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Spring Boot Dependency Injection

Overview

Use this skill to keep Spring Boot wiring explicit, testable, and predictable.

The core model is simple:

• constructor injection for mandatory collaborators
• explicit strategies for optional collaborators
• configuration classes that make bean selection obvious

This skill is about shaping code so it remains easy to test without depending on the Spring container for every unit test.

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

Typical trigger phrases include spring dependency injection, constructor injection, replace field injection, qualifier vs primary, and optional bean.

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 with the smallest useful test

Test in this order:

• direct constructor-based unit tests for service behavior
• slice tests when Spring MVC, JPA, or messaging integration matters
• full @SpringBootTest only when container-wide wiring must be proven

This keeps failures localized and makes dependency problems obvious.

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

Weekly Installs

360

Repository

giuseppe-trisci…oper-kit

GitHub Stars

173

First Seen

Feb 3, 2026

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