diff --git a/dependency-injection/README.md b/dependency-injection/README.md index 90edd4061..abf647b50 100644 --- a/dependency-injection/README.md +++ b/dependency-injection/README.md @@ -9,12 +9,78 @@ tags: --- ## 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 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 brands. + +```java +public abstract class Tobacco { + + private static final Logger LOGGER = LoggerFactory.getLogger(Tobacco.class); + + 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 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 ![alt text](./etc/dependency-injection.png "Dependency Injection") @@ -22,5 +88,12 @@ inversion of control and single responsibility principles. ## 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 +* 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 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)