证明runtimeClass满足Scala中的Bound类型

Question

[我有一个方法以Parquet形式编写我的一个类Foo，该类定义为Thrift。

  import Foo
  import org.apache.spark.rdd.RDD
  import org.apache.thrift.TBase
  import org.apache.hadoop.mapreduce.Job
  import org.apache.parquet.hadoop.ParquetOutputFormat
  import org.apache.parquet.hadoop.thrift.ParquetThriftOutputFormat

  def writeThriftParquet(rdd: RDD[Foo], outputPath: String): Unit = {
    val job = Job.getInstance()
    ParquetThriftOutputFormat.setThriftClass(job, classOf[Foo])
    ParquetOutputFormat.setWriteSupportClass(job, classOf[Foo])

    rdd
      .map(x => (null, x))
      .saveAsNewAPIHadoopFile(
        outputPath,
        classOf[Void],
        classOf[Foo],
        classOf[ParquetThriftOutputFormat[Foo]],
        job.getConfiguration)
  }

这很好，但是我更愿意编写一个更通用的方法。我尝试了（相对）简单的方法：

  def writeThriftParquetGeneral[A <: TBase[_, _]](rdd: RDD[A], outputPath: String): Unit = {
    val job = Job.getInstance()
    ParquetThriftOutputFormat.setThriftClass(job, classOf[A])
    ParquetOutputFormat.setWriteSupportClass(job, classOf[A])

    rdd
      .map(x => (null, x))
      .saveAsNewAPIHadoopFile(
        outputPath,
        classOf[Void],
        classOf[A],
        classOf[ParquetThriftOutputFormat[A]],
        job.getConfiguration)
  }

但是失败，并显示以下错误：

 class type required but A found ParquetThriftOutputFormat.setThriftClass(job, classOf[A])
 class type required but A found ParquetOutputFormat.setWriteSupportClass(job, classOf[A])

为了解决这个问题，我使用了ClassTag，但是还没有编译的内容。

  import scala.reflect._
  implicit val ct = ClassTag[Foo](classOf[Foo])

  def writeThriftParquetGeneral[A <: TBase[_, _]](rdd: RDD[A], outputPath: String)(
    implicit tag: ClassTag[A]): Unit = {
    val job = Job.getInstance()

    // The problem line
    ParquetThriftOutputFormat.setThriftClass(job, tag.runtimeClass)

    // Seems OK from here
    ParquetOutputFormat.setWriteSupportClass(job, tag.runtimeClass)

    rdd
      .map(x => (null, x))
      .saveAsNewAPIHadoopFile(
        outputPath,
        classOf[Void],
        tag.runtimeClass,
        classOf[ParquetThriftOutputFormat[A]],
        job.getConfiguration)
  }

此行失败：ParquetThriftOutputFormat.setThriftClass(job, tag.runtimeClass)

[error]  found   : Class[_$1] where type _$1
[error]  required: Class[_ <: org.apache.thrift.TBase[_, _]]

[令我惊讶的是，编译器（Scala 2.11）没有意识到tag.runtimeClass必须是classOf[A]，并且A满足了定义所定义的类型。

Answer 1

[ClassTag#runtimeClass仅返回一个Class[_]

https://github.com/scala/scala/blob/2.13.x/src/library/scala/reflect/ClassTag.scala#L55

[Class[_ <: TBase[_, _]]是不同于Class[_]的存在类型（实际上是其子类型）

implicitly[Class[_ <: TBase[_, _]] <:< Class[_]]

尝试用替换问题行>>

ParquetThriftOutputFormat.setThriftClass(job, classTag.runtimeClass.asSubclass(classOf[TBase[_, _]]))