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Explain Spring Dependency Injection and Choose an Injection Style

SchwabJava Developer · Onsite
EasyJava DeveloperOnsiteSoftware Engineering Fundamentals

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Tests core knowledge of Spring's Inversion of Control (IoC) container, trade-offs between constructor, setter, and field injection, bean lifecycle, and dependency resolution strategies.

Scenario

In enterprise Java applications, managing object creation and decoupling business logic from infrastructure requires a robust dependency management strategy. As a Java developer working on mission-critical financial services, you are asked to review an architectural standard for Spring-based microservices and explain core Spring Dependency Injection (DI) principles.

Discussion Points

Structure your explanation to address the following technical topics:

1. Spring IoC and Bean Lifecycle

  • Explain the role of the ApplicationContext in discovering, instantiating, and wiring beans.
  • Describe the phase where dependency injection takes place relative to bean initialization hooks (such as @PostConstruct or InitializingBean).

2. Injection Styles Comparison

Compare the three standard styles of dependency injection in Spring:

  • Constructor Injection
  • Setter Injection
  • Field Injection (e.g., @Autowired directly on private fields)

Evaluate each style against the following criteria:

  • Handling required vs. optional dependencies.
  • Immutability (e.g., using final fields).
  • Ease of unit testing without starting a Spring application context.
  • Protection against NullPointerException during runtime initialization.

3. Disambiguating Multiple Implementations

  • When an interface has multiple bean implementations (for example, different payment gateways or auditing sinks), explain how to control which bean is injected using @Primary, @Qualifier, or custom qualifier annotations.

4. Bean Scopes and Thread Safety

  • Differentiate between singleton (default) and prototype scopes.
  • Explain the thread-safety risks of storing mutable state in singleton beans and how to properly design stateless services.

5. Circular Dependencies

  • What causes a circular dependency (BeanA depends on BeanB, which depends on BeanA)?
  • What architectural code smells does a circular dependency typically indicate, and how should it be refactored instead of relying on lazy resolution (@Lazy)?

Hints

Hint 1

Consider how constructor injection enables you to mark class fields as `final` and easily pass mock objects in plain JUnit tests without reflection.

Hint 2

Think about how the Spring container initializes singleton beans once on startup and shares that single instance across all concurrent HTTP worker threads.

Hint 3

When two classes depend on each other, it often indicates a violation of the Single Responsibility Principle that can be solved by extracting a third shared service.

Follow-up questions

  • How does Spring resolve the scope mismatch when a singleton bean injects a prototype-scoped bean, and what is the recommended way to get a new instance each time (e.g., `@Lookup` or `ObjectProvider`)?
  • When is explicit Java-based configuration (`@Configuration` with `@Bean`) preferred over component scanning (`@Component`, `@Service`)?
  • How does Spring Boot 2.6+ handle circular dependencies by default, and why was this change introduced?