UpdatedAug 5, 2026

Spring Boot Dependency Injection

Question Variations

"Why is constructor injection preferred over field injection?"
"How do you inject one of several implementations?"
"When should a bean be request-scoped?"

Why This Is Asked

Dependency injection is central to Spring design. This question evaluates constructor injection, bean ownership, and how to resolve multiple implementations safely.

Key Concepts

Constructor injection makes required dependencies explicit and testable.
Components and @Bean methods define container-managed objects.
@Primary and @Qualifier disambiguate multiple candidates.
Bean scope should match lifecycle and concurrency needs.

Question Variations

“Why is constructor injection preferred over field injection?”
“How do you inject one of several implementations?”
“When should a bean be request-scoped?”

Answers by Technology

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Expected Answer (Spring Boot 4.1.0 / Java 17+)

Prefer constructor injection for required dependencies. It makes the object valid after construction, supports immutable fields, and permits a plain unit test to provide fakes without Spring. Spring discovers stereotype components or invokes @Bean methods to create managed instances. If several beans implement the same interface, use @Qualifier for an explicit choice or @Primary for a sensible default.

Why It Matters

Explicit dependencies make component lifecycles, tests, and wiring failures much easier to reason about.

Code Example

@Service
class InvoiceService {
  private final TaxCalculator taxes;
  InvoiceService(@Qualifier("standardTax") TaxCalculator taxes) { this.taxes = taxes; }
}

Common Mistakes

Using field injection: Dependencies are hidden and tests need reflection or a container.
Resolving ambiguity by injecting a random implementation: Behavior changes as more beans are added.

Follow-up Questions

When use @Primary? (Answer: When one implementation is the normal default.)
Why might request scope be needed? (Answer: For state that must exist once per HTTP request.)