Scala — class declared fields and access modifiers

问题

I have been experimenting with Manifests and such in Scala, and I am having a very hard finding a way to use an object's fields when accessed via the getDeclaredFields method...

Here is an example:

class Woah(val x: String, val y: String)

val w = new Woah("w_x", "w_y")
classOf[Woah].getDeclaredFields foreach (field => println(field.get(w))

I have tried many variations, such as creating a method inside of the class Woah that performs that same action as the third line of code, but replace field.get(w) with field.get(this), and it shows the same exception. The exception thrown is:

java.lang.IllegalAccessException: Class Fun$Woah$$anonfun$1 can not access a member of class Fun$Woah with modifiers "private final"
    at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
    at java.lang.reflect.Field.doSecurityCheck(Field.java:960)
    at java.lang.reflect.Field.getFieldAccessor(Field.java:896)
    at java.lang.reflect.Field.get(Field.java:358)
    at Fun$Woah$$anonfun$1.apply(Fun.scala:17)
    at Fun$Woah$$anonfun$1.apply(Fun.scala:17)
    at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:34)
    at scala.collection.mutable.ArrayOps.foreach(ArrayOps.scala:35)
    at Fun$Woah.<init>(Fun.scala:17)
    at Fun$.main(Fun.scala:20)
    at Fun.main(Fun.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at scala.tools.nsc.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:81)
    at scala.tools.nsc.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:24)
    at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:86)
    at scala.tools.nsc.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:81)
    at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:86)
    at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:83)
    at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

I have done as much searching as I can, and I am not finding any new ideas at the moment. From what I could find, the vals "x" and "y" in the class Woah are declared as private final because they are used outside of the constructor. So I also tried this:

class Woah(val x: String, val y: String) {
  def printParams = classOf[Woah].getDeclaredFields foreach { field =>
    println(field.get(this))
  }
}

Unfortunately the same exception is thrown. Is there any way to have the two vals be public? Or is it possible to accomplish the same goal through another avenue? I am simply interested in accessing a collection of a class's fields' values.

Thank you in advance for your help!

回答1:

class Woah(val x: String, val y: String) {
  def printParams = classOf[Woah].getDeclaredFields foreach { field =>
    field.setAccessible(true)
    println(field.get(this))
  }
}

回答2:

All JVM fields in Scala are private. Access to them is made only through getters, in Scala, so that there's no distinction between them and a parameterless method.

来源：https://stackoverflow.com/questions/6756442/scala-class-declared-fields-and-access-modifiers