scala - 如何通过反射调用伴生对象的方法？

Question

我有一个类名和一个我知道类型参数的方法，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，都会收到一个空引用错误。

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)
                                      ^

作为来自 "Dynamic" method invocation with new Scala reflection API 的示例这似乎不再起作用

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

引用 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