Specs2: how to test a class with more than one injected dependency?

Question

Play 2.4 app, using dependency injection for service classes.

I found that Specs2 chokes when a service class being tested has more than one injected dependency. It fails with "Can't find a constructor for class ..."

$ test-only services.ReportServiceSpec
[error] Can't find a constructor for class services.ReportService
[error] Error: Total 1, Failed 0, Errors 1, Passed 0
[error] Error during tests:
[error]         services.ReportServiceSpec
[error] (test:testOnly) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 2 s, completed Dec 8, 2015 5:24:34 PM

Production code, stripped to bare minimum to reproduce this problem:

package services

import javax.inject.Inject

class ReportService @Inject()(userService: UserService, supportService: SupportService) {  
   // ...  
}

class UserService {  
   // ...  
}

class SupportService {  
   // ...  
}

Test code:

package services

import javax.inject.Inject

import org.specs2.mutable.Specification

class ReportServiceSpec @Inject()(service: ReportService) extends Specification {

  "ReportService" should {
    "Work" in {
      1 mustEqual 1
    }
  }

}

If I remove either UserService or SupportService dependency from ReportService, the test works. But obviously the dependencies are in the production code for a reason. Question is, how do I make this test work?

Edit: When trying to run the test inside IntelliJ IDEA, the same thing fails, but with different messages: "Test framework quit unexpectedly", "This looks like a specs2 exception..."; see full output with stacktrace. I opened a Specs2 issue as instructed in the output, though I have no idea if the problem is in Play or Specs2 or somewhere else.

My library dependencies below. (I tried specifying Specs2 version explicitly, but that didn't help. Looks like I need specs2 % Test as is, for Play's test classes like WithApplication to work.)

resolvers += "scalaz-bintray" at "https://dl.bintray.com/scalaz/releases"
libraryDependencies ++= Seq(
  specs2 % Test,
  jdbc,
  evolutions,
  filters,
  "com.typesafe.play" %% "anorm" % "2.4.0",
  "org.postgresql" % "postgresql" % "9.4-1205-jdbc42"
)

Answer 1

There is limited support for dependency injection in specs2, mostly for execution environments or command-line arguments.

There is nothing preventing you from just using a lazy val and your favourite injection framework:

class MySpec extends Specification with Inject {
  lazy val reportService = inject[ReportService]

  ...
}

With Play and Guice, you could have a test helper such as this:

import play.api.inject.guice.GuiceApplicationBuilder
import scala.reflect.ClassTag    

trait Inject {
  lazy val injector = (new GuiceApplicationBuilder).injector()

  def inject[T : ClassTag]: T = injector.instanceOf[T]
}

Answer 2

If you really need runtime dependency injection, then it's better to use Guice loading, I guess:

package services

import org.specs2.mutable.Specification

import scala.reflect.ClassTag
import com.google.inject.Guice

// Something you'd like to share between your tests
// or maybe not
object Inject {
  lazy val injector = Guice.createInjector()

  def apply[T <: AnyRef](implicit m: ClassTag[T]): T = 
    injector.getInstance(m.runtimeClass).asInstanceOf[T]
}

class ReportServiceSpec  extends Specification {
  lazy val reportService: ReportService = Inject[ReportService]

  "ReportService" should {
    "Work" in {
      reportService.foo mustEqual 2
    }
  }
}

Alternatively you can implement Inject object as

import scala.reflect.ClassTag
import play.api.inject.guice.GuiceApplicationBuilder  

object Inject {
  lazy val injector = (new GuiceApplicationBuilder).injector()
  def apply[T : ClassTag]: T = injector.instanceOf[T]
}

It depends whether you want to use Guice directly, or thru play wrappers.

---

Looks like you are out of luck ATM: The comment says

Try to create an instance of a given class by using whatever constructor is available and trying to instantiate the first parameter recursively if there is a parameter for that constructor.

val constructors = klass.getDeclaredConstructors.toList.filter(_.getParameterTypes.size <= 1).sortBy(_.getParameterTypes.size)

i.e. Specs2 doesn't provide own DI out-of-the box,

---

Or you can reimplement the functionality yourself, if Guice isn't working for you.

App code:

package services

import javax.inject.Inject

class ReportService @Inject()(userService: UserService, supportService: SupportService) {
  val foo: Int = userService.foo + supportService.foo
}

class UserService  {  
   val foo: Int = 1
}
class SupportService {  
    val foo: Int = 41
}

Test code

package services

import org.specs2.mutable.Specification

import scala.reflect.ClassTag
import java.lang.reflect.Constructor

class Trick {
  val m: ClassTag[ReportService] = implicitly
  val classLoader: ClassLoader = m.runtimeClass.getClassLoader

  val trick: ReportService = Trick.createInstance[ReportService](m.runtimeClass, classLoader)
}

object Trick {
  def createInstance[T <: AnyRef](klass: Class[_], loader: ClassLoader)(implicit m: ClassTag[T]): T = {
    val constructors = klass.getDeclaredConstructors.toList.sortBy(_.getParameterTypes.size)
    val constructor = constructors.head

    createInstanceForConstructor(klass, constructor, loader)
  }

  private def createInstanceForConstructor[T <: AnyRef : ClassTag]
    (c: Class[_], constructor: Constructor[_], loader: ClassLoader): T = {
    constructor.setAccessible(true)

    // This can be implemented generically, but I don't remember how to deal with variadic functions
    // generically. IIRC even more reflection.
    if (constructor.getParameterTypes.isEmpty)
      constructor.newInstance().asInstanceOf[T]

    else if (constructor.getParameterTypes.size == 1) {
      // not implemented
      null.asInstanceOf[T]
    } else if (constructor.getParameterTypes.size == 2) {
      val types = constructor.getParameterTypes.toSeq
      val param1 = createInstance(types(0), loader)
      val param2 = createInstance(types(1), loader)
      constructor.newInstance(param1, param2).asInstanceOf[T]
    } else {
      // not implemented
      null.asInstanceOf[T]
    }
  }
}

// NB: no need to @Inject here. The specs2 framework does it for us.
// It sees spec with parameter, and loads it for us.
class ReportServiceSpec (trick: Trick) extends Specification {
  "ReportService" should {
    "Work" in {
      trick.trick.foo mustEqual 2
    }
  }
}

And that expectedly fails with

[info] ReportService should
[error]   x Work
[error]    '42' is not equal to '2' (FooSpec.scala:46)

---

If you don't need runtime dependency injection, then it's better to use cake pattern, and forget reflection all-together.

Answer 3

My colleague suggested a "low-tech" workaround. In the test, instantiate service classes with new:

class ReportServiceSpec extends Specification {
  val service = new ReportService(new UserService, new SupportService)
  // ...
}

This also works:

class ReportServiceSpec @Inject()(userService: UserService) extends Specification {
  val service = new ReportService(userService, new SupportService) 
  // ...    
}

Feel free to post more elegant solutions. I've yet to see a simple DI solution that works (with Guice, Play's default).

Does anyone else find it curious that Play's default test framework does not play well with Play's default DI mechanism?

---

Edit: In the end I went with an "Injector" test helper, almost the same as what Eric suggested:

Injector:

package testhelpers

import play.api.inject.guice.GuiceApplicationBuilder    
import scala.reflect.ClassTag

/**
 * Provides dependency injection for test classes.
 */
object Injector {
  lazy val injector = (new GuiceApplicationBuilder).injector()

  def inject[T: ClassTag]: T = injector.instanceOf[T]
}

Test:

class ReportServiceSpec extends Specification {
  val service = Injector.inject[ReportService]
  // ...
}