2.6. Dependency Injection¶
2.6.1. Purpose¶
To implement a loosely coupled architecture in order to get better testable, maintainable and extendable code.
2.6.2. Usage¶
DatabaseConfiguration
gets injected and DatabaseConnection
will get all that it
needs from $config
. Without DI, the configuration would be created
directly in DatabaseConnection
, which is not very good for testing and
extending it.
2.6.3. Examples¶
- The Doctrine2 ORM uses dependency injection e.g. for configuration
that is injected into a
Connection
object. For testing purposes, one can easily create a mock object of the configuration and inject that into theConnection
object - many frameworks already have containers for DI that create objects via a configuration array and inject them where needed (i.e. in Controllers)
2.6.4. UML Diagram¶

2.6.5. Code¶
You can also find this code on GitHub
DatabaseConfiguration.php
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 | <?php
declare(strict_types=1);
namespace DesignPatterns\Structural\DependencyInjection;
class DatabaseConfiguration
{
public function __construct(
private string $host,
private int $port,
private string $username,
private string $password
) {
}
public function getHost(): string
{
return $this->host;
}
public function getPort(): int
{
return $this->port;
}
public function getUsername(): string
{
return $this->username;
}
public function getPassword(): string
{
return $this->password;
}
}
|
DatabaseConnection.php
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | <?php
declare(strict_types=1);
namespace DesignPatterns\Structural\DependencyInjection;
class DatabaseConnection
{
public function __construct(private DatabaseConfiguration $configuration)
{
}
public function getDsn(): string
{
// this is just for the sake of demonstration, not a real DSN
// notice that only the injected config is used here, so there is
// a real separation of concerns here
return sprintf(
'%s:%s@%s:%d',
$this->configuration->getUsername(),
$this->configuration->getPassword(),
$this->configuration->getHost(),
$this->configuration->getPort()
);
}
}
|
2.6.6. Test¶
Tests/DependencyInjectionTest.php
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | <?php
declare(strict_types=1);
namespace DesignPatterns\Structural\DependencyInjection\Tests;
use DesignPatterns\Structural\DependencyInjection\DatabaseConfiguration;
use DesignPatterns\Structural\DependencyInjection\DatabaseConnection;
use PHPUnit\Framework\TestCase;
class DependencyInjectionTest extends TestCase
{
public function testDependencyInjection()
{
$config = new DatabaseConfiguration('localhost', 3306, 'domnikl', '1234');
$connection = new DatabaseConnection($config);
$this->assertSame('domnikl:1234@localhost:3306', $connection->getDsn());
}
}
|