Dependency Injection in Java Use Cases and Examples in Java

DI use case: A situation where externalizing collaborator selection improves ownership, substitution, or lifecycle clarity.

Dependency injection helps most where infrastructure choices and domain behavior should evolve separately.

Good Java Use Cases

Dependency injection is especially useful for:

• services that depend on repositories or clients
• application layers that vary by environment
• code that needs focused tests without booting the whole stack
• decorators such as logging, metrics, tracing, or caching around a stable contract

Example:

1public final class PaymentService {
2    private final PaymentGateway gateway;
3
4    public PaymentService(PaymentGateway gateway) {
5        this.gateway = gateway;
6    }
7}

The service can use a real gateway in production and a fake or stubbed one in tests.

Weak Use Cases

DI is a weak trade when:

• the dependency is a stateless utility method
• the class only wraps one trivial collaborator with no additional behavior
• interfaces exist only because “DI requires them”
• wiring becomes more complex than the actual logic

Not every new call is an architectural problem.

Example: Metrics Decorator

Dependency injection makes layered behavior easier:

1PaymentGateway gateway = new MetricsPaymentGateway(
2        new RetryingPaymentGateway(realGateway));

The service still depends on the PaymentGateway abstraction. Wiring decides which concrete composition to supply.

Design Review Questions

When reviewing DI use cases, ask:

• What change is DI isolating?
• Is the dependency a meaningful seam or just a small helper?
• Would direct construction actually be simpler here?
• Is wiring complexity proportional to the real need?

The best DI use cases have a clear reason for separating choice of collaborator from use of collaborator.