Constructor injection makes required collaborators explicit and supports ordinary object construction in tests. It also reveals excessive responsibility.
Before you start
You should know Java classes, dependency injection and basic HTTP requests. Identify where a call crosses a framework-managed boundary. The snippets illustrate a focused mechanism; database configuration, application wiring and authentication must be supplied by the surrounding application when applicable.
The practical goal is to reason through this situation: An order service receives repository and payment gateway dependencies through its constructor. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: List required collaborators
Put mandatory dependencies in the constructor rather than hidden field injection.
Step 2: Construct a focused unit
Supply controlled repository and payment adapters without starting Spring.
Step 3: Review responsibility
Many unrelated dependencies suggest the service needs narrower responsibilities.
Worked scenario
An order service receives repository and payment gateway dependencies through its constructor.
An order service receives a fake repository and gateway. The test makes the gateway fail, then inspects whether the service persisted the correct domain outcome. Ordinary construction tests business decisions; a separate framework integration test verifies that Spring wires the intended collaborators.
Common mistake
A huge constructor often indicates too many responsibilities rather than a reason to hide dependencies.
Verify the behavior
Test success and collaborator failure, then verify application wiring independently.
Interview exercise
Test without starting the application.
Answer and reasoning
Construct the service using controlled collaborators and verify its domain behavior directly.
Continue learning
Compare the scenario with the Spring Boot interview questions and test your understanding with the Spring Boot MCQs. For terminology and implementation details, consult the reference material.