100 lines
3.4 KiB
Markdown
100 lines
3.4 KiB
Markdown
---
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layout: pattern
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title: Dependency Injection
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folder: dependency-injection
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permalink: /patterns/dependency-injection/
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categories: Creational
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tags:
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- Decoupling
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---
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## Intent
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Dependency Injection is a software design pattern in which one or more dependencies (or services) are injected, or
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passed by reference, into a dependent object (or client) and are made part of the client's state. The pattern separates
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the creation of a client's dependencies from its own behavior, which allows program designs to be loosely coupled and
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to follow the inversion of control and single responsibility principles.
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## Explanation
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Real world example
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> 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.
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In plain words
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> Dependency Injection separates creation of client's dependencies from its own behavior.
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Wikipedia says
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> 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.
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**Programmatic Example**
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Let's first introduce the tobacco brands.
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```java
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public abstract class Tobacco {
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private static final Logger LOGGER = LoggerFactory.getLogger(Tobacco.class);
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public void smoke(Wizard wizard) {
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LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
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this.getClass().getSimpleName());
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}
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}
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public class SecondBreakfastTobacco extends Tobacco {
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}
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public class RivendellTobacco extends Tobacco {
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}
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public class OldTobyTobacco extends Tobacco {
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}
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```
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Next here's the wizard class hierarchy.
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```java
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public interface Wizard {
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void smoke();
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}
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public class AdvancedWizard implements Wizard {
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private final Tobacco tobacco;
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public AdvancedWizard(Tobacco tobacco) {
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this.tobacco = tobacco;
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}
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@Override
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public void smoke() {
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tobacco.smoke(this);
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}
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}
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```
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And lastly we can show how easy it is to give the old wizard any brand of tobacco.
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```java
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var advancedWizard = new AdvancedWizard(new SecondBreakfastTobacco());
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advancedWizard.smoke();
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```
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## Class diagram
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## Applicability
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Use the Dependency Injection pattern when
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* When you need to remove knowledge of concrete implementation from object
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* To enable unit testing of classes in isolation using mock objects or stubs
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## Credits
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* [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)
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* [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)
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* [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)
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* [Google Guice Tutorial: Open source Java based dependency injection framework](https://www.amazon.com/gp/product/B083P7DZ8M/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B083P7DZ8M&linkId=04f0f902c877921e45215b624a124bfe)
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