java - 为什么这段 Scala 代码会在运行时抛出 IllegalAccessError？

Question

我有一个简单的应用程序:

object Test extends App {
  implicit def t2mapper[X, X0 <: X, X1 <: X](t: (X0, X1)) = new {
    def map[R](f: X => R) = (f(t._1), f(t._2))
  }
  println("Hello!")
  val (foo, bar) = (1, 2) map (_ * 2)
  println((foo, bar))
}

(t2mapper 来自 this answer。)

代码编译正常:

$ scalac -version
Scala compiler version 2.9.1 -- Copyright 2002-2011, LAMP/EPFL
$ scalac -unchecked Test.scala
$ 

但是在运行时，它会抛出一个IllegalAccessError(之前 Hello! 被打印):

$ java -version
java version "1.6.0_24"
OpenJDK Runtime Environment (IcedTea6 1.11.1) (6b24-1.11.1-4ubuntu3)
OpenJDK Server VM (build 20.0-b12, mixed mode)
$ scala Test
java.lang.IllegalAccessError: tried to access field Test$.reflParams$Cache1 from class Test$delayedInit$body
        at Test$delayedInit$body.(Test.scala:6)
        at Test$.(Test.scala:1)
        at Test$.(Test.scala)
        at Test.main(Test.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:616)
        at scala.tools.nsc.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:78)
        at scala.tools.nsc.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:24)
        at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:88)
        at scala.tools.nsc.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:78)
        at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:101)
        at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:33)
        at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:40)
        at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:56)
        at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:80)
        at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:89)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

Note that with the last two lines replaced with

  println((1, 2) map (_ * 2))

或

  val (foo, bar) = (2, 4)
  println((foo, bar))

或

  val intermediate = (1, 2) map (_ * 2)
  val (foo, bar) = intermediate
  println((foo, bar))

它按预期打印 (2,4)。但是当包裹在一个 block 中时

  {
    val intermediate = (1, 2) map (_ * 2)
    val (foo, bar) = intermediate
    println((foo, bar))
  }

或

  private val blah = {
    val intermediate = (1, 2) map (_ * 2)
    val (foo, bar) = intermediate
    println((foo, bar))
  }

它抛出异常。

为什么第一种和最后一种方法会导致 JVM 在运行时抛出错误？

Answer 1

看起来有一些相关的 Unresolved 错误。例如，这一个可能是相关的:

https://issues.scala-lang.org/browse/SI-5251?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

请注意，您也可以创建一个 main 方法而不是扩展 App，它会起作用。

编辑:

当你使用这一行时(我扩展了隐式):

val (foo, bar) = t2mapper((1, 2)) map (_ * 2)

然后在 Eclipse 中将鼠标悬停在 foo 或 bar 上，它会显示 private[this] val foo。

因此，它看起来与SI-5251非常相似。