Constructor injection (preferred) in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is constructor injection in Spring Boot?

Constructor injection is a way to provide dependencies to a class by passing them through its constructor. Spring Boot automatically calls the constructor and injects the required beans.

intermediate

Why is constructor injection preferred over field injection in Spring Boot?

Constructor injection is preferred because it makes dependencies explicit, helps with immutability, supports easier testing, and avoids issues with uninitialized fields.

intermediate

How does Spring Boot know which constructor to use for injection?

Spring Boot uses the constructor with the most parameters that can be satisfied by beans in the context. If there is only one constructor, it uses that by default.

beginner

Show a simple example of constructor injection in a Spring Boot service class.

import org.springframework.stereotype.Service;

@Service
public class MyService {
  private final MyRepository repo;

  public MyService(MyRepository repo) {
    this.repo = repo;
  }

  // service methods
}

intermediate

What happens if a required dependency is missing when using constructor injection?

Spring Boot will fail to start and throw an error because it cannot find a bean to inject into the constructor parameter.

Which of these is a benefit of constructor injection in Spring Boot?

A Automatically injects private fields without constructor

B Allows setting dependencies after object creation

C Requires less code than field injection

D Dependencies are clearly required and immutable

Constructor injection makes dependencies explicit and supports immutability, unlike field injection.

How does Spring Boot choose which constructor to use for injection?

A The constructor with the most parameters that can be injected

B The constructor with no parameters

C The constructor annotated with @Autowired only

D It randomly picks one constructor

Spring Boot uses the constructor with the most parameters that can be satisfied by beans.

What happens if a dependency is missing for constructor injection?

A Spring Boot throws an error and fails to start

B Spring Boot ignores the missing dependency

C Spring Boot uses a default value

D Spring Boot logs a warning but continues

Missing dependencies cause Spring Boot to fail startup with an error.

Which annotation is required on the constructor for injection in Spring Boot 2.6+?

A @Component is required on constructor

B No annotation needed if only one constructor

C @Inject is mandatory

D @Autowired is always required

If there is only one constructor, Spring Boot injects dependencies without needing @Autowired.

Constructor injection helps with which of the following?

A Reducing constructor parameters

B Allowing circular dependencies easily

C Making dependencies immutable

D Injecting dependencies after object creation

Constructor injection supports immutability by setting dependencies once at creation.

Explain constructor injection in Spring Boot and why it is preferred over other injection methods.

Think about how dependencies are passed and why that helps code quality.

Describe what happens if Spring Boot cannot find a bean to inject into a constructor parameter.

Consider how Spring Boot handles required dependencies.