访问 Companion 对象中的私有字段答案

【问题标题】：Access private field in Companion object访问 Companion 对象中的私有字段
【发布时间】：2017-11-30 05:54:09
【问题描述】：

我有一个带有伴随对象的类TestClass。当从类中设置私有字段时，如何使用scala中的运行时反射访问伴生对象中的私有字段xyz，如下所示。

class TestClass {  TestClass.xyz = 100 }
object TestClass { private var xyz: Int = _ }

我尝试了以下

import scala.reflect.runtime.{currentMirror, universe => ru}
val testModuleSymbol = ru.typeOf[TestClass.type].termSymbol.asModule
val moduleMirror = currentMirror.reflectModule(testModuleSymbol)
val instanceMirror = currentMirror.reflect(moduleMirror.instance)
val xyzTerm = ru.typeOf[TestClass.type].decl(ru.TermName("xyz")).asTerm.accessed.asTerm
val fieldMirror = instanceMirror.reflectField(xyzTerm)
val context = fieldMirror.get.asInstanceOf[Int]

但我收到以下错误。

scala> val fieldMirror = instanceMirror.reflectField(xyzTerm)
scala.ScalaReflectionException: Scala field xyz  of object TestClass isn't represented as a Java field, nor does it have a
Java accessor method. One common reason for this is that it may be a private class parameter
not used outside the primary constructor.
  at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$abort(JavaMirrors.scala:115)
  at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$ErrorNonExistentField(JavaMirrors.scala:127)
  at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:242)
  at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:233)
  ... 29 elided

仅当我在TestClass 中引用变量xyz（即TestClass.xyz = 100）时才会引发此异常。如果从类中删除此引用，那么我的示例代码就可以正常工作。

【问题讨论】：

标签： scala reflection

【解决方案1】：

让这个工作：

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

val runMirror = ru.runtimeMirror(getClass.getClassLoader)
val objectDef = Class.forName("org.myorg.TestClass")
val objectTypeModule = runMirror.moduleSymbol(objectDef).asModule
val objectType = objectTypeModule.typeSignature

val methodMap = objectType.members
  .filter(_.isMethod)
  .map(d => {
    d.name.toString -> d.asMethod
  })
  .toMap

// get the scala Object
val instance = runMirror.reflectModule(objectTypeModule).instance
val instanceMirror = runMirror.reflect(instance)
// get the private value
val result = instanceMirror.reflectMethod(methodMap("xyz")).apply()
assert(result == 100)

【讨论】：