# Dependency injection (DI)

Aurelia ships with a lightweight but powerful dependency injection (DI) container. It wires classes together so you can focus on behavior instead of manual service lookups.

> **Before you start:** Make sure your project is set up using [App configuration and startup](https://docs.aurelia.io/getting-to-know-aurelia/startup-and-enhancement/app-configuration-and-startup); DI registrations usually happen during boot or feature composition.

## Why DI matters

* **Loose coupling** – swap implementations without rewriting consumers.
* **Testability** – inject doubles or fakes and keep constructors clean.
* **Lifetime control** – opt into singleton, transient, and scoped registrations.
* **Discoverability** – centralise service configuration instead of scattering `new` statements across the app.

## Quick start

Declare the dependencies your class needs using the `resolve()` function:

```typescript
import { resolve, ILogger } from '@aurelia/kernel';
import { ApiClient } from './services/api-client';

export class OrdersPage {
  private api = resolve(ApiClient);
  private logger = resolve(ILogger);

  async binding() {
    const orders = await this.api.load();
    this.logger.debug('Loaded orders', orders);
  }
}
```

Most Aurelia apps use the default root container created by the runtime, but you can always create your own for testing or specialised scenarios:

```typescript
import { DI, Registration } from '@aurelia/kernel';

const container = DI.createContainer();
container.register(
  Registration.singleton(ApiClient, ApiClient),
  Registration.instance('config', runtimeConfig)
);
```

Resolve services directly when you need to:

```typescript
const api = container.get(ApiClient);
const loggers = container.getAll(ILogger);
```

## Key concepts at a glance

| Concept              | Summary                                                                                      | Dive deeper                                                                                                                                 |
| -------------------- | -------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------- |
| Containers & scopes  | Create child containers to override or extend registrations per feature or component.        | [Container overview](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/overview)              |
| Registration helpers | `Registration.singleton`, `.transient`, `.instance`, and more control lifetime and creation. | [Creating services](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/creating-services)      |
| Interfaces & tokens  | Use symbols or `DI.createInterface()` to inject by contract instead of class.                | [What is DI?](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/what-is-dependency-injection) |
| Resolvers            | Fine tune how dependencies are delivered (lazy, optional, all, new instance, etc.).          | [Resolvers](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/resolvers)                      |

## Usage tips

* Keep constructors small; if a class needs more than a few services, consider extracting collaborators.
* Register feature-specific services at the component or route boundary rather than globally when possible.
* Combine DI with [App tasks](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/app-tasks) to hook container configuration into the application lifecycle.
* When integrating with other frameworks, create a bridge service that the container can hand out instead of reaching through globals.

## Next steps

1. Walk through the [DI overview](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/overview) for a conceptual refresher.
2. Review [creating services](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/creating-services) to choose the right registration helper.
3. Learn how [resolvers](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/resolvers) influence injection behavior for advanced scenarios.
4. Explore [What is dependency injection?](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/dependency-injection/what-is-dependency-injection) if you’re new to DI patterns in general.

Once these fundamentals feel natural, pair them with Aurelia features such as [App tasks](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/app-tasks) and the [task queue](https://docs.aurelia.io/getting-to-know-aurelia/services-and-runtime-hooks/task-queue) to orchestrate complex startup flows.


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