Dependency injection is an important application design pattern. Angular has its own dependency injection framework, and you really can't build an Angular application without it. It's used so widely that almost everyone just calls it DI.
This page covers what DI is, why it's so useful, and how to use it in an Angular app.
- Why dependency injection?
- Angular dependency injection
- Configuring the injector
- Registering providers in an
- Registering providers in a component
- When to use
NgModuleversus an application component
- Preparing the
- Implicit injector creation
- Singleton services
- Testing the component
- When the service needs a service
- Creating and registering a logger service
- Injector providers
- Dependency injection tokens
- Optional dependencies
- Appendix: Working with injectors directly
Why dependency injection?
To understand why dependency injection is so important, consider an example without it. Imagine writing the following code:
Car class creates everything it needs inside its constructor.
What's the problem?
The problem is that the
Car class is brittle, inflexible, and hard to test.
Car needs an engine and tires. Instead of asking for them,
Car constructor instantiates its own copies from
the very specific classes
What if the
Engine class evolves and its constructor requires a parameter?
That would break the
Car class and it would stay broken until you rewrote it along the lines of
this.engine = new Engine(theNewParameter).
Engine constructor parameters weren't even a consideration when you first wrote
You may not anticipate them even now.
But you'll have to start caring because
when the definition of
Engine changes, the
Car class must change.
What if you want to put a different brand of tires on your
Car? Too bad.
You're locked into whatever brand the
Tires class creates. That makes the
Car class inflexible.
Right now each new car gets its own
engine. It can't share an
engine with other cars.
While that makes sense for an automobile engine,
surely you can think of other dependencies that should be shared, such as the onboard
wireless connection to the manufacturer's service center. This
Car lacks the flexibility
to share services that have been created previously for other consumers.
When you write tests for
Car you're at the mercy of its hidden dependencies.
Is it even possible to create a new
Engine in a test environment?
Engine depend upon? What does that dependency depend on?
Will a new instance of
Engine make an asynchronous call to the server?
You certainly don't want that going on during tests.
What if the
Car should flash a warning signal when tire pressure is low?
How do you confirm that it actually does flash a warning
if you can't swap in low-pressure tires during the test?
You have no control over the car's hidden dependencies. When you can't control the dependencies, a class becomes difficult to test.
How can you make
Car more robust, flexible, and testable?
That's super easy. Change the
Car constructor to a version with DI:
See what happened? The definition of the dependencies are
now in the constructor.
Car class no longer creates an
It just consumes them.
This example leverages TypeScript's constructor syntax for declaring parameters and properties simultaneously.
Now you can create a car by passing the engine and tires to the constructor.
How cool is that?
The definition of the
tire dependencies are
decoupled from the
You can pass in any kind of
tires you like, as long as they
conform to the general API requirements of an
Now, if someone extends the
Engine class, that is not
The consumer of
Car has the problem. The consumer must update the car creation code to
something like this:
The critical point is this: the
Car class did not have to change.
You'll take care of the consumer's problem shortly.
Car class is much easier to test now because you are in complete control
of its dependencies.
You can pass mocks to the constructor that do exactly what you want them to do
during each test:
You just learned what dependency injection is.
It's a coding pattern in which a class receives its dependencies from external sources rather than creating them itself.
Cool! But what about that poor consumer?
Anyone who wants a
Car must now
create all three parts: the
Car class shed its problems at the consumer's expense.
You need something that takes care of assembling these parts.
You could write a giant class to do that:
It's not so bad now with only three creation methods. But maintaining it will be hairy as the application grows. This factory is going to become a huge spiderweb of interdependent factory methods!
Wouldn't it be nice if you could simply list the things you want to build without having to define which dependency gets injected into what?
This is where the dependency injection framework comes into play. Imagine the framework had something called an injector. You register some classes with this injector, and it figures out how to create them.
When you need a
Car, you simply ask the injector to get it for you and you're good to go.
Everyone wins. The
Car knows nothing about creating an
The consumer knows nothing about creating a
You don't have a gigantic factory class to maintain.
Car and consumer simply ask for what they need and the injector delivers.
This is what a dependency injection framework is all about.
Now that you know what dependency injection is and appreciate its benefits, read on to see how it is implemented in Angular.
Angular dependency injection
Angular ships with its own dependency injection framework. This framework can also be used as a standalone module by other applications and frameworks.
To see what it can do when building components in Angular,
start with a simplified version of the
that from the The Tour of Heroes.
HeroesComponent is the root component of the Heroes feature area.
It governs all the child components of this area.
This stripped down version has only one child,
which displays a list of heroes.
HeroListComponent gets heroes from
HEROES, an in-memory collection
defined in another 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 want to get your heroes data from a remote server,
you'll have to change the implementation of
fix every other use of the
HEROES mock data.
It's better to make a service that hides how the app gets hero data.
Given that the service is a separate concern, consider writing the service code in its own file.
See this note for details.
HeroService exposes a
getHeroes method that returns
the same mock data as before, but none of its consumers need to know that.
@Injectable() decorator above the service class is
Of course, this isn't a real service. If the app were actually getting data from a remote server, the API would have to be asynchronous, perhaps returning a Promise. You'd also have to rewrite the way components consume the service. This is important in general, but not in this example.
A service is nothing more than a class in Angular. It remains nothing more than a class until you register it with an Angular injector.
Configuring the injector
You don't have to create an Angular injector. Angular creates an application-wide injector for you during the bootstrap process.
You do have to configure the injector by registering the providers that create the services the application requires. This guide explains what providers are later.
You can either register a provider within an NgModule or in application components.
Registering providers in an NgModule
AppModule that registers two providers,
UserService and an
HeroService is used only within the
and its subcomponents, the top-level
HeroesComponent is the ideal
place to register it.
Registering providers in a component
Here's a revised
HeroesComponent that registers the
HeroService in its
When to use NgModule versus an application component
On the one hand, a provider in an
NgModule is registered in the root injector. That means that every provider
registered within an
NgModule will be accessible in the entire application.
On the other hand, a provider registered in an application component is available only on that component and all its children.
APP_CONFIG service needs to be available all across the application, so it's
registered in the
But since the
HeroService is only used within the Heroes
feature area and nowhere else, it makes sense to register it in
Also see "Should I add app-wide providers to the root
AppComponent?" in the NgModule FAQ.
Preparing the HeroListComponent for injection
HeroListComponent should get heroes from the injected
Per the dependency injection pattern, the component must ask for the service in its
constructor, as discussed earlier.
It's a small change:
Focus on the constructor
Adding a parameter to the constructor isn't all that's happening here.
Note that the constructor parameter has the type
HeroService, and that
HeroListComponent class has an
(scroll up to confirm that fact).
Also recall that the parent component (
providers information for
The constructor parameter type, the
and the parent's
providers information combine to tell the
Angular injector to inject an instance of
HeroService whenever it creates a new
Implicit injector creation
You saw how to use an injector to create a new
Car earlier in this guide.
You could create such an injector
You won't find code like that in the Tour of Heroes or any of the other
You could write code that explicitly creates an injector if you had to,
but it's not always the best choice.
Angular takes care of creating and calling injectors
when it creates components for you—whether through HTML markup, as in
or after navigating to a component with the router.
If you let Angular do its job, you'll enjoy the benefits of automated dependency injection.
Dependencies are singletons within the scope of an injector.
In this guide's example, a single
HeroService instance is shared among the
HeroesComponent and its
However, Angular DI is a hierarchical injection system, which means that nested injectors can create their own service instances. For more information, see Hierarchical Injectors.
Testing the component
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.
For example, you can create a new
HeroListComponent with a mock service that you can manipulate
Learn more in Testing.
When the service needs a service
HeroService is very simple. It doesn't have any dependencies of its own.
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
Here is the revision compared to the original.
The constructor now asks for an injected instance of a
Logger and stores it in a private property called
You call that property within the
getHeroes() method when anyone asks for heroes.
@Injectable() marks a class as available to an
injector for instantiation. Generally speaking, an injector reports an
error when trying to instantiate a class that is not marked as
As it happens, you could have omitted
@Injectable() from the first
HeroService because it had no injected parameters.
But you must have it now that the service has an injected dependency.
You need it because Angular requires constructor parameter metadata
in order to inject a
@Injectable() to every service class, even those that don't have dependencies
and, therefore, do not technically require it. Here's why:
- Future proofing: No need to remember
@Injectable()when you add a dependency later.
- Consistency: All services follow the same rules, and you don't have to wonder why a decorator is missing.
Injectors are also responsible for instantiating components
HeroesComponent. So why doesn't
You can add it if you really want to. It isn't necessary because the
HeroesComponent is already marked with
@Component, and this
decorator class (like
@Pipe, which you learn about later)
is a subtype of @Injectable(). It is in
@Injectable() decorators that
identify a class as a target for instantiation by an injector.
The TypeScript compiler discards metadata by default.
emitDecoratorMetadata compiler option is true
(as it should be in the
for every class with at least one decorator.
While any decorator will trigger this effect, mark the service class with the @Injectable() decorator to make the intent clear.
@Injectable(), not just
The application will fail mysteriously if you forget the parentheses.
Creating and registering a logger service
Inject a logger into
HeroService in two steps:
- Create the logger service.
- Register it with the application.
The logger service is quite simple:
You're likely to need the same logger service everywhere in your application,
so put it in the project's
app folder and
register it in the
providers array of the application module,
If you forget to register the logger, Angular throws an exception when it first looks for the logger:
That's Angular telling you that the dependency injector couldn't find the provider for the logger.
It needed that provider to create a
Logger to inject into a new
HeroService, which it needed to
create and inject into a new
The chain of creations started with the
Logger provider. Providers are the subject of the next section.
A 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 and other services.
You must register a service provider with the injector, or it won't know how to create the service.
Earlier you registered the
Logger service in the
providers array of the metadata for the
AppModule like this:
There are many ways to provide something that looks and behaves like a
Logger class itself is an obvious and natural provider.
But it's not the only way.
You can configure the injector with alternative providers that can deliver an object that behaves like a
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.
What matters is that the injector has a provider to go to when it needs a
The Provider class and provide object literal
You wrote the
providers array like this:
This is actually a shorthand expression for a provider registration using a provider object literal with two properties:
The first is the token that serves as the key for both locating a dependency value and registering the provider.
The second is 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.
Alternative class providers
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
Class provider with dependencies
EvenBetterLogger could display the user name in the log message.
This logger gets the user from the injected
which is also injected at the application level.
Configure it like
Aliased class providers
Suppose an old component depends upon an
OldLogger has the same interface as the
NewLogger, but for some reason
you can't update the old component to use it.
When the old component logs a message with
you'd like the singleton instance of
NewLogger to handle it instead.
The dependency injector should inject that singleton instance
when a component asks for either the new or the old logger.
OldLogger should be an alias for
You certainly do not want two different
NewLogger instances in your app.
Unfortunately, that's what you get if you try to alias
The solution: alias with the
Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class.
Then you register a provider with the
which makes this object play the logger role.
useValue examples in the
Non-class dependencies and
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.
Suppose also that the injectable service has no independent access to the source of this information.
This situation calls for a factory provider.
To illustrate the point, add a new business requirement:
HeroService must hide secret heroes from normal users.
Only authorized users should see secret heroes.
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.
EvenBetterLogger, you can't inject the
UserService into the
HeroService won't have direct access to the user information to decide
who is authorized and who is not.
HeroService constructor takes a boolean flag to control display of secret heroes.
You can inject the
Logger, but you can't inject the boolean
You'll have to take over the creation of new instances of this
HeroService with a factory provider.
A factory provider needs a factory function:
HeroService has no access to the
UserService, the factory function does.
You inject both the
Logger and the
UserService into the factory provider
and let the injector pass them along to the factory function:
useFactory field tells Angular that the provider is a factory function
whose implementation is the
deps property is an array of provider tokens.
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.
Notice that you captured the factory provider in an exported variable,
This extra step makes the factory provider reusable.
You can register the
HeroService with this variable wherever you need it.
In this sample, you need it only in the
where it replaces the previous
HeroService registration in the metadata
Here you see the new and the old implementation side-by-side:
Dependency injection tokens
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.
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:
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:
This is especially convenient when you consider that most dependency values are provided by classes.
What if the dependency value isn't a class? Sometimes the thing you want to inject is a span string, function, or object.
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:
What if you'd like to make this configuration object available for injection? You know you can register an object with a value provider.
But what should you use as the token?
You don't have a class to serve as a token.
There is no
TypeScript interfaces aren't valid tokens
HERO_DI_CONFIG constant has an interface,
AppConfig. Unfortunately, you
cannot use a TypeScript interface as a token:
That seems strange if you're used to dependency injection in strongly typed languages, where an interface is the preferred dependency lookup key.
One solution to choosing a provider token for non-class dependencies is to define and use an OpaqueToken. The definition looks like this:
Register the dependency provider using the
Now you can inject the configuration object into any constructor that needs it, with
the help of an
AppConfig interface plays no role in dependency injection,
it supports typing of the configuration object within the class.
Aternatively, you can provide and inject the configuration object in an ngModule like
HeroService requires a
Logger, but what if it could get by without
You can tell Angular that the dependency is optional by annotating the
constructor argument with
@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
logger to null.
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.
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.
Appendix: Working with injectors directly
Developers rarely work directly with an injector, but
InjectorComponent that does.
Injector is itself an injectable service.
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.
Note that the services themselves are not injected into the component.
They are retrieved by calling
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.
The technique is an example of the service locator pattern.
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.
Framework developers may take this approach when they must acquire services generically and dynamically.
Appendix: Why have one class per file
Having multiple classes in the same file is confusing and best avoided. Developers expect one class per file. Keep them happy.
If you combine the
HeroService class with
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.
You actually can define the component first with the help of the
forwardRef() method as explained
in this blog post.
But why flirt with trouble?
Avoid the problem altogether by defining components and services in separate files.