在 Scala 中，如何从可序列化的类型创建 TypeTag?

Question

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

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，这与从编译时推理创建的typeTag不同：

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)

指示第二个类型标记不可序列化，而第一个类型标记不可序列化。

所以我的问题是：如何使第二个 TypeTag 像第一个一样可序列化？

我目前正在使用 scala 2.12.10。

Answer 1

基于 如何手动创建类型标签？

如果您需要可序列化的TypeTag并且性能不是您的主要内容 关注

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)
...