Spring Boot Dependency Injection Skill

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:

Verify the bean loads with a minimal context test:

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

Run constructor-based unit tests for service behavior (no Spring needed).
Add slice tests only when MVC, JPA, or messaging integration must be verified.
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 and Warnings

Field injection hides dependencies and makes tests harder to write.
Circular dependencies are usually a design problem, not a wiring trick to solve with @Lazy.
Overusing qualifiers can make the codebase hard to reason about; prefer better abstractions or clearer configuration.
Optional collaborators still need deterministic behavior when absent.
Full-context tests can hide the real source of wiring failures if used too early.

References

references/reference.md
references/examples.md
references/spring-official-dependency-injection.md

Related Skills

spring-boot-crud-patterns
spring-boot-rest-api-standards
unit-test-service-layer

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