Table of Contents
Item 5: PHP Best Practices - Prefer dependency injection to hardwiring resources
Introduction to Dependency Injection in [[PHP]]
In PHP, dependency injection (DI) is a design pattern that promotes loose coupling between components by injecting dependencies (such as services, objects, or resources) into a class, rather than hardwiring these dependencies directly within the class. This approach contrasts with hardwiring, where resources and dependencies are created or managed directly inside the class, leading to tightly coupled code that is harder to test, extend, and maintain. By preferring dependency injection over hardwiring resources, you can achieve more modular, testable, and maintainable code.
Advantages of Dependency Injection in [[PHP]]
Preferring dependency injection over hardwiring resources offers several key advantages: 1. **Improved Testability**: DI allows you to easily replace real implementations with mocks or stubs during testing, making unit tests more isolated and reliable. 2. **Loose Coupling**: DI decouples classes from their dependencies, allowing them to evolve independently. This results in a more flexible and maintainable codebase. 3. **Simplified Configuration Management**: DI frameworks or patterns allow centralized management of dependencies, reducing complexity and making configuration changes easier. 4. **Better Separation of Concerns**: By separating the creation of dependencies from their usage, you adhere to the single responsibility principle, leading to more focused and maintainable classes.
Example 1: Hardwiring vs. Dependency Injection in a Service Class
- Hardwiring Example
```php class UserService {
private $dbConnection;
public function __construct() {
// Hardwiring the dependency
$this->dbConnection = new DatabaseConnection('localhost', 'mydb');
}
public function addUser(User $user): void {
$this->dbConnection->save($user);
}
}
```
In this example, the `UserService` class is responsible for creating its `DatabaseConnection` dependency. This tight coupling makes the `UserService` class harder to test, extend, and maintain.
- Dependency Injection Example
```php class UserService {
private $dbConnection;
public function __construct(DatabaseConnection $dbConnection) {
// Injecting the dependency
$this->dbConnection = $dbConnection;
}
public function addUser(User $user): void {
$this->dbConnection->save($user);
}
}
```
Here, the `UserService` class receives its `DatabaseConnection` dependency through its constructor. This loose coupling allows for greater flexibility and makes the class easier to test and modify.
Example 2: Using a Dependency Injection Container
In the PHP ecosystem, there are several dependency injection containers available, such as PHP-DI, Symfony DI, and Pimple. These containers help manage dependencies more effectively.
- Setting Up PHP-DI
```php use DI\ContainerBuilder;
$containerBuilder = new ContainerBuilder(); $containerBuilder→addDefinitions([
DatabaseConnection::class => \DI\create(DatabaseConnection::class)
->constructor('localhost', 'mydb'),
UserService::class => \DI\autowire(UserService::class),
]);
$container = $containerBuilder→build();
// Using the container to resolve dependencies $userService = $container→get(UserService::class); $userService→addUser(new User('John Doe')); ```
In this example, PHP-DI is used to define and resolve dependencies. The `UserService` class receives its `DatabaseConnection` dependency automatically when it is resolved from the PHP-DI container.
Example 3: Constructor Injection vs. Setter Injection
Dependency injection in PHP can be implemented in different ways, with constructor injection and setter injection being the most common methods.
- Constructor Injection (Preferred)
```php class OrderService {
private $paymentService;
public function __construct(PaymentService $paymentService) {
$this->paymentService = $paymentService;
}
public function processOrder(Order $order): void {
$this->paymentService->processPayment($order);
}
}
```
- Setter Injection
```php class OrderService {
private $paymentService;
public function setPaymentService(PaymentService $paymentService): void {
$this->paymentService = $paymentService;
}
public function processOrder(Order $order): void {
$this->paymentService->processPayment($order);
}
}
```
Constructor injection is generally preferred over setter injection because it makes dependencies explicit and ensures that the class is never in an invalid state. Constructor injection also promotes immutability, as the dependencies are typically set only once via the constructor.
Example 4: Testing with Dependency Injection
One of the main benefits of dependency injection is the ability to test classes more effectively by injecting mock or stub dependencies.
- Testing a Class with Mock Dependencies
```php use PHPUnit\Framework\TestCase;
class UserServiceTest extends TestCase {
public function testAddUserCallsSaveOnDatabaseConnection(): void {
$mockDbConnection = $this->createMock(DatabaseConnection::class);
$mockDbConnection->expects($this->once())
->method('save')
->with($this->isInstanceOf(User::class));
$userService = new UserService($mockDbConnection);
$user = new User('John Doe');
$userService->addUser($user); }} ```
In this example, a mock `DatabaseConnection` is injected into the `UserService` for testing purposes. This allows you to test the `UserService` without relying on a real database connection, making your tests faster and more reliable.
When to Prefer Dependency Injection in [[PHP]]
Dependency injection is particularly useful in the following scenarios: - **Complex Applications**: In large or complex applications, DI helps manage the interdependencies between classes more effectively. - **Test-Driven Development (TDD)**: If you follow TDD practices, DI makes it easier to create testable classes by allowing dependencies to be injected as mocks or stubs. - **Web Applications**: When building web applications with frameworks like Symfony or Laravel, DI helps manage configuration and external resources like databases or external APIs. - **Modular Architectures**: DI is beneficial in systems designed with modular components, where dependencies need to be loosely coupled and easily interchangeable.
Conclusion
In PHP, preferring dependency injection over hardwiring resources is a best practice that leads to more maintainable, testable, and flexible code. By injecting dependencies, you decouple your classes from their dependencies, making it easier to manage and extend your application. This approach aligns well with modern PHP development practices, especially when using frameworks like Symfony, Laravel, or PHP-DI that support DI.