Dependency Injection

You are a software architect. The user wants to set up dependency injection in their application to decouple dependencies and improve testability.

What to check first

• Verify your project has a package manager initialized (package.json for Node.js, pom.xml for Java, etc.)
• Identify all classes or modules that currently have hard dependencies on concrete implementations
• Determine whether you need a DI framework (tsyringe, inversify, Spring, Guice) or will implement manual injection

Steps

1. Choose a DI container—for TypeScript/Node.js use tsyringe or inversify; for Java use Spring Framework or Google Guice
2. Install the framework: npm install tsyringe reflect-metadata (or equivalent for your ecosystem)
3. Import reflect-metadata at your application entry point before any other code runs
4. Create an interface that defines your service contract (e.g., IUserRepository)
5. Implement concrete classes that satisfy the interface (e.g., UserRepository, MockUserRepository)
6. Register implementations in the container using decorators (@injectable(), @inject()) or manual registration
7. Inject dependencies into constructors using the decorator pattern or container.get() calls
8. Update your bootstrap/main file to resolve root services from the container instead of instantiating directly

Code

import 'reflect-metadata';
import { Container, injectable, inject } from 'tsyringe';

// Step 1: Define service interfaces
interface ILogger {
  log(message: string): void;
}

interface IUserRepository {
  getUserById(id: string): Promise<{ id: string; name: string }>;
}

// Step 2: Implement concrete classes with @injectable decorator
@injectable()
class ConsoleLogger implements ILogger {
  log(message: string): void {
    console.log(`[LOG] ${message}`);
  }
}

@injectable()
class UserRepository implements IUserRepository {
  async getUserById(id: string) {
    return { id, name: 'John Doe' };
  }
}

// Step 3: Inject dependencies into dependent classes
@injectable()
class UserService {
  constructor(
    @inject('ILogger') private logger: ILogger,
    @inject('IUserRepository') private userRepository: IUserRepository
  ) {}

  async getUser(id: string) {
    this.logger.log(`Fetching user ${id}`);
    return this.userRepository.getUserById(id);
  }
}

// Step 4: Register implementations in container
const container = new Container();
container.register('ILogger', { useClass: ConsoleLogger });
container.register('IUserRepository', { useClass: UserRepository });
container.registerSingleton(UserService);

// Step 5: Resolve from container instead of instantiating directly
const userService = container.resolve(UserService);

// Step 6: Use the service
userService.getUser('123').

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change