从 TypeTag[T] 获取伴随对象的字段值

Question

I have a quite rare use case where a trait is being implemented by 3rd party (think of a plugin architecture) and I want to get a field of each trait's companion object.

A simple trait implementation looks like this:

trait Plugin {
  val ID: String
}
class HelloWorldPlugin extends Plugin {
  val ID = HelloWorldPlugin.ID
}

object HelloWorldPlugin {
  val ID = "hello world"
}

In the plugin registry - I'd like to get the ID of each plugin. Since we're dealing with type erasure - Reflection seems like my only option. I tried the following to no avail:

object CompanionReflectionDemo {
  import scala.reflect.runtime.{universe => ru}
  private lazy val universeMirror = ru.runtimeMirror(getClass.getClassLoader)

  def registerPlugin[T <: Plugin](implicit tt: ru.TypeTag[T])  = {
    val companionMirror = universeMirror.reflectModule(ru.typeOf[T].typeSymbol.companion.asModule)
    val m = universeMirror.reflect(companionMirror.instance)
    val field = m.reflectField(ru.typeOf[T].decl(ru.TermName("ID")).asTerm.accessed.asTerm)
    field.get
  }

  def main(args: Array[String]) {
    val x = registerPlugin[HelloWorldPlugin]
    println(x) // expecting "hello world"
  }
}

But the problem here is that typeOf[T] always returns the Class type and not the Module type - hence I get the following runtime error:

Exception in thread "main" scala.ScalaReflectionException: expected a member of object HelloWorldPlugin, you provided value org.reflect.HelloWorldPlugin.ID
    at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$abort(JavaMirrors.scala:115)
    at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$ErrorNotMember(JavaMirrors.scala:121)
    at scala.reflect.runtime.JavaMirrors$JavaMirror$$anonfun$scala$reflect$runtime$JavaMirrors$JavaMirror$$checkMemberOf$1.apply(JavaMirrors.scala:214)
    at scala.reflect.runtime.JavaMirrors$JavaMirror.ensuringNotFree(JavaMirrors.scala:204)
    at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$checkMemberOf(JavaMirrors.scala:213)
    at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:236)
    at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:233)
    at org.reflect.CompanionReflectionDemo$.registerPlugin(Blah.scala:21)
    at org.reflect.CompanionReflectionDemo$.main(Blah.scala:26)

What's the best way to get the ID value for each Plugin from its TypeTag ?

Answer 1

It really depends on your use case, but I would highly advice against the approach you have taken to create your plugin design pattern. The problem is that for each plugin, you have to define a class, a corresponding object, and finally to 'register' each plugin. This creates a huge overhead and danger to create runtime errors. Furthermore, the correspondence between the ID field in the companion object and its class is not enforced by the compiler, but its left to the mercy and the diligence of the developer.

I believe that a more elegant approach would be to use the TypeTag[T] for each object that represents one of your plugins. That way, you enforce the isomorphism between all implemented plugins, and all registered plugins, and avoid the creation of companion objects.

Nevertheless, even with the given constraints, you can extract the string value from the companion object. Using Scala 2.11.8, we have:

object CompanionReflectionDemo {

  import scala.reflect.runtime.{universe => ru}
  private lazy val rootMirror = ru.runtimeMirror(getClass.getClassLoader)

  def registerPlugin[T <: Plugin](implicit tt: ru.TypeTag[T]) : String = {

    val classMirror       = rootMirror.reflectClass(tt.tpe.typeSymbol.asClass)
    val companionSymbol   = classMirror.symbol.companion
    val companionInstance = rootMirror.reflectModule(companionSymbol.asModule)
    val companionMirror   = rootMirror.reflect(companionInstance.instance)

    val fieldSymbol = companionSymbol.typeSignature.decl(ru.TermName("ID")).asTerm
    val fieldMirror = companionMirror.reflectField(fieldSymbol)

    fieldMirror.get.asInstanceOf[String]
  }

  def main(args: Array[String]) {
    val x = registerPlugin[HelloWorldPlugin]
    println(x) // expecting "hello world"
  }
}