mirror of
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Ilkka Seppälä
135 lines
5.5 KiB
Markdown
135 lines
5.5 KiB
Markdown
---
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title: Dependency Injection
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category: Structural
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language: en
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tag:
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- Decoupling
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- Dependency management
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- Inversion of control
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---
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## Also known as
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* Inversion of Control (IoC)
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* Dependency Inversion
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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 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](https://java-design-patterns.com/principles/#inversion-of-control) and [Single Responsibility](https://java-design-patterns.com/principles/#single-responsibility-principle) 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` interface and the concrete brands.
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```java
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@Slf4j
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public abstract class Tobacco {
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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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* When aiming to reduce the coupling between classes and increase the modularity of the application.
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* In scenarios where the object creation process is complex or should be separated from the class usage.
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* In applications requiring easier unit testing by allowing dependencies to be mocked or stubbed.
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* Within frameworks or libraries that manage object lifecycles and dependencies, such as Spring or Jakarta EE (formerly Java EE).
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## Known Uses
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* Frameworks like Spring, Jakarta EE, and Google Guice use DI extensively to manage component lifecycles and dependencies.
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* Desktop and web applications that require flexible architecture with easily interchangeable components.
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## Consequences
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Benefits:
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* Enhances modularity and separation of concerns.
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* Simplifies unit testing by allowing for easy mocking of dependencies.
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* Increases flexibility and maintainability by promoting loose coupling.
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Trade-offs:
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* Can introduce complexity in the configuration, especially in large projects.
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* Might increase the learning curve for developers unfamiliar with DI patterns or frameworks.
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* Requires careful management of object lifecycles and scopes.
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## Related Patterns
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* [Factory Method](https://java-design-patterns.com/patterns/factory-method/) and [Abstract Factory](https://java-design-patterns.com/patterns/abstract-factory/): Used to create instances that the DI mechanism will inject.
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* [Service Locator](https://java-design-patterns.com/patterns/service-locator/): An alternative to DI for locating services or components, though it does not decouple the lookup process as effectively as DI.
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* [Singleton](https://java-design-patterns.com/patterns/singleton/): Often used in conjunction with DI to provide a single instance of a service across the application.
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## Credits
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* [Spring in Action](https://amzn.to/4asnpSG)
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* [Dependency Injection: Design patterns using Spring and Guice](https://amzn.to/4aMyHkI)
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* [Java Design Pattern Essentials](https://amzn.to/3xtPPxa)
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* [Pro Java EE Spring Patterns: Best Practices and Design Strategies Implementing Java EE Patterns with the Spring Framework](https://amzn.to/3J6Teoh)
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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: 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)
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