I have a 3rd party library that I am trying to inject the configuration into the constructor.
This is what I need to do:
class MyModule(configuration: Configuration) extends AbstractModule {
override def configure(): Unit = {
bind(classOf[TwitterApi])
.to(classOf[MyTwitterApi])
.asEagerSingleton
}
}
The constructor of MyTwitterApi doesn't take a Play.api.Configuration but a typesafe.config.Config
class MyTwitterApi(config: Config) ...
So I need to do pass configuration.underlying to my constructor, how is this possible using DI in this AbstractModule?
I need this instance to be a singleton also.
You can use provider to setup your module with eagerSingleton
import com.google.inject.{AbstractModule, Provider}
class MyModule(configuration: Configuration) extends AbstractModule {
override def configure(): Unit = {
val twitterApiProvider: Provider[TwitterApi] =
() => new MyTwitterApi(configuration.underlying)
bind(classOf[TwitterApi])
.toProvider(twitterApiProvider)
.asEagerSingleton
}
}
You can find a working example with sample classes at - https://scastie.scala-lang.org/sarveshseri/ujwvJJNnTpiWDqdkBJQoFw/2
I think you want something like this:
class MyModule(configuration: Configuration) extends AbstractModule {
override def configure(): Unit = {
val myTwitterApiInstance = new MyTwitterApi(configuration.underlying)
bind(classOf[TwitterApi])
.toInstance(myTwitterApiInstance)
}
}
Or another approach would be to provide a binding for Config but if your MyTwitterApi doesn't have #Inject annotation this won't help.
Is it possible to instantiate the dependency first and then bind it in the module config method?
Currently I have the following config:
class PersonServiceImpl #Inject()(addressService: AddressService) {
...
}
class AppModule extends AbstractModule with ScalaModule {
def configure() {
bind[PersonService].to[PersonServiceImpl]
bind[AddressBook].to[AddressBookImpl]
}
#Provides #Singleton
def provideAddressService(addressBook: AddressBook): AddressService = {
new AddressServiceImpl(addressBook) with SecureAddressView
}
}
... which works fine. What I want to do now is to move the instantiation of the AddressServiceImpl into a separate module. So, the problem is that in order to create an instance of AddressServiceImpl I need Guice to inject the addressBook parameter for me, but I also want to create the instance myself so I can mix SecureAddressView in:
class AddressModule extends AbstractModule with ScalaModule {
def configure() {
bind[AddressService].to[AddressServiceImpl]
}
#Provides #Singleton
def provideAddressService(addressBook: AddressBook): AddressService = {
new AddressServiceImpl(addressBook) with SecureAddressView
}
}
This fails, though, as Guice comes back complaining about the provideAddressService method. It basically says that A binding to AddressService was already configured and points to the bind[AddressService].to[AddressServiceImpl] line in the configure method.
Any idea how to create an instance and mix in a trait while still delegating the resolution of downstream parameter dependencies to Guice?
OK, quite an obvious one but I was misled by the fact that I had to override the configure method. So, all I had to do is provide a dummy implementation for configure.
class AddressModule extends AbstractModule with ScalaModule {
override def configure(): Unit = ()
#Provides #Singleton
def provideAddressService(addressBook: AddressBook): AddressService = {
new AddressServiceImpl(addressBook) with SecureAddressView
}
}
Although this still looks quite dodgy as I have to provide explicitly all the parameters to the AddressService constructor. There must be a more elegant way of mixin traits. Or maybe not...
I am trying to create a service that runs in the background of my app (reads and writes to a queue) that I want to build with the actor system. However I am getting an error when I try to inject an ActorSystem into my class:
play.api.UnexpectedException: Unexpected exception[CreationException: Unable to create injector, see the following errors:
1) Error injecting constructor, java.lang.IllegalArgumentException: no matching constructor found on class services.Indexer$IndexActor for arguments []
at services.Indexer.<init>(Indexer.scala:21)
at Module.configure(Module.scala:6) (via modules: com.google.inject.util.Modules$OverrideModule -> Module)
while locating services.Indexer
Here is my setup:
// Module.scala
class Module extends AbstractModule {
override def configure() = {
bind(classOf[Indexer]).asEagerSingleton() // I suspect this needs to change
}
}
// Indexer.scala
#Singleton
class Indexer #Inject() (appLifecycle: ApplicationLifecycle, system: ActorSystem) (implicit ec: ExecutionContext) { ... }
In the play documentation there is an example of injecting an actor system, but that only seems to work when you inject into a class which extends Controller:
#Singleton
class MyController #Inject()(system: ActorSystem)(implicit exec: ExecutionContext) extends Controller {
// This works
}
Found the problem:
In Module.scala I am using the com.google.inject package
import com.google.inject.AbstractModule // <-------
import services.Indexer
class Module extends AbstractModule {
override def configure() = {
bind(classOf[Indexer]).asEagerSingleton()
}
}
But in my Indexer.scala service I was using the javax.inject package.
I have switched all of my files to now:
import com.google.inject._
I'm using codingwell/scala-guice and trying to inject DAO-classes into constructors of other components/classes.
In the first attempt, I only used one DAO-class to see if it works:
class DaoModule extends AbstractModule with ScalaModule {
override def configure() {
val dao1 = new FirstDaoImpl
bind(new TypeLiteral[FirstDaoTrait] {}).toInstance(dao1)
}
}
The binding works as expected, it can be used for constructor injection.
In the second step, I wanted to add another DAO class to the module. However, that DAO-class depends on the first DAO:
class SecondDaoImpl #Inject()(firstDao: FirstDaoTrait) extends SecondDaoTrait
I'm not sure how to add the necessary binding to the existing module. Repeating the first step would result in this:
val dao2 = new SecondDaoImpl(???)
bind(new TypeLiteral[SecondDaoTrait] {}).toInstance(dao2)
But of course this class can only be instantiated by providing the first DAO (therefore the "???"). How can I do this?
Use bind and let scala-guice resolve the dependencies for you:
class DaoModule extends AbstractModule with ScalaModule {
override def configure() {
bind[FirstDaoTrait].to[FirstDaoImpl]
bind[SecondDaoTrait].to[SecondDaoImpl]
}
}
And now using the injector:
val injector = Guice.createInjector(new DaoModule())
val secondDao = injector.instance[SecondDaoTrait]
I'm trying to migrate from Play 2.4 to 2.5 avoiding deprecated stuff.
I had an abstract class Microservice from which I created some objects. Some functions of the Microservice class used play.api.libs.ws.WS to make HTTP requests and also play.Play.application.configuration to read the configuration.
Previously, all I needed was some imports like:
import play.api.libs.ws._
import play.api.Play.current
import play.api.libs.concurrent.Execution.Implicits.defaultContext
But now you should use dependency injection to use WS and also to use access the current Play application.
I have something like this (shortened):
abstract class Microservice(serviceName: String) {
// ...
protected lazy val serviceURL: String = play.Play.application.configuration.getString(s"microservice.$serviceName.url")
// ...and functions using WS.url()...
}
An object looks something like this (shortened):
object HelloWorldService extends Microservice("helloWorld") {
// ...
}
Unfortunately I don't understand how I get all the stuff (WS, configuration, ExecutionContect) into the abstract class to make it work.
I tried to change it to:
abstract class Microservice #Inject() (serviceName: String, ws: WSClient, configuration: play.api.Configuration)(implicit context: scala.concurrent.ExecutionContext) {
// ...
}
But this doesn't solve the problem, because now I have to change the object too, and I can't figure out how.
I tried to turn the object into a #Singleton class, like:
#Singleton
class HelloWorldService #Inject() (implicit ec: scala.concurrent.ExecutionContext) extends Microservice ("helloWorld", ws: WSClient, configuration: play.api.Configuration) { /* ... */ }
I tried all sorts of combinations, but I'm not getting anywhere and I feel I'm not really on the right track here.
Any ideas how I can use things like WS the proper way (not using deprecated methods) without making things so complicated?
This is more related to how Guice handles inheritance and you have to do exactly what you would do if you were not using Guice, which is declaring the parameters to the superclass and calling the super constructor at your child classes. Guice even suggest it at its docs:
Wherever possible, use constructor injection to create immutable objects. Immutable objects are simple, shareable, and can be composed.
Constructor injection has some limitations:
Subclasses must call super() with all dependencies. This makes constructor injection cumbersome, especially as the injected base class changes.
In pure Java, it will means doing something like this:
public abstract class Base {
private final Dependency dep;
public Base(Dependency dep) {
this.dep = dep;
}
}
public class Child extends Base {
private final AnotherDependency anotherDep;
public Child(Dependency dep, AnotherDependency anotherDep) {
super(dep); // guaranteeing that fields at superclass will be properly configured
this.anotherDep = anotherDep;
}
}
Dependency injection won't change that and you will just have to add the annotations to indicate how to inject the dependencies. In this case, since Base class is abstract, and then no instances of Base can be created, we may skip it and just annotate Child class:
public abstract class Base {
private final Dependency dep;
public Base(Dependency dep) {
this.dep = dep;
}
}
public class Child extends Base {
private final AnotherDependency anotherDep;
#Inject
public Child(Dependency dep, AnotherDependency anotherDep) {
super(dep); // guaranteeing that fields at superclass will be properly configured
this.anotherDep = anotherDep;
}
}
Translating to Scala, we will have something like this:
abstract class Base(dep: Dependency) {
// something else
}
class Child #Inject() (anotherDep: AnotherDependency, dep: Dependency) extends Base(dep) {
// something else
}
Now, we can rewrite your code to use this knowledge and avoid deprecated APIs:
abstract class Microservice(serviceName: String, configuration: Configuration, ws: WSClient) {
protected lazy val serviceURL: String = configuration.getString(s"microservice.$serviceName.url")
// ...and functions using the injected WSClient...
}
// a class instead of an object
// annotated as a Singleton
#Singleton
class HelloWorldService(configuration: Configuration, ws: WSClient)
extends Microservice("helloWorld", configuration, ws) {
// ...
}
The last point is the implicit ExecutionContext and here we have two options:
Use the default execution context, which will be play.api.libs.concurrent.Execution.Implicits.defaultContext
Use other thread pools
This depends on you, but you can easily inject an ActorSystem to lookup the dispatcher. If you decide to go with a custom thread pool, you can do something like this:
abstract class Microservice(serviceName: String, configuration: Configuration, ws: WSClient, actorSystem: ActorSystem) {
// this will be available here and at the subclass too
implicit val executionContext = actorSystem.dispatchers.lookup("my-context")
protected lazy val serviceURL: String = configuration.getString(s"microservice.$serviceName.url")
// ...and functions using the injected WSClient...
}
// a class instead of an object
// annotated as a Singleton
#Singleton
class HelloWorldService(configuration: Configuration, ws: WSClient, actorSystem: ActorSystem)
extends Microservice("helloWorld", configuration, ws, actorSystem) {
// ...
}
How to use HelloWorldService?
Now, there are two things you need to understand in order to proper inject an instance of HelloWorldService where you need it.
From where HelloWorldService gets its dependencies?
Guice docs has a good explanation about it:
Dependency Injection
Like the factory, dependency injection is just a design pattern. The core principle is to separate behaviour from dependency resolution.
The dependency injection pattern leads to code that's modular and testable, and Guice makes it easy to write. To use Guice, we first need to tell it how to map our interfaces to their implementations. This configuration is done in a Guice module, which is any Java class that implements the Module interface.
And then, Playframework declare modules for WSClient and for Configuration. Both modules gives Guice enough information about how to build these dependencies, and there are modules to describe how to build the dependencies necessary for WSClient and Configuration. Again, Guice docs has a good explanation about it:
With dependency injection, objects accept dependencies in their constructors. To construct an object, you first build its dependencies. But to build each dependency, you need its dependencies, and so on. So when you build an object, you really need to build an object graph.
In our case, for HelloWorldService, we are using constructor injection to enable Guice to set/create our object graph.
How HelloWorldService is injected?
Just like WSClient has a module to describe how an implementation is binded to an interface/trait, we can do the same for HelloWorldService. Play docs has a clear explanation about how to create and configure modules, so I won't repeat it here.
But after creating an module, to inject a HelloWorldService to your controller, you just declare it as a dependency:
class MyController #Inject() (service: Microservice) extends Controller {
def index = Action {
// access "service" here and do whatever you want
}
}
In scala,
-> If you do not want to explicitly forward all the injected parameters to the base constructor, you can do it like that :
abstract class Base {
val depOne: DependencyOne
val depTwo: DependencyTwo
// ...
}
case class Child #Inject() (param1: Int,
depOne: DependencyOne,
depTwo: DependencyTwo) extends Base {
// ...
}