IoC container mental model in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when Spring Boot starts the IoC container?

When a Spring Boot application starts, what does the IoC container do first?

A It deletes all existing beans and starts with an empty container.

B It scans for classes annotated with @Component, @Service, @Repository, and creates their instances as beans.

C It waits for user input before loading any beans.

D It immediately runs all @PostConstruct methods before creating any beans.

💡 Hint

Think about how Spring finds and prepares the parts of your app it manages.

The IoC container scans the classpath for classes marked with special annotations like @Component and creates their instances as beans to manage them.

❓ state_output

intermediate

What is the state of a bean after Spring IoC container finishes initialization?

After the Spring IoC container finishes initializing, what is true about the beans it manages?

A Beans are created but their lifecycle methods like @PostConstruct are not called.

B Beans are only created when first requested by the application code.

C Beans exist but have no dependencies injected yet.

D All singleton beans are created and ready to use, with dependencies injected.

💡 Hint

Consider the default scope of beans and when dependencies are set.

By default, singleton beans are created eagerly at startup with all dependencies injected and lifecycle methods called.

📝 Syntax

advanced

Which code snippet correctly defines a Spring bean with constructor injection?

Choose the correct way to define a Spring bean using constructor injection in a @Component class.

A

@Component
public class ServiceA {
  private final Repo repo;
  public ServiceA(Repo repo) { this.repo = repo; }
}

B

@Component
public class ServiceA {
  @Autowired
  private Repo repo;
  public ServiceA() {}
}

C

@Component
public class ServiceA {
  private Repo repo;
  public void setRepo(Repo repo) { this.repo = repo; }
}

D

@Component
public class ServiceA {
  private Repo repo;
  public ServiceA() { repo = new Repo(); }
}

💡 Hint

Constructor injection means passing dependencies through the constructor parameters.

Option A uses constructor injection correctly by defining a constructor with the dependency parameter. Spring will inject the Repo bean automatically.

🔧 Debug

advanced

Why does this Spring bean fail to initialize?

Given this Spring bean code, why does the application fail to start?

@Component
public class MyService {
  private final MyRepository repo;
  public MyService() {}
}

A Because the constructor does not accept MyRepository, Spring cannot inject the dependency.

B Because @Component is missing, Spring does not detect the class.

C Because MyRepository is not annotated with @Service, injection fails.

D Because the class has no @Autowired annotation on the constructor.

💡 Hint

Think about how Spring injects dependencies when constructors are present.

Spring uses constructor injection by default if a constructor with parameters exists. Here, the no-arg constructor prevents injection, so the dependency is never set.

🧠 Conceptual

expert

How does Spring IoC container manage bean lifecycle callbacks?

Which statement best describes how the Spring IoC container manages lifecycle callbacks like @PostConstruct and DisposableBean?

A Spring ignores lifecycle annotations and requires manual calls to lifecycle methods.

B Spring calls @PostConstruct methods before creating the bean instance, and DisposableBean methods immediately after creation.

C Spring calls @PostConstruct methods after creating and injecting dependencies, and calls DisposableBean methods before destroying singleton beans.

D Spring only calls lifecycle methods if the bean is prototype scoped.

💡 Hint

Think about when initialization and destruction happen in the bean lifecycle.

Spring calls @PostConstruct after the bean is fully created and dependencies injected. It calls DisposableBean methods before destroying singleton beans during container shutdown.