Dependency Injection API Rules

name: Dependency Injection Rules
description: Rules and guidance for applying the Dependency Injection design pattern.
version: 1.0.0
created: 2026-04-28
modified: 2026-04-28
rules:
  - id: DI-001
    name: Prefer Constructor Injection
    severity: warning
    description: Inject required collaborators through the constructor by default.
    rationale: Required dependencies become explicit and the resulting object is always in a valid state.

  - id: DI-002
    name: Use Setter Injection for Optional Dependencies
    severity: info
    description: Reserve setter injection for genuinely optional collaborators or post-construction reconfiguration.
    rationale: Optional setters keep the constructor focused while signaling that the dependency may be absent.

  - id: DI-003
    name: Depend on Abstractions
    severity: error
    description: Type constructor parameters and fields against interfaces, not concrete classes.
    rationale: Programming to interfaces preserves substitutability and unlocks test doubles.

  - id: DI-004
    name: No new Inside Business Logic
    severity: warning
    description: Avoid instantiating collaborators with new inside methods that hold business logic.
    rationale: Hidden construction couples logic to specific implementations and frustrates testing.

  - id: DI-005
    name: Avoid Service Locator in Domain Code
    severity: warning
    description: Domain and application services should not call a service locator to fetch collaborators.
    rationale: Locators hide dependencies and reintroduce the coupling DI removes.

  - id: DI-006
    name: Composition Root at the Edge
    severity: error
    description: Wire the object graph in a single composition root at application startup.
    rationale: Centralizing wiring keeps the rest of the codebase free of container concerns.

  - id: DI-007
    name: Limit Constructor Parameter Count
    severity: warning
    description: A constructor that requires more than five collaborators signals a violation of the single responsibility principle.
    rationale: A bloated constructor reveals an over-scoped class that should be decomposed.

  - id: DI-008
    name: Forbid Field Injection in Production Code
    severity: warning
    description: Avoid annotation-driven field injection on private fields outside frameworks that require it.
    rationale: Field injection hides dependencies, prevents immutability, and breaks plain-object construction in tests.

  - id: DI-009
    name: Scope Lifetimes Explicitly
    severity: info
    description: Declare singleton, scoped, and transient lifetimes intentionally; do not rely on container defaults.
    rationale: Implicit lifetimes produce subtle threading and state-sharing bugs.

  - id: DI-010
    name: Avoid Circular Dependencies
    severity: error
    description: Detect and break circular dependencies between injected components.
    rationale: Cycles indicate a layering mistake and often manifest as initialization order bugs.

  - id: DI-011
    name: Configuration Separate from Usage
    severity: warning
    description: Externalize wiring configuration from the classes that consume the dependencies.
    rationale: Decoupling configuration from logic is the foundational principle that DI exists to express.

  - id: DI-012
    name: Test Without the Container
    severity: info
    description: Unit tests should construct objects directly with test doubles rather than booting the DI container.
    rationale: Container-free tests run faster and prove that injected classes can be instantiated explicitly.