/spring-boot-dependency-injection

name: spring-boot-dependency-injection
description: Provides dependency injection patterns for Spring Boot projects, including constructor-first design, optional collaborator handling, bean selection, and wiring validation. Use when creating services and configurations, replacing field injection, or troubleshooting ambiguous or fragile Spring wiring.
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Spring Boot Dependency Injection

Overview

Provides constructor-first dependency injection patterns for Spring Boot:

• mandatory collaborators via constructor injection
• optional collaborators via `ObjectProvider` or no-op fallbacks
• bean selection via `@Primary` and `@Qualifier`
• validation via minimal context tests before full integration

When to Use

Use this skill when:

• creating a new `@Service`, `@Component`, `@Repository`, or `@Configuration` class
• replacing field injection in legacy Spring code
• resolving multiple beans of the same type with qualifiers or primary beans
• handling optional features, adapters, or integrations without null-driven wiring
• reviewing circular dependencies or brittle context startup failures
• preparing code for direct constructor-based unit testing

Instructions

1. Separate mandatory and optional collaborators

For each class, identify:

• mandatory collaborators required for correct behavior
• optional collaborators that enable integrations, caching, notifications, or feature-flagged behavior

Mandatory collaborators belong in the constructor. Optional ones need an explicit strategy such as `ObjectProvider`, conditional beans, or a no-op implementation.

2. Default to constructor injection

For application services and adapters:

• inject mandatory dependencies through the constructor
• keep injected fields `final`
• instantiate the class directly in unit tests without starting Spring

A single constructor is usually enough; `@Autowired` is unnecessary in that case.

3. Resolve optional behavior intentionally

Good options include:

• `ObjectProvider<T>` when lazy access is useful
• `@ConditionalOnProperty` or `@ConditionalOnMissingBean` when wiring should change by configuration
• a no-op implementation when the caller should not care whether the feature is enabled

Avoid nullable collaborators that leave runtime behavior ambiguous.

4. Use bean selection annotations only when needed

When multiple beans share the same type:

• use `@Primary` for the default implementation
• use `@Qualifier` for named variants
• keep the qualifier names stable and easy to grep

If selection rules become complex, move them into a dedicated configuration class instead of spreading them across services.

5. Keep wiring in configuration, not business code

Use `@Configuration` and `@Bean` methods when:

• the object comes from a third-party library
• conditional creation logic is needed
• you need environment-specific wiring or explicit composition

Business services should not know how infrastructure collaborators are instantiated.

6. Validate wiring explicitly

After writing a new service or configuration:

1. **Verify the bean loads** with a minimal context test:

   @SpringBootTest
   @ContextConfiguration(classes = UserService.class)
   class UserServiceWiringTest {
       @Autowired UserService userService;
       @Test void serviceIsInstantiated() { assertNotNull(userService); }
   }

2. **Run constructor-based unit tests** for service behavior (no Spring needed). 3. **Add slice tests** only when MVC, JPA, or messaging integration must be verified. 4. **Reserve `@SpringBootTest`** for container-wide wiring validation.

Failures at step 1 indicate wiring issues before business logic is added.

Examples

Example 1: Constructor-first application service

@Service
public class UserService {

    private final UserRepository userRepository;
    private final EmailSender emailSender;

    public UserService(UserRepository userRepository, EmailSender emailSender) {
        this.userRepository = userRepository;
        this.emailSender = emailSender;
    }

    public User register(UserRegistrationRequest request) {
        User user = userRepository.save(User.from(request));
        emailSender.sendWelcome(user);
        return user;
    }
}

This class is easy to instantiate directly in a unit test with mocks.

Example 2: Optional dependency with a no-op fallback

@Service
public class ReportService {

    private final ReportRepository reportRepository;
    private final NotificationGateway notificationGateway;

    public ReportService(
        ReportRepository reportRepository,
        ObjectProvider<NotificationGateway> notificationGatewayProvider
    ) {
        this.reportRepository = reportRepository;
        this.notificationGateway = notificationGatewayProvider.getIfAvailable(NotificationGateway::noOp);
    }
}

This keeps optional behavior explicit without leaking `null` handling through the rest of the class.

Example 3: Multiple beans with clear selection

@Configuration
public class PaymentConfiguration {

    @Bean
    @Primary
    PaymentGateway stripeGateway() {
        return new StripePaymentGateway();
    }

    @Bean
    @Qualifier("fallbackGateway")
    PaymentGateway mockGateway() {
        return new MockPaymentGateway();
    }
}

Use `@Primary` for the default path and `@Qualifier` only where a specific variant is required.

Best Practices

• Prefer constructor injection for mandatory dependencies.
• Keep service constructors small; if a class needs too many collaborators, the design probably wants another abstraction.
• Use no-op or conditional beans instead of nullable optional dependencies.
• Keep framework-specific creation logic in configuration classes.
• Test services without Spring first, then add container tests only where they add value.
• Remove field injection during refactors instead of extending it.

Constraints a