Scala 反射：如何在运行时定义一个案例 class 然后引用它？

Question

我想在运行时定义一个案例 class ，例如

val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()
val myClass: ClassDef = q"case class Authentication(email: String)".asInstanceOf[ClassDef]
val definedClass  = tb.define(myClass)

然后能够在另一个反射中引用它

  // Actor code that recognise the defined case object
  val actorCode = q"""
  import akka.actor._
  object HelloActor {
    def props() = Props(new HelloActor())
  }
  class HelloActor() extends Actor {
    def receive = {
      case $definedClass(emailParam)  => println("case object instance has been received!")
      case _       => println("None received!")
    }
  }
  return HelloActor.props()
  """

关于如何做到这一点的任何想法？

Answer 1

您展示的示例在没有任何编译时反射的情况下是可行的：

import akka.actor._

// define extractor object: https://docs.scala-lang.org/tour/extractor-objects.html
sealed trait EmailExtractor[A] {
  def unapply(value: A): Option[String]
}
object EmailExtractor {
  
  def of[A](pf: PartialFunction[A, String]) = new EmailExtractor[A] {
    def unapply(value: A): Option[String] = pf.lift(value)
  }
}

// inject difference in behavior via constructor
class HelloActor[A](emailExtractor: EmailExtractor[A]) extends Actor {

  def receive = {
    case emailExtractor(emailParam) =>
      println("case class instance has been received!")
    case _ => println("None received!")
  }
}
object HelloActor {
  
  def props[A](emailExtractor: EmailExtractor[A]) =
    Props(new HelloActor(emailExtractor))
}

implicit val actorSystem: ActorSystem = ActorSystem()

// you'd have to define message somewhere available to both:
//  * actor you are creating
//  * place where you are sending a message
// anyway, there is no reason to generate a whole new actor because of it

def oneTimeCaseClass1 = {
  case class Message(email: String)
  
  actorSystem.actorOf(HelloActor.props(EmailExtractor.of[Message] {
    case Message(string) => string
  })) ! Message("test@test.com")
}

def oneTimeCaseClass2 = {
  case class Message(email: String)
  
  actorSystem.actorOf(HelloActor.props(EmailExtractor.of[Message] {
    case Message(string) => string
  })) ! Message("test2@test2.com")
}

oneTimeCaseClass1
oneTimeCaseClass2

scala.concurrent.Await.result(
  actorSystem.terminate(),
  scala.concurrent.duration.Duration.Inf
)

（见斯卡斯蒂）

此外：

• return是一种不好的代码实践
• Akka Classic（无类型）不鼓励使用 Akka Typed
• 编译时反射的价值恰恰在于它发生在编译时 - 准引号不能在运行时用于定义新代码，这需要成熟的编译器结合将其 output 提供给 class 加载程序的特定方式（所以基本上嵌入 REPL） - 这很危险（基本上是任意代码执行），容易出现 OOM（因为如果 REPL 将其整个历史记录存储在内存中，则无法收集 memory）并且没有必要

Answer 2

您的代码似乎几乎没问题。

您只是错过了.companion

val actorCode = q"""
   import akka.actor._
   object HelloActor {
     def props() = Props(new HelloActor())
   }
   class HelloActor() extends Actor {
     def receive = {
       case ${definedClass.companion}(emailParam)  => println("case object instance has been received!")
       case _       => println("None received!")
     }
   }
   ActorSystem("hellokernel").actorOf(HelloActor.props()) ! ${definedClass.companion}("abc")
 """

tb.eval(actorCode) // case object instance has been received!

Scala 2.13.8