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我正在寻找一种不使用编译时工具（完全依赖运行时）来创建可序列化 TypeTag 的方法。这是所有反射语言的基本特征。

这篇文章中的答案提出了几种方法：

在 Scala 中，如何从可序列化的类型创建 TypeTag？

他们都没有工作：

package com.tribbloids.spike.scala_spike

import java.io.{
  ByteArrayInputStream,
  ByteArrayOutputStream,
  ObjectInputStream,
  ObjectOutputStream
}

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

class TypeTagFromType extends FunSpec {

  import ScalaReflection.universe._

  it("create TypeTag from reflection") {

    val ttg = typeTag[String]
    val ttg2 = TypeUtils.createTypeTag_fast(ttg.tpe, ttg.mirror)
    val ttg3 = TypeUtils.createTypeTag_slow(ttg.tpe, ttg.mirror)

    Seq(
      ttg -> "from static inference",
      ttg2 -> "from dynamic type - fast",
      ttg3 -> "from dynamic type - slow"
    ).map {
      case (tt, k) =>
        println(k)

        try {

          val bytes = serialise(tt)
          val tt2 = deserialise(bytes)
          assert(tt.tpe =:= tt2.tpe)
        } catch {
          case e: Throwable =>
            e.printStackTrace()
        }

    }
  }

  def serialise(tt: universe.TypeTag[_]): Array[Byte] = {
    val bos = new ByteArrayOutputStream()
    try {
      val out = new ObjectOutputStream(bos)
      out.writeObject(tt)
      out.flush()
      val array = bos.toByteArray
      array
    } finally {
      bos.close()
    }
  }

  def deserialise(tt: Array[Byte]): TypeTag[_] = {

    val bis = new ByteArrayInputStream(tt)

    try {
      val in = new ObjectInputStream(bis)
      in.readObject().asInstanceOf[TypeTag[_]]

    } finally {
      bis.close()
    }

  }
}

object TypeUtils {

  import ScalaReflection.universe._

  def createTypeTag_fast[T](
      tpe: Type,
      mirror: Mirror
  ): TypeTag[T] = {
    TypeTag.apply(
      mirror,
      NaiveTypeCreator(tpe)
    )
  }

  def createTypeTag_slow[T](
      tpe: Type,
      mirror: Mirror
  ): TypeTag[T] = {

    val toolbox = scala.tools.reflect.ToolBox(mirror).mkToolBox()

    val tree = toolbox.parse(s"scala.reflect.runtime.universe.typeTag[$tpe]")
    val result = toolbox.eval(tree).asInstanceOf[TypeTag[T]]

    result
  }

  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]
    }
  }
}

For ttg2and ttg3created in runtime, 在序列化或反序列化时遇到错误，ttg2遇到错误：

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)
...

ttg3遇到错误：

java.lang.ClassNotFoundException: __wrapper$1$71de08de01364321af52d1563247025d.__wrapper$1$71de08de01364321af52d1563247025d$$typecreator1$1
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at java.io.ObjectInputStream.resolveClass(ObjectInputStream.java:686)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1868)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1751)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2042)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1573)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2287)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2211)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2069)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1573)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:431)
...

如果你熟悉 scala 反射的设计，你能给出一个“规范”的实现来产生一个正常工作的 TypeTag 吗？

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0 回答 0