在Scala中，如何从可序列化的类型创建TypeTag?

问题描述

在Scala反射中，通常可以使用TypeCreator从Type构造TypeTag:

In Scala reflection, the TypeTag can usually be constructed from a Type using a TypeCreator:

object TypeUtils {

  import ScalaReflection.universe._

  def createTypeTag[T](
      tpe: Type,
      mirror: reflect.api.Mirror[reflect.runtime.universe.type]
  ): TypeTag[T] = {
    TypeTag.apply(
      mirror,
      NaiveTypeCreator(tpe)
    )
  }

  case class NaiveTypeCreator(tpe: Type) extends reflect.api.TypeCreator {

    def apply[U <: reflect.api.Universe with Singleton](
        m: reflect.api.Mirror[U]): U#Type = {
      //          assert(m eq mirror, s"TypeTag[$tpe] defined in $mirror cannot be migrated to $m.")
      tpe.asInstanceOf[U#Type]
    }
  }

不幸的是，事实证明createTypeTag的输出不可序列化，这与通过编译时推断创建的typeTag不同:

Unfortunately, it turns out the output of createTypeTag is not serializable, unlike the typeTag created from compile-time inference:

import java.io.{ByteArrayOutputStream, ObjectOutputStream}

import org.apache.spark.sql.catalyst.ScalaReflection
import org.scalatest.FunSpec

class TypeTagFromType extends FunSpec {

  import ScalaReflection.universe._

  it("create TypeTag from reflection") {

    val ttg = typeTag[String]

    val ttg2 = TypeUtils.createTypeTag(ttg.tpe, ttg.mirror)

    Seq(
      ttg -> "from static inference",
      ttg2 -> "from dynamic type"
    ).foreach {
      case (tt, k) =>
        println(k)

        val bos = new ByteArrayOutputStream()
        try {
          val out = new ObjectOutputStream(bos)
          out.writeObject(tt)
          out.flush()
          val array = bos.toByteArray
        } finally {
          bos.close()
        }
    }
  }
}

这给出了输出:

from static inference
from dynamic type


scala.reflect.runtime.JavaMirrors$JavaMirror$$anon$2
java.io.NotSerializableException: scala.reflect.runtime.JavaMirrors$JavaMirror$$anon$2
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)

表示第二个TypeTag是不可序列化的，而第一个是不可序列化的.

Indicating that the second TypeTag is not serializable, while the first one is.

所以我的问题是:如何使第二个TypeTag像第一个一样可序列化?

So my question is: how to make the second TypeTag to be serializable like the first one?

此刻我正在使用Scala 2.12.10.

I'm using scala 2.12.10 at the moment.

推荐答案

基于如何手动创建TypeTag?

然后考虑

import scala.reflect.runtime.universe._
import scala.reflect.runtime.currentMirror
import scala.tools.reflect.ToolBox

def createTypeTag(tp: Type): TypeTag[_] = {
  val toolbox = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()
  val ttree = toolbox.parse(s"scala.reflect.runtime.universe.typeTag[$tp]")
  toolbox.eval(ttree).asInstanceOf[TypeTag[_]]
}

val ttg = typeTag[List[String]]
val ttg2 = createTypeTag(ttg.tpe)
...

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