Java Spring Best Practices: Dependency Injection Module

This module demonstrates the best practices for using Dependency Injection in a Spring Boot application.

---

📌 Overview

This module illustrates how to effectively use Dependency Injection in a Spring Boot application. Dependency Injection ( DI) is a fundamental principle in Spring that promotes loose coupling and enhances testability by injecting dependencies rather than instantiating them directly within the classes.

Key Features:

• Constructor Injection: Preferred method for injecting dependencies, ensuring immutability and making the code easier to test.
• Separation of Concerns: Keeps your business logic separate from dependency management.
• Clean and Maintainable Code: Promotes cleaner code architecture by adhering to SOLID principles.

---

🏛️ Module Structure

src/
├── main/
│   ├── java/
│   │   └── dev/
│   │       └── agitrubard/
│   │           └── dependencyinjection/
│   │               ├── controller/
│   │               │   └── SomeController.java
│   │               └── service/
│   │                   ├── SomeService.java
│   │                   └── impl/
│   │                       └── SomeServiceImpl.java
│   └── resources/
│       └── application.properties
└── test/
    └── java/
        └── dev/
            └── agitrubard/
                └── dependencyinjection/
                    └── SomeControllerTests.java

---

🚀 Getting Started

Prerequisites

• Java 21
• Maven 3.x.x
• Spring Boot 3.x.x

Installation

1. Clone the repository:

git clone https://github.com/agitrubard/java-spring-best-practices.git
cd java-spring-best-practices

2. Build the project:

mvn clean install

3. Run the application:

mvn spring-boot:run

---

📄 Usage

Asking Something

To ask something, make a GET request to /something/ask. This endpoint will use the SomeService to respond with a friendly message.

Example Response

"Slm, nbr?"

---

Controller

SomeController handles the HTTP requests. It uses constructor injection to receive an instance of SomeService.

@RestController
@RequestMapping("/something")
class SomeController {

    private final SomeService someService;

  public SomeController(SomeService someService) {
        this.someService = someService;
    }

    @GetMapping("/ask")
    ResponseEntity<String> askSomething() {
        String something = someService.askSomething();
        return ResponseEntity.ok(something);
    }

}

Service

SomeService is an interface that defines the contract for asking something.

public interface SomeService {

    String askSomething();

}

Service Implementation

SomeServiceImpl provides the actual implementation of SomeService.

@Service
class SomeServiceImpl implements SomeService {

    @Override
    public String askSomething() {
        return "Slm, nbr?";
    }

}

---

📙 Dependency Injection Best Practices

• Use Constructor Injection: Preferred over field or setter injection as it promotes immutability and ensures that the dependency is provided when the object is created.
• Avoid Circular Dependencies: Design your services to avoid circular references, which can lead to complicated dependency graphs and potential application startup issues.
• Use Interfaces: Inject interfaces rather than concrete implementations to adhere to the Dependency Inversion Principle and make your code more flexible and testable.
• Promote Single Responsibility: Each class should have one responsibility. For example, the controller handles HTTP requests, and the service handles business logic.

Class Diagram

┌──────────────────────┐
│ SomeController       │
├──────────────────────┤
│ - someService        │
└───────────┬──────────┘
            │
            │
┌───────────┴──────────────┐
│ SomeService              │
├──────────────────────────┤
│ + askSomething(): String │
└───────────┬──────────────┘
            │
            │
┌───────────┴──────────────┐
│ SomeServiceImpl          │
├──────────────────────────┤
│ + askSomething(): String │
└──────────────────────────┘

In this diagram:

• SomeController depends on SomeService.
• SomeServiceImpl implements SomeService.