如何在Scala中获取运行时的泛型类型

Question

I have a following class

package myapp.model

case class Person(
 name: String,
 age: Option[Int]
)

I would like to implement following function:

def getFieldClass(className: String, fieldName:String): java.lang.Class[_] = {
    // case normal field return its class
    // case Option field return generic type of Option
}

So that for following input:

• className="myapp.model.Person"
• fieldName="age"

the function will return class object: scala.Int

---

Solution with Java Reflection API doesn't work well, it returns java.lang.Object for Option[Int]:

def getFieldClass(className: String, fieldName:String): java.lang.Class[_] = {
    val cls = java.lang.Class.forName(className)
    val pt = cls.getDeclaredField(fieldName).getGenericType.asInstanceOf[java.lang.reflect.ParameterizedType]
    val tpe = pt.getActualTypeArguments()(0);
    java.lang.Class.forName(tpe.getTypeName)
}

I'm writing part of deserializing feature and I don't have the object to check it's type, I have only a class name.

Answer 1

You can accomplish this with Scala's reflection library.

It's not especially pretty though:

import scala.reflect.runtime.{ universe => u }
import scala.reflect.runtime.universe._

object ReflectionHelper {

  val classLoader = Thread.currentThread().getContextClassLoader

  val mirror = u.runtimeMirror(classLoader)

  def getFieldType(className: String, fieldName: String): Option[Type] = {

    val classSymbol = mirror.staticClass(className)

    for {
      fieldSymbol <- classSymbol.selfType.members.collectFirst({
        case s: Symbol if s.isPublic && s.name.decodedName.toString() == fieldName => s
      })
    } yield {

      fieldSymbol.info.resultType
    }
  }

  def maybeUnwrapFieldType[A](fieldType: Type)(implicit tag: TypeTag[A]): Option[Type] = {
    if (fieldType.typeConstructor == tag.tpe.typeConstructor) {
      fieldType.typeArgs.headOption
    } else {
      Option(fieldType)
    }
  }

  def getFieldClass(className: String, fieldName: String): java.lang.Class[_] = {

    // case normal field return its class
    // case Option field return generic type of Option

    val result = for {
      fieldType <- getFieldType(className, fieldName)
      unwrappedFieldType <- maybeUnwrapFieldType[Option[_]](fieldType)
    } yield {
      mirror.runtimeClass(unwrappedFieldType)
    }

    // Consider changing return type to: Option[Class[_]]
    result.getOrElse(null)
  }
}

Then:

ReflectionHelper.getFieldClass("myapp.model.Person", "age")  // int
ReflectionHelper.getFieldClass("myapp.model.Person", "name") // class java.lang.String

I would recommend changing the return type of getFieldClass to be optional in case the field value doesn't make sense!

Answer 2

Maybe this can help you. TypeTags and manifests.

For example, we can write a method which takes some arbitrary object, and using a TypeTag, prints information about that object's type arguments:

 import scala.reflect.runtime.universe._ def paramInfo[T](x: T)(implicit tag: TypeTag[T]): Unit = { val targs = tag.tpe match { case TypeRef(_, _, args) => args } println(s"type of $x has type arguments $targs") } 

Here, we write a generic method paramInfo parameterized on T, and we supply an implicit parameter (implicit tag: TypeTag[T]). We can then directly access the type (of type Type) that tag represents using method tpe of TypeTag.

We can then use our method paramInfo as follows:

 scala> paramInfo(42) type of 42 has type arguments List() scala> paramInfo(List(1, 2)) type of List(1, 2) has type arguments List(Int)