在Scala中基于String选择案例类 [英] Select case class based on String in Scala

问题描述

如何基于 String 值选择 case类?

我的代码是

val spark = SparkSession.builder()...
val rddOfJsonStrings: RDD[String] = // some json strings as RDD
val classSelector: String = ??? // could be "Foo" or "Bar", or any other String value
case class Foo(foo: String)
case class Bar(bar: String)


if (classSelector == "Foo") {
  val df: DataFrame = spark.read.json(rddOfJsonStrings)
  df.as[Foo]
} else if (classSelector == "Bar") {
  val df: DataFrame = spark.read.json(rddOfJsonStrings)
  df.as[Bar]
} else {
  throw ClassUnknownException //custom Exception
}

变量 classSeletector 是一个简单的String，应用于指向同名的case类.

The variable classSeletector is a simple String that should be used to point to the case class of the same name.

想象一下，我不仅有 Foo 和 Bar 作为案例类，而且还有更多.如何基于String调用 df.as [] 语句(如果可能的话)?

Imagine I don't only have Foo and Bar as case classes but more then those two. How is it possible to call the df.as[] statement based on the String (if possible at all)?

还是Scala中提供了一种完全不同的方法?

Or is there a completely different approach available in Scala?

推荐答案

如何基于String调用df.as []语句(如果可能的话)?

How is it possible to call the df.as[] statement based on the String (if possible at all)?

不是(或基于任何运行时值).您可能会注意到，所有答案仍然需要:

It isn't (or based on any runtime value). You may note that all answers still need to:

1. 对 Foo 和 Bar 有一个单独的分支(对于要添加的每个类，还有一个分支)；

1. have a separate branch for Foo and Bar (and one more branch for each class you'll want to add);

在分支中重复两次类名.

repeat the class name twice in the branch.

您可以避免第二个:

import scala.reflect.{classTag, ClassTag}

val df: DataFrame = spark.read.json(rddOfJsonStrings)
// local function defined where df and classSelector are visible
def dfAsOption[T : Encoder : ClassTag] =
  Option.when(classSelector == classTag[T].runtimeClass.simpleName)(df.as[T])

dfAsOption[Foo].dfAsOption(asOption[Bar]).getOrElse(throw ClassUnknownException)

但是首先，如果可能的话，您将需要一个宏.我猜不是.

But for the first you'd need a macro if it's possible at all. I would guess it isn't.

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