Dependency Injection & IoC

The flow

1. 1.Define contract
2. 2.Create beans
3. 3.Inject constructor
4. 4.Use service
5. 5.Close context

Invert construction responsibilities

A service defines the repository contract it needs; configuration chooses the implementation. The service does not instantiate a database client or look up the application context. Constructor injection makes required collaborators visible and supports final fields.

In the example, TopicService is ordinary Java. A small configuration creates its repository and wires the service. A unit test supplies a lambda without starting Spring; a context test checks actual bean wiring.

Scope is not a lock

The default singleton scope means one bean per Spring container, not one instance across every process and not automatic synchronization. Keep request-specific identity outside mutable singleton fields. A shared ChatClient can be safe while a shared conversation ID still leaks context.

Use qualifiers or explicit bean names when several implementations exist. A circular constructor dependency often signals responsibilities that should be separated rather than hidden through field injection.

Test the appropriate boundary

Test service decisions without a container when possible. Add a focused context test for bean selection and lifecycle. Starting a full web server for every domain test increases feedback time without necessarily exercising a new risk.

Try it

package com.interviewpreparator.lessons;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
public final class DependencyInjectionExample {
    public interface TopicRepository { String findTitle(String id); }
    public static final class TopicService {
        private final TopicRepository repository;
        public TopicService(TopicRepository repository) { this.repository = repository; }
        public String title(String id) { return repository.findTitle(id); }
    }
    @Configuration
    public static class Wiring {
        @Bean TopicRepository topics() { return id -> "Java records"; }
        @Bean TopicService service(TopicRepository repository) { return new TopicService(repository); }
    }
}

examples/spring-ai-lab/src/main/java/com/interviewpreparator/lessons/DependencyInjectionExample.java · mvn -f examples/spring-ai-lab/pom.xml test. Java 21+, Spring Boot 4.0.5, Spring AI 2.0.2 where applicable. The demo uses synthetic models; live inference quality is a separate check.

Interview questions

Official references

• Spring Framework — DI

Continue learning

• Spring Boot Configuration →
• Spring AI ChatClient →