allows program designs to be loosely coupled and to follow the inversion of control and single
|
|
responsibility principles.
|
|
|
|
## Explanation
|
|
|
|
Real world example
|
|
|
|
> The old wizard likes to fill his pipe and smoke tobacco once in a while. However, he doesn't want
|
|
> to depend on a single tobacco brand only but likes to be able to enjoy them all interchangeably.
|
|
|
|
In plain words
|
|
|
|
> Dependency Injection separates creation of client's dependencies from its own behavior.
|
|
|
|
Wikipedia says
|
|
|
|
> In software engineering, dependency injection is a technique in which an object receives other
|
|
> objects that it depends on. These other objects are called dependencies.
|
|
|
|
**Programmatic Example**
|
|
|
|
Let's first introduce the `Tobacco` interface and the concrete brands.
|
|
|
|
```java
|
|
@Slf4j
|
|
public abstract class Tobacco {
|
|
|
|
public void smoke(Wizard wizard) {
|
|
LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
|
|
this.getClass().getSimpleName());
|
|
}
|
|
}
|
|
|
|
public class SecondBreakfastTobacco extends Tobacco {
|
|
}
|
|
|
|
public class RivendellTobacco extends Tobacco {
|
|
}
|
|
|
|
public class OldTobyTobacco extends Tobacco {
|
|
}
|
|
```
|
|
|
|
Next here's the `Wizard` class hierarchy.
|
|
|
|
```java
|
|
public interface Wizard {
|
|
|
|
void smoke();
|
|
}
|
|
|
|
public class AdvancedWizard implements Wizard {
|
|
|
|
private final Tobacco tobacco;
|
|
|
|
public AdvancedWizard(Tobacco tobacco) {
|
|
this.tobacco = tobacco;
|
|
}
|
|
|
|
@Override
|
|
public void smoke() {
|
|
tobacco.smoke(this);
|
|
}
|
|
}
|
|
```
|
|
|
|
And lastly we can show how easy it is to give the old wizard any brand of tobacco.
|
|
|
|
```java
|
|
var advancedWizard = new AdvancedWizard(new SecondBreakfastTobacco());
|
|
advancedWizard.smoke();
|
|
```
|
|
|
|
## Class diagram
|
|
|
|
|
|
|
|
## Applicability
|
|
|
|
Use the Dependency Injection pattern when:
|
|
|
|
* When you need to remove knowledge of concrete implementation from object.
|
|
* To enable unit testing of classes in isolation using mock objects or stubs.
|
|
|
|
## Credits
|
|
|
|
* [Dependency Injection Principles, Practices, and Patterns](https://www.amazon.com/gp/product/161729473X/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=161729473X&linkId=57079257a5c7d33755493802f3b884bd)
|
|
* [Clean Code: A Handbook of Agile Software Craftsmanship](https://www.amazon.com/gp/product/0132350882/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0132350882&linkCode=as2&tag=javadesignpat-20&linkId=2c390d89cc9e61c01b9e7005c7842871)
|
|
* [Java 9 Dependency Injection: Write loosely coupled code with Spring 5 and Guice](https://www.amazon.com/gp/product/1788296257/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=1788296257&linkId=4e9137a3bf722a8b5b156cce1eec0fc1)
|
|
* [Google Guice: Agile Lightweight Dependency Injection Framework](https://www.amazon.com/gp/product/1590599977/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1590599977&linkId=3b10c90b7ba480a1b7777ff38000f956)
|