@Autowired vs Constructor Injection - which is better in Spring Boot?

Question

I've seen two ways to inject dependencies in Spring Boot:

```java
// Field Injection
@Autowired
private UserService userService;

// Constructor Injection
private final UserService userService;

public MyClass(UserService userService) {
    this.userService = userService;
}
```

Which one is recommended and why? I see field injection used a lot in tutorials but some people say it's bad practice.

micro_king · Accepted Answer

**Constructor Injection is strongly recommended.** Here's why:

**Advantages of Constructor Injection:**

1. **Testability** - Easy to instantiate with mocks in unit tests
2. **Immutability** - Can declare fields as `final`
3. **Null Safety** - Dependencies must be provided (can't be null accidentally)
4. **Circular Dependency Detection** - Detected at startup, not runtime
5. **No Reflection** - No need for @Autowired reflection

**Example with Lombok:**
```java
@Service
@RequiredArgsConstructor
public class UserService {
    private final UserRepository userRepository;
    private final EmailService emailService;
    // Constructor generated by Lombok
}
```

**Field Injection problems:**
```java
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;  // Can be null!

// Hard to unit test - need reflection or Spring
}
```

**Spring Boot 4.x+ Recommendation:**
```java
// Single constructor - @Autowired is optional
@Service
public class UserService {
    private final UserRepository repository;

public UserService(UserRepository repository) {
        this.repository = repository;
    }
}
```

**Summary:** Always prefer constructor injection. Use Lombok's @RequiredArgsConstructor to reduce boilerplate.

1 Answer