10. Spring・Spring Framework・Spring Boot・Bean・DI コンテナ

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Constructor;
import java.util.LinkedHashMap;
import java.util.Map;

// アノテーション: クラスに付ける印。これ自体は何もしない
@Retention(RetentionPolicy.RUNTIME)
@interface Component {}

@Component
class Clock {
    String now() {
        return "12:00";
    }
}

@Component
class GreetingService {
    private final Clock clock;

    // 必要な相手を自分で new せず、コンストラクタの引数で受け取る(依存性の注入)
    GreetingService(Clock clock) {
        this.clock = clock;
    }

    String greet() {
        return "hello at " + clock.now();
    }
}

// 印が無いクラス
class Helper {}

// DI コンテナの極小の模型(Spring の実装ではない)。
// 印の付いたクラスだけを作り、名前を付けて持っておく
class MiniContainer {
    private final Map<String, Object> beans = new LinkedHashMap<>();

    void register(Class<?>... candidates) throws Exception {
        for (Class<?> c : candidates) {
            if (!c.isAnnotationPresent(Component.class)) {
                System.out.println("skip     " + c.getSimpleName() + " (no @Component)");
                continue;
            }
            Constructor<?> ctor = c.getDeclaredConstructors()[0];
            Class<?>[] types = ctor.getParameterTypes();
            Object[] args = new Object[types.length];
            for (int i = 0; i < types.length; i++) {
                args[i] = getBean(types[i]); // 引数の型で、持っている Bean を探して渡す
            }
            String name = Character.toLowerCase(c.getSimpleName().charAt(0)) + c.getSimpleName().substring(1);
            beans.put(name, ctor.newInstance(args));
            System.out.println("register " + name);
        }
    }

    <T> T getBean(Class<T> type) {
        for (Object bean : beans.values()) {
            if (type.isInstance(bean)) {
                return type.cast(bean);
            }
        }
        throw new IllegalStateException("no bean of type " + type.getSimpleName());
    }

    boolean contains(Object o) {
        return beans.containsValue(o);
    }
}

class Main {
    public static void main(String[] args) throws Exception {
        MiniContainer container = new MiniContainer();
        container.register(Clock.class, GreetingService.class, Helper.class);

        GreetingService fromContainer = container.getBean(GreetingService.class);
        System.out.println(fromContainer.greet());

        // 同じクラスでも、自分で new したものはコンテナの管理の外にある
        GreetingService byHand = new GreetingService(new Clock());
        System.out.println("fromContainer is a bean: " + container.contains(fromContainer));
        System.out.println("byHand is a bean       : " + container.contains(byHand));
        System.out.println("same object each time  : "
                + (container.getBean(GreetingService.class) == fromContainer));
    }
}