A Tiny DI Container in Plain Java

How Does Spring Do It?

Spring's real container is large, but the core idea fits in a page: scan for annotated classes, read each constructor with reflection, create the dependencies first (recursively), then create the bean and keep it as a singleton.

Find classes marked with @Component.
For each, look at the constructor parameter types.
Get (or create) a bean for each parameter type. For an interface, find the component that implements it.
Call the constructor with those beans and store the result.

MiniSpring

Example

A working DI container in about 50 lines, using our own @Component annotation and reflection. Note the creation order: dependencies are built before the beans that need them.

import java.lang.annotation.*;
import java.lang.reflect.Constructor;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        MiniContainer ctx = new MiniContainer(List.of(
            OrderService.class, ProductRepository.class, EmailNotifier.class, NotAComponent.class));

        OrderService orders = ctx.getBean(OrderService.class);
        orders.placeOrder(1L, 2);
        System.out.println("Same OrderService instance? " + (orders == ctx.getBean(OrderService.class)));
        System.out.println("Beans in container: " + ctx.beanNames());
    }
}

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE)
@interface Component {}

class MiniContainer {
    private final Map<Class<?>, Object> singletons = new LinkedHashMap<>();
    private final List<Class<?>> components;

    MiniContainer(List<Class<?>> scanned) throws Exception {
        components = scanned.stream().filter(c -> c.isAnnotationPresent(Component.class)).toList();
        for (Class<?> c : components) getBean(c);                 // eager creation, like Spring singletons
    }

    <T> T getBean(Class<T> type) throws Exception {
        Class<?> impl = components.stream().filter(type::isAssignableFrom).findFirst()
            .orElseThrow(() -> new IllegalStateException("No bean of type " + type.getSimpleName()));
        if (singletons.containsKey(impl)) return type.cast(singletons.get(impl));

        Constructor<?> ctor = impl.getDeclaredConstructors()[0];
        Object[] deps = new Object[ctor.getParameterCount()];
        for (int i = 0; i < deps.length; i++) deps[i] = getBean(ctor.getParameterTypes()[i]);   // recursion

        Object bean = ctor.newInstance(deps);
        singletons.put(impl, bean);
        System.out.println("Created bean: " + impl.getSimpleName());
        return type.cast(bean);
    }

    List<String> beanNames() { return singletons.keySet().stream().map(Class::getSimpleName).toList(); }
}

interface Notifier { void send(String msg); }

@Component class EmailNotifier implements Notifier {
    public void send(String msg) { System.out.println("EMAIL: " + msg); }
}

@Component class ProductRepository {
    String findName(long id) { return id == 1L ? "Boat Rockerz 450" : "Unknown"; }
}

@Component class OrderService {
    private final ProductRepository repo;
    private final Notifier notifier;
    OrderService(ProductRepository repo, Notifier notifier) { this.repo = repo; this.notifier = notifier; }
    void placeOrder(long productId, int qty) { notifier.send("Order placed: " + qty + " x " + repo.findName(productId)); }
}

class NotAComponent {}       // no annotation: ignored by the scan

Output

Created bean: ProductRepository
Created bean: EmailNotifier
Created bean: OrderService
EMAIL: Order placed: 2 x Boat Rockerz 450
Same OrderService instance? true
Beans in container: [ProductRepository, EmailNotifier, OrderService]

What Real Spring Adds on Top

Classpath scanning (finds classes in packages without a list).
Circular dependency detection with a clear error message.
@Qualifier, @Primary, scopes, lifecycle callbacks, @Value injection.
Proxies around beans so that @Transactional, @Cacheable, @Async and security work without changing your code.

Exercise: Make the container fail clearly when a class needs a dependency that is not a component. What happens if you remove EmailNotifier from the list?

Show answer

// getBean(Notifier.class) finds no component implementing Notifier, so it throws:
// java.lang.IllegalStateException: No bean of type Notifier
// Spring gives the same kind of error at startup:
// "Parameter 1 of constructor in OrderService required a bean of type 'Notifier' that could not be found."