# Angular Dependency Injection **Dependency Injection (DI)** is a way to create objects that depend upon other objects. A Dependency Injection system supplies the dependent objects (called the _dependencies_) when it creates an instance of an object. The [Dependency Injection pattern](guide/dependency-injection-pattern) page describes this general approach. _The guide you're reading now_ explains how Angular's own Dependency Injection system works. ## DI by example You'll learn Angular Dependency Injection through a discussion of the sample app that accompanies this guide. Run the anytime. Start by reviewing this simplified version of the _heroes_ feature from the [The Tour of Heroes](tutorial/).

The `HeroesComponent` is the top-level heroes component. It's only purpose is to display the `HeroListComponent` which displays a list of hero names. This version of the `HeroListComponent` gets its `heroes` from the `HEROES` array, an in-memory collection defined in a separate `mock-heroes` file. That may suffice in the early stages of development, but it's far from ideal. As soon as you try to test this component or get heroes from a remote server, you'll have to change the implementation of `HerosListComponent` and replace every other use of the `HEROES` mock data. It's better to hide these details inside a _service_ class, [defined in its own file](#one-class-per-file). ## Create an injectable _HeroService_ The [**Angular CLI**](https://cli.angular.io/) can generate a new `HeroService` class in the `src/app/heroes` folder with this command. ng generate service heroes/hero That command creates the following `HeroService` skeleton. Assume for now that the [`@Injectable` decorator](#injectable) is an essential ingredient in every Angular service definition. The rest of the class has been rewritten to expose a `getHeroes` method that returns the same mock data as before. Of course, this isn't a real data service. If the app were actually getting data from a remote server, the `getHeroes` method signature would have to be asynchronous. That's a defect we can safely ignore in this guide where our focus is on _injecting the service_ into the `HeroList` component. {@a injector-config} {@a bootstrap} ## Register a service provider A _service_ is just a class in Angular until you register it with an Angular dependency injector. An Angular injector is responsible for creating service instances and injecting them into classes like the `HeroListComponent`. You rarely create an Angular injector yourself. Angular creates injectors for you as it executes the app, starting with the _root injector_ that it creates during the [bootstrap process](guide/bootstrapping). You do have to register _providers_ with an injector before the injector can create that service. **Providers** tell the injector _how to create the service_. Without a provider, the injector would not know that it is responsible for injecting the service nor be able to create the service.

You'll learn much more about _providers_ [below](#providers). For now it is sufficient to know that they create services and must be registered with an injector.

You can register a provider with any Angular decorator that supports the **`providers` array property**. Many Angular decorators accept metadata with a `providers` property. The two most important examples are `@Component` and `@NgModule`. {@a register-providers-component} ### _@Component_ providers ### 在组件中注册提供商 Here's a revised `HeroesComponent` that registers the `HeroService` in its `providers` array. 下面是修改过的`HerosComponent`，把`HeroService`注册到了它的`providers`数组中。 {@a register-providers-ngmodule} ### _@NgModule_ providers In the following excerpt, the root `AppModule` registers two providers in its `providers` array. The first entry registers the `UserService` class (_not shown_) under the `UserService` _injection token_. The second registers a value (`HERO_DI_CONFIG`) under the `APP_CONFIG` _injection token_. Thanks to these registrations, Angular can inject the `UserService` or the `HERO_DI_CONFIG` value into any class that it creates.

You'll learn about _injection tokens_ and _provider_ syntax [below](#providers).

{@a ngmodule-vs-comp} ### _@NgModule_ or _@Component_? Should you register a service with an Angular module or with a component? The two choices lead to differences in service _scope_ and service _lifetime_. **Angular module providers** (`@NgModule.providers`) are registered with the application's root injector. Angular can inject the corresponding services in any class it creates. Once created, a service instance lives for the life of the app and Angular injects this one service instance in every class that needs it. You're likely to inject the `UserService` in many places throughout the app and will want to inject the same service instance every time. Providing the `UserService` with an Angular module is a good choice.

To be precise, Angular module providers are registered with the root injector _unless the module is_ [lazy loaded](guide/lazy-loading-ngmodules). In this sample, all modules are _eagerly loaded_ when the application starts, so all module providers are registered with the app's root injector.

**A component's providers** (`@Component.providers`) are registered with each component instance's own injector. Angular can only inject the corresponding services in that component instance or one of its descendant component instances. Angular cannot inject the same service instance anywhere else. Note that a component-provided service may have a limited lifetime. Each new instance of the component gets its own instance of the service and, when the component instance is destroyed, so is that service instance. In this sample app, the `HeroComponent` is created when the application starts and is never destroyed so the `HeroService` created for the `HeroComponent` also live for the life of the app. If you want to restrict `HeroService` access to the `HeroComponent` and its nested `HeroListComponent`, providing the `HeroService` in the `HeroComponent` may be a good choice.

The scope and lifetime of component-provided services is a consequence of [the way Angular creates component instances](#component-child-injectors).

## Inject a service The `HeroListComponent` should get heroes from the `HeroService`. The component shouldn't create the `HeroService` with `new`. It should ask for the `HeroService` to be injected. You can tell Angular to inject a dependency in the component's constructor by specifying a **constructor parameter with the dependency type**. Here's the `HeroListComponent` constructor, asking for the `HeroService` to be injected. Of course, the `HeroListComponent` should do something with the injected `HeroService`. Here's the revised component, making use of the injected service, side-by-side with the previous version for comparison.

Notice that the `HeroListComponent` doesn't know where the `HeroService` comes from. _You_ know that it comes from the parent `HeroesComponent`. But if you decided instead to provide the `HeroService` in the `AppModule`, the `HeroListComponent` wouldn't change at all. The _only thing that matters_ is that the `HeroService` is provided in some parent injector. {@a singleton-services} ## Singleton services Services are singletons _within the scope of an injector_. There is at most one instance of a service in a given injector. There is only one root injector and the `UserService` is registered with that injector. Therefore, there can be just one `UserService` instance in the entire app and every class that injects `UserService` get this service instance. However, Angular DI is a [hierarchical injection system](guide/hierarchical-dependency-injection), which means that nested injectors can create their own service instances. Angular creates nested injectors all the time. {@a component-child-injectors} ## Component child injectors For example, when Angular creates a new instance of a component that has `@Component.providers`, it also creates a new _child injector_ for that instance. Component injectors are independent of each other and each of them creates its own instances of the component-provided services. When Angular destroys one of these component instance, it also destroys the component's injector and that injector's service instances. Thanks to [injector inheritance](guide/hierarchical-dependency-injection), you can still inject application-wide services into these components. A component's injector is a child of its parent component's injector, and a descendent of its parent's parent's injector, and so on all the way back to the application's _root_ injector. Angular can inject a service provided by any injector in that lineage. For example, Angular could inject a `HeroListComponent` with both the `HeroService` provided in `HeroComponent` and the `UserService` provided in `AppModule`. {@a testing-the-component} ## Testing the component ## 测试组件 Earlier you saw that designing a class for dependency injection makes the class easier to test. Listing dependencies as constructor parameters may be all you need to test application parts effectively. 前面强调过，设计一个适合依赖注入的类，可以让这个类更容易测试。要有效的测试应用中的一部分，只需要在构造函数的参数中列出依赖。 For example, you can create a new `HeroListComponent` with a mock service that you can manipulate under test: 例如，新建的`HeroListComponent`实例使用一个模拟 (mock) 服务，以便可以在测试中操纵它：

Learn more in the [Testing](guide/testing) guide. 要学习更多知识，参见[测试](guide/testing)一章。

{@a service-needs-service} ## When the service needs a service ## 当服务需要别的服务时 The `HeroService` is very simple. It doesn't have any dependencies of its own. 这个`HeroService`非常简单。它本身不需要任何依赖。 What if it had a dependency? What if it reported its activities through a logging service? You'd apply the same *constructor injection* pattern, adding a constructor that takes a `Logger` parameter. 如果它也有依赖，该怎么办呢？例如，它需要通过日志服务来汇报自己的活动。我们同样用*构造函数注入*模式，来添加一个带有`Logger`参数的构造函数。 Here is the revised `HeroService` that injects the `Logger`, side-by-side with the previous service for comparison.

The constructor asks for an injected instance of a `Logger` and stores it in a private field called `logger`. The `getHeroes()` method logs a message when asked to fetch heroes. 这个构造函数要求注入一个`Logger`类的实例，并把它存到名为`logger`的私有字段中。当请求英雄数据时，`getHeroes()`中就会记录一个消息。 {@a logger-service} #### The dependent _Logger_ service The sample app's `Logger` service is quite simple: If the app didn't provide this `Logger`, Angular would throw an exception when it looked for a `Logger` to inject into the `HeroService`. ERROR Error: No provider for Logger! Because a singleton logger service is useful everywhere, it's provided in the root `AppModule`. {@a injectable} ## _@Injectable()_ The **[@Injectable()](api/core/Injectable)** decorator identifies a service class that _might_ require injected dependencies. The `HeroService` must be annotated with `@Injectable()` because it requires an injected `Logger`.

Always write `@Injectable()` with parentheses, not just `@Injectable`.

When Angular creates a class whose constructor has parameters, it looks for type and injection metadata about those parameters so that it can inject the right service. If Angular can't find that parameter information, it throws an error. Angular can only find the parameter information _if the class has a decorator of some kind_. While _any_ decorator will do, the `@Injectable()` decorator is the standard decorator for service classes.

The decorator requirement is imposed by TypeScript. TypeScript normally discards parameter type information when it _transpiles_ the code to JavaScript. It preserves this information if the class has a decorator and the `emitDecoratorMetadata` compiler option is set `true` in TypeScript's `tsconfig.json` configuration file, . The CLI configures `tsconfig.json` with `emitDecoratorMetadata: true` It's your job to put `@Injectable()` on your service classes.

The `Logger` service is annotated with `@Injectable()` decorator too, although it has no constructor and no dependencies. In fact, _every_ Angular service class in this app is annotated with the `@Injectable()` decorator, whether or not it has a constructor and dependencies. `@Injectable()` is a required coding style for services. {@a providers} ## Providers ## 服务提供商们 A service provider *provides* the concrete, runtime version of a dependency value. The injector relies on **providers** to create instances of the services that the injector injects into components, directives, pipes, and other services. 服务提供商*提供*依赖值的一个具体的、运行时的版本。注入器依靠**提供商**来创建服务的实例，注入器再将服务的实例注入组件、管道或其它服务。 You must register a service *provider* with an injector, or it won't know how to create the service. 必须为注入器注册一个服务的*提供商*，否则它就不知道该如何创建该服务。 The next few sections explain the many ways you can specify a provider. Almost all of the accompanying code snippets are extracts from the sample app's `providers.component.ts` file. ### The class as its own provider There are many ways to *provide* something that looks and behaves like a `Logger`. The `Logger` class itself is an obvious and natural provider. 有很多方式可以*提供*一些实现 `Logger`类的东西。 `Logger`类本身是一个显而易见而且自然而然的提供商。 But it's not the only way. 但它不是唯一的途径。 You can configure the injector with alternative providers that can deliver an object that behaves like a `Logger`. You could provide a substitute class. You could provide a logger-like object. You could give it a provider that calls a logger factory function. Any of these approaches might be a good choice under the right circumstances. 可以用其它备选提供商来配置注入器，只要它们能交付一个行为类似于`Logger`的对象就可以了。可以提供一个替代类。你可以提供一个类似日志的对象。可以给它一个提供商，让它调用可以创建日志服务的工厂函数。所有这些方法，只要用在正确的场合，都可能是一个好的选择。 What matters is that the injector has a provider to go to when it needs a `Logger`. 重点是，当注入器需要一个`Logger`时，它得先有一个提供商。 {@a provide} ### The _provide_ object literal ### *provide* 对象字面量 Here's the class-provider syntax again. This is actually a shorthand expression for a provider registration using a _provider_ object literal with two properties: 这其实是用于注册提供商的简写表达式。使用的是一个带有两个属性的_提供商_对象字面量： The `provide` property holds the [token](guide/dependency-injection#token) that serves as the key for both locating a dependency value and registering the provider. `provide`属性保存的是[令牌 (token)](guide/dependency-injection#token)，它作为键值 (key) 使用，用于定位依赖值和注册提供商。 The second property is always a provider definition object, which you can think of as a *recipe* for creating the dependency value. There are many ways to create dependency values just as there are many ways to write a recipe. 第二个是一个提供商定义对象。可以把它看做是指导如何创建依赖值的*配方*。有很多方式创建依赖值…… 也有很多方式可以写配方。 {@a class-provider} ### Alternative class providers ### 备选的类提供商 Occasionally you'll ask a different class to provide the service. The following code tells the injector to return a `BetterLogger` when something asks for the `Logger`. 某些时候，我们会请求一个不同的类来提供服务。下列代码告诉注入器，当有人请求`Logger`时，返回`BetterLogger`。 {@a class-provider-dependencies} ### Class provider with dependencies ### 带依赖的类提供商 Maybe an `EvenBetterLogger` could display the user name in the log message. This logger gets the user from the injected `UserService`, which is also injected at the application level. 假设`EvenBetterLogger`可以在日志消息中显示用户名。这个日志服务从注入的`UserService`中取得用户， `UserService`通常也会在应用级注入。 Configure it like `BetterLogger`. 就像之前在`BetterLogger`中那样配置它。 {@a aliased-class-providers} ### Aliased class providers ### 别名类提供商 Suppose an old component depends upon an `OldLogger` class. `OldLogger` has the same interface as the `NewLogger`, but for some reason you can't update the old component to use it. 假设某个旧组件依赖一个`OldLogger`类。 `OldLogger`和`NewLogger`具有相同的接口，但是由于某些原因，我们不能升级这个旧组件并使用它。 When the *old* component logs a message with `OldLogger`, you'd like the singleton instance of `NewLogger` to handle it instead. 当*旧*组件想使用`OldLogger`记录消息时，我们希望改用`NewLogger`的单例对象来记录。 The dependency injector should inject that singleton instance when a component asks for either the new or the old logger. The `OldLogger` should be an alias for `NewLogger`. 不管组件请求的是新的还是旧的日志服务，依赖注入器注入的都应该是同一个单例对象。也就是说，`OldLogger`应该是`NewLogger`的别名。 You certainly do not want two different `NewLogger` instances in your app. Unfortunately, that's what you get if you try to alias `OldLogger` to `NewLogger` with `useClass`. 我们当然不会希望应用中有两个不同的`NewLogger`实例。不幸的是，如果尝试通过`useClass`来把`OldLogger`作为`NewLogger`的别名，就会导致这样的后果。 The solution: alias with the `useExisting` option. 解决方案：使用`useExisting`选项指定别名。 {@a value-provider} ### Value providers ### 值提供商 Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class. 有时，提供一个预先做好的对象会比请求注入器从类中创建它更容易。 Then you register a provider with the `useValue` option, which makes this object play the logger role. 于是可以通过`useValue`选项来注册提供商，它会让这个对象直接扮演 logger 的角色。 See more `useValue` examples in the [Non-class dependencies](guide/dependency-injection#non-class-dependencies) and [InjectionToken](guide/dependency-injection#injection-token) sections. 查看更多`useValue`的例子，见[非类依赖](guide/dependency-injection#non-class-dependencies)和 [InjectionToken](guide/dependency-injection#injection-token)部分。 {@a factory-provider} ### Factory providers ### 工厂提供商 Sometimes you need to create the dependent value dynamically, based on information you won't have until the last possible moment. Maybe the information changes repeatedly in the course of the browser session. 有时，我们需要动态创建这个依赖值，因为它所需要的信息直到最后一刻才能确定。也许这个信息会在浏览器的会话中不停地变化。 Suppose also that the injectable service has no independent access to the source of this information. 还假设这个可注入的服务没法通过独立的源访问此信息。 This situation calls for a **factory provider**. 这种情况下，请调用**工厂提供商**。 To illustrate the point, add a new business requirement: the `HeroService` must hide *secret* heroes from normal users. Only authorized users should see secret heroes. 下面通过添加新的业务需求来说明这一点： `HeroService` 必须对普通用户隐藏掉*秘密*英雄。只有授权用户才能看到秘密英雄。 Like the `EvenBetterLogger`, the `HeroService` needs a fact about the user. It needs to know if the user is authorized to see secret heroes. That authorization can change during the course of a single application session, as when you log in a different user. 就像`EvenBetterLogger`那样，`HeroService`需要了解此用户的身份。它需要知道，这个用户是否有权看到隐藏英雄。这个授权可能在单一的应用会话中被改变，例如，改用另一个用户的身份登录时。 Unlike `EvenBetterLogger`, you can't inject the `UserService` into the `HeroService`. The `HeroService` won't have direct access to the user information to decide who is authorized and who is not. 与`EvenBetterLogger`不同，不能把`UserService`注入到`HeroService`中。 `HeroService`无权访问用户信息，来决定谁有授权谁没有授权。 Instead, the `HeroService` constructor takes a boolean flag to control display of secret heroes. 让`HeroService`的构造函数带上一个布尔型的标志，来控制是否显示隐藏的英雄。 You can inject the `Logger`, but you can't inject the boolean `isAuthorized`. You'll have to take over the creation of new instances of this `HeroService` with a factory provider. 我们可以注入`Logger`，但是不能注入逻辑型的`isAuthorized`。我们不得不通过通过工厂提供商创建这个`HeroService`的新实例。 A factory provider needs a factory function: 工厂提供商需要一个工厂方法： Although the `HeroService` has no access to the `UserService`, the factory function does. 虽然`HeroService`不能访问`UserService`，但是工厂方法可以。 You inject both the `Logger` and the `UserService` into the factory provider and let the injector pass them along to the factory function: 同时把`Logger`和`UserService`注入到工厂提供商中，并且让注入器把它们传给工厂方法：

The `useFactory` field tells Angular that the provider is a factory function whose implementation is the `heroServiceFactory`. `useFactory`字段告诉 Angular：这个提供商是一个工厂方法，它的实现是`heroServiceFactory`。 The `deps` property is an array of [provider tokens](guide/dependency-injection#token). The `Logger` and `UserService` classes serve as tokens for their own class providers. The injector resolves these tokens and injects the corresponding services into the matching factory function parameters. `deps`属性是[提供商令牌](guide/dependency-injection#token)数组。 `Logger`和`UserService`类作为它们自身类提供商的令牌。注入器解析这些令牌，把相应的服务注入到工厂函数中相应的参数中去。

Notice that you captured the factory provider in an exported variable, `heroServiceProvider`. This extra step makes the factory provider reusable. You can register the `HeroService` with this variable wherever you need it. 注意，我们在一个导出的变量中捕获了这个工厂提供商：`heroServiceProvider`。这个额外的步骤让工厂提供商可被复用。无论哪里需要，都可以使用这个变量注册`HeroService`。 In this sample, you need it only in the `HeroesComponent`, where it replaces the previous `HeroService` registration in the metadata `providers` array. Here you see the new and the old implementation side-by-side: 在这个例子中，只在`HeroesComponent`中需要它，这里，它代替了元数据`providers`数组中原来的`HeroService`注册。对比一下新的和旧的实现：

{@a token} ## Dependency injection tokens ## 依赖注入令牌 When you register a provider with an injector, you associate that provider with a dependency injection token. The injector maintains an internal *token-provider* map that it references when asked for a dependency. The token is the key to the map. 当向注入器注册提供商时，实际上是把这个提供商和一个 DI 令牌关联起来了。注入器维护一个内部的*令牌-提供商*映射表，这个映射表会在请求依赖时被引用到。令牌就是这个映射表中的键值。 In all previous examples, the dependency value has been a class *instance*, and the class *type* served as its own lookup key. Here you get a `HeroService` directly from the injector by supplying the `HeroService` type as the token: 在前面的所有例子中，依赖值都是一个类*实例*，并且类的*类型*作为它自己的查找键值。在下面的代码中，`HeroService`类型作为令牌，直接从注入器中获取`HeroService` 实例： You have similar good fortune when you write a constructor that requires an injected class-based dependency. When you define a constructor parameter with the `HeroService` class type, Angular knows to inject the service associated with that `HeroService` class token: 编写需要基于类的依赖注入的构造函数对我们来说是很幸运的。只要定义一个`HeroService`类型的构造函数参数， Angular 就会知道把跟`HeroService`类令牌关联的服务注入进来： This is especially convenient when you consider that most dependency values are provided by classes. 这是一个特殊的规约，因为大多数依赖值都是以类的形式提供的。 {@a non-class-dependencies} ### Non-class dependencies ### 非类依赖 What if the dependency value isn't a class? Sometimes the thing you want to inject is a string, function, or object. 如果依赖值不是一个类呢？有时候想要注入的东西是一个字符串，函数或者对象。 Applications often define configuration objects with lots of small facts (like the title of the application or the address of a web API endpoint) but these configuration objects aren't always instances of a class. They can be object literals such as this one: 应用程序经常为很多很小的因素定义配置对象（例如应用程序的标题或网络API终点的地址）。但是这些配置对象不总是类的实例，它们可能是对象，如下面这个： What if you'd like to make this configuration object available for injection? You know you can register an object with a [value provider](guide/dependency-injection#value-provider). 我们想让这个配置对象在注入时可用，而且知道可以使用[值提供商](guide/dependency-injection#value-provider)来注册一个对象。 But what should you use as the token? You don't have a class to serve as a token. There is no `AppConfig` class. 但是，这种情况下用什么作令牌呢？我们没办法找一个类来当作令牌，因为没有`Config`类。

### TypeScript interfaces aren't valid tokens ### TypeScript 接口不是一个有效的令牌 The `HERO_DI_CONFIG` constant conforms to the `AppConfig` interface. Unfortunately, you cannot use a TypeScript interface as a token: `CONFIG`常量有一个接口：`AppConfig`。不幸的是，不能把 TypeScript 接口用作令牌： That seems strange if you're used to dependency injection in strongly typed languages, where an interface is the preferred dependency lookup key. 对于习惯于在强类型的语言中使用依赖注入的开发人员，这会看起来很奇怪，因为在强类型语言中，接口是首选的用于查找依赖的主键。 It's not Angular's doing. An interface is a TypeScript design-time artifact. JavaScript doesn't have interfaces. The TypeScript interface disappears from the generated JavaScript. There is no interface type information left for Angular to find at runtime. 这不是 Angular 的错。接口只是 TypeScript 设计时 (design-time) 的概念。JavaScript 没有接口。 TypeScript 接口不会出现在生成的 JavaScript 代码中。在运行期，没有接口类型信息可供 Angular 查找。

{@a injection-token} ### _InjectionToken_ ### _InjectionToken_ 值 One solution to choosing a provider token for non-class dependencies is to define and use an [*InjectionToken*](api/core/InjectionToken). The definition of such a token looks like this: 解决方案是为非类依赖定义和使用InjectionToken作为提供商令牌。定义方式是这样的： The type parameter, while optional, conveys the dependency's type to developers and tooling. The token description is another developer aid. 类型参数，虽然是可选的，但可以向开发者和开发工具传达类型信息。而且这个令牌的描述信息也可以为开发者提供帮助。 Register the dependency provider using the `InjectionToken` object: 使用这个`InjectionToken`对象注册依赖的提供商： Now you can inject the configuration object into any constructor that needs it, with the help of an `@Inject` decorator: 现在，在`@Inject`装饰器的帮助下，这个配置对象可以注入到任何需要它的构造函数中：

Although the `AppConfig` interface plays no role in dependency injection, it supports typing of the configuration object within the class. 虽然`AppConfig`接口在依赖注入过程中没有任何作用，但它为该类中的配置对象提供了强类型信息。

Alternatively, you can provide and inject the configuration object in an ngModule like `AppModule`. 或者在 ngModule 中提供并注入这个配置对象，如`AppModule`。 {@a optional} ## Optional dependencies ## 可选依赖 The `HeroService` *requires* a `Logger`, but what if it could get by without a `logger`? You can tell Angular that the dependency is optional by annotating the constructor argument with `@Optional()`: `HeroService`*需要*一个`Logger`，但是如果想不提供 Logger 也能得到它，该怎么办呢？可以把构造函数的参数标记为`@Optional()`，告诉 Angular 该依赖是可选的： When using `@Optional()`, your code must be prepared for a null value. If you don't register a `logger` somewhere up the line, the injector will set the value of `logger` to null. 当使用`@Optional()`时，代码必须准备好如何处理空值。如果其它的代码没有注册一个 `logger`，注入器会设置该`logger`的值为空 null。 ## Summary ## 小结 You learned the basics of Angular dependency injection in this page. You can register various kinds of providers, and you know how to ask for an injected object (such as a service) by adding a parameter to a constructor. 本章，我们学习了 Angular 依赖注入的基础知识。我们可以注册很多种类的提供商，知道如何通过添加构造函数的参数来请求一个注入对象（例如一个服务）。 Angular dependency injection is more capable than this guide has described. You can learn more about its advanced features, beginning with its support for nested injectors, in [Hierarchical Dependency Injection](guide/hierarchical-dependency-injection). Angular 依赖注入比前面描述的更能干。学习更多高级特性，如对嵌套注入器的支持，见[多级依赖注入](guide/hierarchical-dependency-injection)一章。 {@a explicit-injector} ## Appendix: Working with injectors directly ## 附录：直接使用注入器 Developers rarely work directly with an injector, but here's an `InjectorComponent` that does. 这里的`InjectorComponent`直接使用了注入器，但我们很少直接使用它。 An `Injector` is itself an injectable service. `Injector`本身是可注入的服务。 In this example, Angular injects the component's own `Injector` into the component's constructor. The component then asks the injected injector for the services it wants in `ngOnInit()`. 在这个例子中，Angular 把组件自身的`Injector`注入到了组件的构造函数中。然后，组件在`ngOnInit()`中向注入的注入器请求它所需的服务。 Note that the services themselves are not injected into the component. They are retrieved by calling `injector.get()`. 注意，这些服务本身没有注入到组件，它们是通过调用`injector.get()`获得的。 The `get()` method throws an error if it can't resolve the requested service. You can call `get()` with a second parameter, which is the value to return if the service is not found. Angular can't find the service if it's not registered with this or any ancestor injector. `get()`方法如果不能解析所请求的服务，会抛出异常。调用`get()`时，还可以使用第二个参数，一旦获取的服务没有在当前或任何祖先注入器中注册过，就把它作为返回值。

The technique is an example of the [service locator pattern](https://en.wikipedia.org/wiki/Service_locator_pattern). 刚描述的这项技术是[服务定位器模式](https://en.wikipedia.org/wiki/Service_locator_pattern)的一个范例。 **Avoid** this technique unless you genuinely need it. It encourages a careless grab-bag approach such as you see here. It's difficult to explain, understand, and test. You can't know by inspecting the constructor what this class requires or what it will do. It could acquire services from any ancestor component, not just its own. You're forced to spelunk the implementation to discover what it does. 要**避免使用**此技术，除非确实需要它。它会鼓励鲁莽的方式，就像在这里看到的。它难以解释、理解和测试。仅通过阅读构造函数，没法知道这个类需要什么或者它将做什么。它可以从任何祖先组件中获得服务，而不仅仅是它自己。会迫使我们深入它的实现，才可能明白它都做了啥。 Framework developers may take this approach when they must acquire services generically and dynamically. 框架开发人员必须采用通用的或者动态的方式获取服务时，可能采用这个方法。

{@a one-class-per-file} ## Appendix: one class per file ## 附录：为什么建议每个文件只放一个类 Having multiple classes in the same file is confusing and best avoided. Developers expect one class per file. Keep them happy. 在同一个文件中有多个类容易造成混淆，最好避免。开发人员期望每个文件只放一个类。这会让它们开心点。 If you combine the `HeroService` class with the `HeroesComponent` in the same file, **define the component last**. If you define the component before the service, you'll get a runtime null reference error. 如果我们蔑视这个建议，并且 —— 比如说 —— 把`HeroService`和`HeroesComponent`组合在同一个文件里， **就得把组件定义放在最后面！** 如果把组件定义在了服务的前面，在运行时抛出空指针错误。

You actually can define the component first with the help of the `forwardRef()` method as explained in this [blog post](http://blog.thoughtram.io/angular/2015/09/03/forward-references-in-angular-2.html). 在`forwardRef()`方法的帮助下，实际上也可以先定义组件，具体说明见这篇[博客](http://blog.thoughtram.io/angular/2015/09/03/forward-references-in-angular-2.html)。 But it's best to avoid the problem altogether by defining components and services in separate files. 但是为什么要先给自己找麻烦呢？还是通过在独立的文件中定义组件和服务，完全避免此问题吧。