How can I transform a Map to a case class in Scala

问题:

If I have a Map[String,String]("url" -> "xxx", "title" -> "yyy"), is there an way to generically transform it into a case class Image(url:String, title:String)?

I can write a helper:

object Image{
  def fromMap(params:Map[String,String]) = Image(url=params("url"), title=params("title"))
}

but is there a way to generically write this once for a map to any case class?

回答1:

First off, there are some safe alternatives you could do if you just want to shorten your code. The companion object can be treated as a function so you could use something like this:

def build2[A,B,C](m: Map[A,B], f: (B,B) => C)(k1: A, k2: A): Option[C] = for {
  v1 <- m.get(k1)
  v2 <- m.get(k2)
} yield f(v1, v2)

build2(m, Image)("url", "title")

This will return an option containing the result. Alternatively you could use the ApplicativeBuilders in Scalaz which internally do almost the same but with a nicer syntax:

import scalaz._, Scalaz._
(m.get("url") |@| m.get("title"))(Image)

If you really need to do this via reflection then the easiest way would be to use Paranamer (as the Lift-Framework does). Paranamer can restore the parameter names by inspecting the bytecode so there is a performance hit and it will not work in all environments due to classloader issues (the REPL for example). If you restrict yourself to classes with only String constructor parameters then you could do it like this:

val pn = new CachingParanamer(new BytecodeReadingParanamer)

def fill[T](m: Map[String,String])(implicit mf: ClassManifest[T]) = for {
  ctor <- mf.erasure.getDeclaredConstructors.filter(m => m.getParameterTypes.forall(classOf[String]==)).headOption
  parameters = pn.lookupParameterNames(ctor)
} yield ctor.newInstance(parameters.map(m): _*).asInstanceOf[T]

val img = fill[Image](m)

(Note that this example can pick a default constructor as it does not check for the parameter count which you would want to do)

回答2:

Here's a solution using builtin scala/java reflection:

  def createCaseClass[T](vals : Map[String, Object])(implicit cmf : ClassManifest[T]) = {
      val ctor = cmf.erasure.getConstructors().head
      val args = cmf.erasure.getDeclaredFields().map( f => vals(f.getName) )
      ctor.newInstance(args : _*).asInstanceOf[T]
  }

To use it:

val image = createCaseClass[Image](Map("url" -> "xxx", "title" -> "yyy"))

回答3:

Not a full answer to your question, but a start…

It can be done, but it will probably get more tricky than you thought. Each generated Scala class is annotated with the Java annotation ScalaSignature, whose bytes member can be parsed to give you the metadata that you would need (including argument names). The format of this signature is not API, however, so you'll need to parse it yourself (and are likely to change the way you parse it with each new major Scala release).

Maybe the best place to start is the lift-json library, which has the ability to create instances of case classes based on JSON data.

Update: I think lift-json actually uses Paranamer to do this, and thus may not parse the bytes of ScalaSignature… Which makes this technique work for non-Scala classes, too.

Update 2: See Moritz's answer instead, who is better informed than I am.

回答4:

There's a hack you can transform map to json and then to case class. I used spray-json

import spray.json._

object MainClass2 extends App {
  val mapData: Map[Any, Any] =
    Map(
      "one" -> "1",
      "two" -> 2,
      "three" -> 12323232123887L,
      "four" -> 4.4,
      "five" -> false
    )

  implicit object AnyJsonFormat extends JsonFormat[Any] {
    def write(x: Any): JsValue = x match {
      case int: Int           => JsNumber(int)
      case long: Long          => JsNumber(long)
      case double: Double        => JsNumber(double)
      case string: String        => JsString(string)
      case boolean: Boolean if boolean  => JsTrue
      case boolean: Boolean if !boolean => JsFalse
    }
    def read(value: JsValue): Any = value match {
      case JsNumber(int) => int.intValue()
      case JsNumber(long) => long.longValue()
      case JsNumber(double) => double.doubleValue()
      case JsString(string) => string
      case JsTrue      => true
      case JsFalse     => false
    }
  }

  import ObjJsonProtocol._
  val json = mapData.toJson
  val result: TestObj = json.convertTo[TestObj]
  println(result)

}

final case class TestObj(one: String, two: Int, three: Long, four: Double, five: Boolean)

object ObjJsonProtocol extends DefaultJsonProtocol {
  implicit val objFormat: RootJsonFormat[TestObj] = jsonFormat5(TestObj)
}

and use this dependency in sbt build:

 "io.spray"          %%   "spray-json"     %   "1.3.3"

回答5:

This cannot be done, since you would need to get the companion object's apply method's parameter names and they simply aren't available via reflection. If you have a lot of these case classes, you could parse their declarations and generate the fromMap methods.