为什么`Future#toString`返回`“" List()“`? [英] Why does `Future#toString` returns `"List()"`?

问题描述

将来调用.toString而不等待完成将导致不确定的结果.我的问题是为什么在未完成的期货中调用.toString在scala 2.10.x和2.11.x中返回"List()"? 实现似乎并不明确关于那个.

Calling .toString on a future without waiting for complession leads to nondeterministic results. My question is why calling .toString on uncompleted futures returns "List()" in scala 2.10.x and 2.11.x? The implementation does not seem to be explicit about that.

从REPL中可以观察到此行为:

This behavior can be observed from the REPL:

scala> import scala.concurrent.Future, scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.ExecutionContext.Implicits.global

scala> Future(1).toString
res0: scala.concurrent.Future[Int] = Success(1)

scala> Future(1).toString
res1: scala.concurrent.Future[Int] = List()

请注意，Scala 2.12.x可能会显式实现Future#toString来返回"Future(<not completed>)"((

Note that Scala 2.12.x will likely explicitly implement Future#toString to return "Future(<not completed>)" instead (source).

证据表明这不是来自REPL的伪像或某些隐藏的隐式"(-Yno-predef删除了所有默认的隐式):

evidence that this is not an artefact comming from the REPL or "some hidden implicit" (-Yno-predef removes all the default implicits):

Future.scala:

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

object Main extends App {
  System.out.println(Future(1).toString)
}

build.sbt:

scalaVersion := "2.11.8"

scalacOptions := Seq(
  "-deprecation",
  "-encoding", "UTF-8",
  "-feature",
  "-unchecked",
  "-Yno-predef",
  "-Xfatal-warnings",
  "-Xlint",
  "-Yinline-warnings",
  "-Yno-adapted-args",
  "-Ywarn-dead-code",
  "-Ywarn-unused-import",
  "-Ywarn-numeric-widen",
  "-Ywarn-value-discard",
  "-Xfuture")

推荐答案

这是删除sun.misc.Unsafe依赖项的不幸副作用.在Scala 2.12和更高版本的Scala 2.11 IIRC中已得到纠正.

It was an unfortunate side effect of removing the dependence of sun.misc.Unsafe. It is corrected in Scala 2.12 and newer versions of Scala 2.11 IIRC.

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