Spring Boot Constructor Injection Explained with Step-by-Step Example

Learn everything about Constructor Injection in Spring Boot, its advantages, and how to implement it with real-world examples. Master Dependency Injection the right way!

In this article, we’ll walk through:

• What Constructor Injection is
• Why is it recommended
• Step-by-step implementation with code
• Real-world use case
• Best practices

What is Constructor Injection in Spring Boot?

Constructor Injection means injecting dependencies through the class constructor.

Instead of:

@Autowired
private UserService userService;

You write:

private final UserService userService;

public UserController(UserService userService) {
    this.userService = userService;
}

Spring automatically recognizes and injects the dependency without needing the @Autowired annotation (since Spring 4.3) — as long as there’s only one constructor.

✅ Why Prefer Constructor Injection?