在任意 scala 对象中调用私有方法

Question

假设我有一个 Scala 对象：

object SomeObject {
  private def someMethod(msg: String): Unit = println(msg)
}

我可以使用以下代码调用someMethod：

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

def invokeObjectPrivateMethod[R](methodName: String, args: AnyRef*): R = {
  val rm = ru.runtimeMirror(getClass.getClassLoader)
  val instanceMirror = rm.reflect(SomeObject)
  val methodSymbol = ru.typeOf[SomeObject.type].decl(ru.TermName(methodName)).asMethod
  val method = instanceMirror.reflectMethod(methodSymbol)
  method(args: _*).asInstanceOf[R]
}
invokeObjectPrivateMethod("someMethod", "it works")

但在上面我已经将SomeObject 硬编码到函数中。我真正想要的是传递任意对象名称/类标签/任何东西，这样我就可以在任何对象中通用地调用私有函数。

这些尝试没有奏效：

// With explicit ClassTag parameter
def invokeObjectPrivateMethod2[R](classTag: ClassTag[_], methodName: String, args: AnyRef*): R = {
  val rm = ru.runtimeMirror(getClass.getClassLoader)
  val instanceMirror = rm.reflect(classTag)
  val methodSymbol = ru.typeOf[classTag.type].decl(ru.TermName(methodName)).asMethod
  val method = instanceMirror.reflectMethod(methodSymbol)
  method(args: _*).asInstanceOf[R]
}
// This fails at runtime with: `scala.ScalaReflectionException: <none> is not a method`
invokeObjectPrivateMethod2[Unit](ClassTag(SomeObject.getClass), "someMethod", "it doesn't work")

// With implicit ClassTag/TypeTag
def invokeObjectPrivateMethod3[T: ClassTag, S: ru.TypeTag, R](methodName: String, args: AnyRef*)(implicit ct: ClassTag[T]): R = {
  val rm = ru.runtimeMirror(getClass.getClassLoader)
  val instanceMirror = rm.reflect(ct)
  val methodSymbol = ru.typeOf[S].decl(ru.TermName(methodName)).asMethod
  val method = instanceMirror.reflectMethod(methodSymbol)
  method(args: _*).asInstanceOf[R]
}
// This fails at compile time: `not found: type SomeObject`
invokeObjectPrivateMethod3[SomeObject, SomeObject, Unit]("someMethod", "it also doesn't work")

我有点不知所措，所以我尝试使用第 3 个选项可能甚至没有意义。

谢谢！

Answer 1

如果您有 String 的对象名称，请尝试以下操作

def invokeObjectPrivateMethod[R](objectName: String, methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val moduleSymbol = rm.staticModule(objectName)
  val classSymbol = moduleSymbol.moduleClass.asClass
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val objectType = classSymbol.toType
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod("com.example.App.SomeObject", "someMethod", "it works") //it works

或者如果你有对象ClassTag

def invokeObjectPrivateMethod[R](classTag: ClassTag[_], methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val clazz = classTag.runtimeClass
  val classSymbol = rm.classSymbol(clazz)
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule //see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val objectType = classSymbol.toType
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}
invokeObjectPrivateMethod(classTag[SomeObject.type], "someMethod", "it works") //it works

或者如果你有对象TypeTag

def invokeObjectPrivateMethod[R](typeTag: TypeTag[_], methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val objectType = typeTag.tpe
  val clazz = rm.runtimeClass(objectType) // see (**)
  val classSymbol = rm.classSymbol(clazz)
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule // see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod(typeTag[SomeObject.type], "someMethod", "it works") //it works

或者如果你有对象Class

def invokeObjectPrivateMethod[R](clazz: Class[_], methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val classSymbol =  rm.classSymbol(clazz)
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule //see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val objectType = classSymbol.toType
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod(SomeObject.getClass, "someMethod", "it works") //it works

或者如果你有对象Type

def invokeObjectPrivateMethod[R](typ: Type, methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val classSymbol =  typ.typeSymbol.asClass
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule //see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val methodSymbol = typ.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod(typeOf[SomeObject.type], "someMethod", "it works") //it works

(*)Get the module symbol, given I have the module class, scala macro

(**)How to get ClassTag form TypeTag, or both at same time?

我假设该方法没有重载版本，否则您应该使用typ.decl(...).alternatives.find(...).get.asMethod 或typ.decl(...).alternatives.head.asMethod。

使用 Java 反射

def invokeObjectPrivateMethod[R](packageName: String, objectName: String, methodName: String, args: AnyRef*): R = {
  val javaClassName = s"$packageName.${objectName.replace('.', '$')}$$"
  val clazz = Class.forName(javaClassName)
  val method = clazz.getDeclaredMethods.find(_.getName == methodName).get
  method.setAccessible(true)
  val field = clazz.getField("MODULE$")
  val instance = field.get(null) // null because field is static
  method.invoke(instance, args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod("com.example", "App.SomeObject", "someMethod", "it works") //it works

使用 Java 方法句柄

def invokeObjectPrivateMethod[R](packageName: String, objectName: String, methodName: String, args: AnyRef*): R = {
  val javaClassName = s"$packageName.${objectName.replace('.', '$')}$$"
  val clazz = Class.forName(javaClassName)
  val lookup = MethodHandles.lookup
  val field = clazz.getField("MODULE$")
  val fieldMethodHandle = lookup.unreflectGetter(field)
  val instance = fieldMethodHandle.invokeWithArguments()
  val method = clazz.getDeclaredMethods.find(_.getName == methodName).get
  method.setAccessible(true)
  val methodHandle = lookup.unreflect(method)
  methodHandle.invokeWithArguments(instance +: args : _*).asInstanceOf[R]
}

invokeObjectPrivateMethod("com.example", "App.SomeObject", "someMethod", "it works") //it works