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 writing a Guice Module and in that module I have a provides method.
class FooModule extends ScalaModule {
#Provides
#Singleton
def providesFoo() : Foo = {
new Foo()
}
}
The problem is that the constructor of new Foo takes a Type Bar as parameter.
I want to know how do I ask guice to give me an instance of Bar so that I can do a new on Foo in the Module
Request those parameters by specifying them as method parameters. From the Provides method wiki documentation:
Dependencies can be passed in as parameters to the method. The injector will exercise the bindings for each of these before invoking the method.
So:
#Provides
#Singleton
def providesFoo(val bar : Bar) : Foo = {
new Foo(bar)
}
I am using Scala + Play and the out of box Guice set up for dependency injection. I am also using Akka Persistence behind the scenes and would like to create a binding for a custom read journal that I can then inject around my application.
Unfortunately, the read journal constructor (which I do not control) requires an explicit reference to the actor system:
PersistenceQuery(actorSystem).readJournalFor[CustomReadJournal]("custom-key")
How do I get a reference to the underlying actorSystem from within a binding definition class (Module)? Is this possible? More generally, is it possible to define interdependent bindings (a la Scaldi?)
My Module class entry currently looks like:
bind(classOf[CustomReadJournal]).toInstance(PersistenceQuery(<what do i put here?>).readJournalFor[CustomReadJournal]("custom-journal"))
Thanks in advance for the help!
If you need to do some kind of logic to create a dependency injection it is useful to use the #Provides annotation. For example:
trait MyProvider {
#Provides
def provideThing(): Thing = {
//make the thing and return it
}
}
class MyModule extends AbstractModule with MyProvider {
override def configure() {
bind(classOf[TraitYYY]).to(classOf[ClassThatTakesThingAsParameter])
}
}
A useful thing to know is that #Provides methods can themselves take parameters and get their arguments injected. For example:
#Provides
def provideThingNeedingParameter(param: P): ThingNeedingParam = {
new ThingNeedingParam(param)
}
Which is relevant to your situation I believe since you want to provide an actor system to an instance of some class.
// You can use #Singleton with #Provides if you need this to be one as well!
#Provides
def provideActorSystem(app: Application): ActorSystem = {
play.api.libs.concurrent.Akka.system(app)
}
#Provides
def providePersistenceQuery(actorSystem: ActorSystem): PersistenceQuery = {
PersistenceQuery(actorSystem)
}
#Provides
def provideCustomReadJournal(persistenceQuery: PersistenceQuery):CustomReadJournal = {
persistenceQuery.readJournalFor[CustomReadJournal]("custom-key")
}
By creating an #Provides annotated method for your CustomReadJournal you can avoid the bind call from configure entirely and control the parameters a bit more. Also, if you need to, #Provides works with #Singleton. I haven't used Akka persistence, but I think this should help you
I'm not sure why this won't compile - I'm trying to use an inner class (or trait) such that other objects can work with the resulting "RequestReturn", but only Trait Request and it's descendants can construct the object in the first place. I may be taking the wrong approach but shouldn't this code logically work? I've flagged the RequestReturn constructor as protected[Request] so it stands to reason that class RequestContinue would have the ability to call the constructor also.
trait Request {
class RequestReturn protected[Request](val x:Any)
def fulfill(item:Boolean):RequestReturn = new RequestReturn(item) //this line compiles
}
trait RequestContinue extends Request{
override def fulfill(item:Boolean):RequestReturn = new RequestReturn(item) //this won't compile
}
Error:(19, 54) constructor RequestReturn in class RequestReturn cannot be accessed in trait RequestContinue
Access to protected constructor RequestReturn not permitted because
enclosing trait RequestContinue in package .... is not a subclass of
class RequestReturn in trait Request where target is defined
override def fulfill(item:Boolean):RequestReturn = new RequestReturn(item)
^
The rules are here:
Access from RequestReturn, descendants and their companion modules; and from Request and its companion module.
You could define a protected factory method in Request for your use case.
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 {
// ...
}