Dependency Injection

package at.aydin.lab.di.legacy;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

// Design Pattern: Service Locator
// Zweck: Zentrale, manuelle Auflösung von Abhängigkeiten in Legacy-Anwendungen.
public final class ServiceRegistry {
    private static final Map<Class<?>, Object> SERVICES = new ConcurrentHashMap<>();
    private ServiceRegistry() {
    }

    public static <T> void register(Class<T> type, T implementation) {
        SERVICES.put(type, implementation);
    }

    public static <T> T resolve(Class<T> type) {
        Object service = SERVICES.get(type);
        if (service == null) {
            throw new IllegalStateException("Kein Service registriert: " + type.getName());
        }
        return type.cast(service);
    }
}

package at.aydin.lab.di.modern;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DiModernApplication {
    public static void main(String[] args) {
        SpringApplication.run(DiModernApplication.class, args);
    }
}

package at.aydin.lab.di.legacy;

public interface GreetingService {
    String greet(String name);
}

package at.aydin.lab.di.modern;

// Design Pattern: Port (Hexagonal Architecture)
// Zweck: Die Anwendung hängt von einem fachlichen Vertrag statt einer konkreten Implementierung ab.
public interface GreetingPort {
    String greet(String name);
}

package at.aydin.lab.di.legacy;

public final class DefaultGreetingService implements GreetingService {
    @Override
    public String greet(String name) {
        return "Willkommen, " + name + "!";
    }
}

package at.aydin.lab.di.modern;

import org.springframework.stereotype.Service;

@Service
public class GermanGreetingService implements GreetingPort {
    @Override
    public String greet(String name) {
        return "Willkommen, " + name + "!";
    }
}

package at.aydin.lab.di.legacy;

public final class DiLegacyApplication {
    private DiLegacyApplication() {
    }

    public static void main(String[] args) {
        ServiceRegistry.register(GreetingService.class, new DefaultGreetingService());
        GreetingService service = ServiceRegistry.resolve(GreetingService.class);
        System.out.println(service.greet("Aydin"));
    }
}

package at.aydin.lab.di.modern;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DiModernApplication {
    public static void main(String[] args) {
        SpringApplication.run(DiModernApplication.class, args);
    }
}

package at.aydin.lab.di.modern;

import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class GreetingRunner implements CommandLineRunner {
    private final GreetingPort greetingPort;
    // Design Pattern: Constructor Injection
    // Zweck: Abhängigkeiten sind sichtbar, unveränderlich und leicht testbar.
    public GreetingRunner(GreetingPort greetingPort) {
        this.greetingPort = greetingPort;
    }

    @Override
    public void run(String ... args) {
        System.out.println(greetingPort.greet("Aydin"));
    }
}