如何通过反射调用伴生对象上的方法？

Question

我有一个类名和一个我知道其类型参数的方法，我想通过反射调用这个方法。

在java中我会做类似的事情：

Class.forName("foo").getMethod("name", ... type args...).invoke(null, ..args)

但是在 Scala 中，每当我尝试调用时，都会收到 null ref 错误。

我正在使用 scala 2.10.4

-- 编辑

我试过了：

$ scala
Welcome to Scala version 2.10.5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class Foo { def bar(x: Int) = x }
defined class Foo

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> val foo = new Foo
foo: Foo = Foo@2db7a79b

scala> runtimeMirror(getClass.getClassLoader).reflect(foo)
res0: reflect.runtime.universe.InstanceMirror = instance mirror for Foo@2db7a79b

scala> res0.symbol.typeSignature.member(newTermName("bar"))
res1: reflect.runtime.universe.Symbol = method bar

scala> res0.reflectMethod(res1.asMethodSymbol)(42)
<console>:15: error: value asMethodSymbol is not a member of reflect.runtime.universe.Symbol
              res0.reflectMethod(res1.asMethodSymbol)(42)
                                      ^

作为"Dynamic" method invocation with new Scala reflection API 的示例，它似乎不再起作用了

这是我正在尝试做的测试：

object Test{
  def run(): Boolean = true
}

class InvokeTests extends FlatSpec with Matchers {
  "invoke" should "do" in {
    import scala.reflect.runtime.universe._

    val mirror = runtimeMirror(this.getClass.getClassLoader)

    val moduleSymbol = mirror.moduleSymbol(Class.forName(Test.getClass.getName))

    val reflected = mirror.reflect(moduleSymbol)

    val methodName = reflected.symbol.typeSignature.member(newTermName("run"))

    reflected.reflectMethod(methodName.asMethod)() shouldBe true
  }
}

Answer 1

参考scala doc reflection overview

// get runtime universe
val ru = scala.reflect.runtime.universe

// get runtime mirror
val rm = ru.runtimeMirror(getClass.getClassLoader)

// define class and companion object
class Boo{def hey(x: Int) = x}; object Boo{def hi(x: Int) = x*x}

// get instance mirror for companion object Boo
val instanceMirror = rm.reflect(Boo)

// get method symbol for the "hi" method in companion object
val methodSymbolHi = ru.typeOf[Boo.type].decl(ru.TermName("hi")).asMethod

// get method mirror for "hi" method
val methodHi = instanceMirror.reflectMethod(methodSymbolHi)

// invoke the method "hi"
methodHi(4)  // 16

Answer 2

下面的 Scala 2.13.3 示例编码在两种不同的情况下调用通用方法 apply，通过查找引用相应示例类的伴随对象：

package spielwiese.reflection_versuche

object Structural_Typing_Companion_Object_Invoke_Testframe {

  // Example 1
  case class Person01(vorname: String, name: String)

  object Person01 {
    def apply(x: Integer): Person01 = new Person01(s"Prename_$x", s"Lastname_$x")
  }

  // Example 2
  case class Person02(vorname: String, name: String)

  object Person02 {
    def apply(x: Integer): Person02 = new Person02(s"Prename_$x", s"Lastname_$x")
  }

  // Invocation demo:

  import scala.reflect.runtime.{universe => ru}

  def callGenericCompanionObjectMethod[T](implicit typetag: ru.TypeTag[T]) = {

    val m = ru.runtimeMirror(getClass.getClassLoader)

    val o1 = m.reflectModule(ru.typeOf[T].typeSymbol.companion.asModule).instance

    val o2 = o1.asInstanceOf[{ def apply(x: Integer): T }] // "structural typing"

    o2.apply(123)

  }

  def main(args: Array[String]): Unit = {

    println(callGenericCompanionObjectMethod[Person01]) 
    // prints: "Person01(Prename_123,Lastname_123)"

    println(callGenericCompanionObjectMethod[Person02]) 
    // prints: "Person02(Prename_123,Lastname_123)"

  }

}

Answer 3

如果您只有Type 的Foo（问题是foo）：

% scala
Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 15-loom).
Type in expressions for evaluation. Or try :help.

scala> :paste
// Entering paste mode (ctrl-D to finish)

import scala.reflect.runtime.currentMirror
import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe.definitions._

def f(tpe: Type): Unit = {
  val m = currentMirror
  val im = m.reflect(
    m.reflectModule(tpe.typeSymbol.asClass.companion.asModule).instance
  )
  val method = tpe.companion.decl(TermName("name")).asMethod
  val v = im.reflectMethod(method)("John", "Doe")
  println(v)
}

// Exiting paste mode, now interpreting.

import scala.reflect.runtime.currentMirror
import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe.definitions._
f: (tpe: reflect.runtime.universe.Type)Unit

scala> :paste
// Entering paste mode (ctrl-D to finish)

case class Foo()
case object Foo { def name(fn: String, ln: String): String = s"$fn $ln" }

// Exiting paste mode, now interpreting.

defined class Foo
defined object Foo

scala> f(typeOf[Foo])
John Doe