docs: update dependency injection

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Ilkka Seppälä
2024-04-08 21:20:50 +03:00
parent c1466eb66e
commit 0b850aa764
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@@ -1,47 +1,49 @@
---
title: Dependency Injection
category: Creational
category: Structural
language: en
tag:
- Decoupling
- Decoupling
- Dependency management
- Inversion of control
---
## Also known as
* Inversion of Control (IoC)
* Dependency Inversion
## 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.
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.
## 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.
> 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.
> 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.
> 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 void smoke(Wizard wizard) {
LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
this.getClass().getSimpleName());
}
}
public class SecondBreakfastTobacco extends Tobacco {
@@ -59,44 +61,73 @@ Next here's the `Wizard` class hierarchy.
```java
public interface Wizard {
void smoke();
void smoke();
}
public class AdvancedWizard implements Wizard {
private final Tobacco tobacco;
private final Tobacco tobacco;
public AdvancedWizard(Tobacco tobacco) {
this.tobacco = tobacco;
}
public AdvancedWizard(Tobacco tobacco) {
this.tobacco = tobacco;
}
@Override
public void smoke() {
tobacco.smoke(this);
}
@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();
var advancedWizard=new AdvancedWizard(new SecondBreakfastTobacco());
advancedWizard.smoke();
```
## Class diagram
![alt text](./etc/dependency-injection.png "Dependency Injection")
![Dependency Injection](./etc/dependency-injection.png "Dependency Injection")
## Applicability
Use the Dependency Injection pattern when:
* When aiming to reduce the coupling between classes and increase the modularity of the application.
* In scenarios where the object creation process is complex or should be separated from the class usage.
* In applications requiring easier unit testing by allowing dependencies to be mocked or stubbed.
* Within frameworks or libraries that manage object lifecycles and dependencies, such as Spring or Jakarta EE (formerly Java EE).
* When you need to remove knowledge of concrete implementation from object.
* To enable unit testing of classes in isolation using mock objects or stubs.
## Known Uses
* Frameworks like Spring, Jakarta EE, and Google Guice use DI extensively to manage component lifecycles and dependencies.
* Desktop and web applications that require flexible architecture with easily interchangeable components.
## Consequences
Benefits:
* Enhances modularity and separation of concerns.
* Simplifies unit testing by allowing for easy mocking of dependencies.
* Increases flexibility and maintainability by promoting loose coupling.
Trade-offs:
* Can introduce complexity in the configuration, especially in large projects.
* Might increase the learning curve for developers unfamiliar with DI patterns or frameworks.
* Requires careful management of object lifecycles and scopes.
## Related Patterns
* [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.
* [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.
* [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.
## Credits
* [Spring in Action](https://amzn.to/4asnpSG)
* [Dependency Injection: Design patterns using Spring and Guice](https://amzn.to/4aMyHkI)
* [Java Design Pattern Essentials](https://amzn.to/3xtPPxa)
* [Pro Java EE Spring Patterns: Best Practices and Design Strategies Implementing Java EE Patterns with the Spring Framework](https://amzn.to/3J6Teoh)
* [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)
@@ -28,7 +28,7 @@ import com.google.inject.Guice;
/**
* Dependency Injection pattern deals with how objects handle their dependencies. The pattern
* implements so called inversion of control principle. Inversion of control has two specific rules:
* implements so-called inversion of control principle. Inversion of control has two specific rules:
* - High-level modules should not depend on low-level modules. Both should depend on abstractions.
* - Abstractions should not depend on details. Details should depend on abstractions.
*
@@ -37,7 +37,7 @@ import com.google.inject.Guice;
* concrete implementation which cannot be changed.
*
* <p>The second and third wizards({@link AdvancedWizard} and {@link AdvancedSorceress}) are more
* flexible. They do not depend on any concrete implementation but abstraction. They utilizes
* flexible. They do not depend on any concrete implementation but abstraction. They utilize
* Dependency Injection pattern allowing their {@link Tobacco} dependency to be injected through
* constructor ({@link AdvancedWizard}) or setter ({@link AdvancedSorceress}). This way, handling
* the dependency is no longer the wizard's responsibility. It is resolved outside the wizard
@@ -58,7 +58,7 @@ class AdvancedSorceressTest {
* her through the setter's parameter
*/
@Test
void testSmokeEveryThing() throws Exception {
void testSmokeEveryThing() {
List<Tobacco> tobaccos = List.of(
new OldTobyTobacco(),
@@ -57,7 +57,7 @@ class AdvancedWizardTest {
* through the constructor parameter
*/
@Test
void testSmokeEveryThing() throws Exception {
void testSmokeEveryThing() {
List<Tobacco> tobaccos = List.of(
new OldTobyTobacco(),
@@ -35,7 +35,6 @@ class AppTest {
/**
* Issue: Add at least one assertion to this test case.
*
* Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
* throws an exception.
*/
@@ -58,7 +58,7 @@ class GuiceWizardTest {
* through the constructor parameter
*/
@Test
void testSmokeEveryThingThroughConstructor() throws Exception {
void testSmokeEveryThingThroughConstructor() {
List<Tobacco> tobaccos = List.of(
new OldTobyTobacco(),
@@ -83,7 +83,7 @@ class GuiceWizardTest {
* through the Guice google inject framework
*/
@Test
void testSmokeEveryThingThroughInjectionFramework() throws Exception {
void testSmokeEveryThingThroughInjectionFramework() {
List<Class<? extends Tobacco>> tobaccos = List.of(
OldTobyTobacco.class,