Scala 对象的初始化是如何工作的？

Question

我是运行一个 sbt 应用程序实例化了一个 class MyObject 的对象，它使用来自其伴随对象的 val。这是主要的 class(object):

object MainClass {

  def main(args: Array[String]) {
   val a = new MyObject()
  }
}

这里是MyObject的定义：

import java.text.SimpleDateFormat
import java.util.Calendar


class MyObject {
  val aValue = MyObject.yesterday
}

object MyObject {

  val yesterday = getDaysAgo(1)

  val dateNumFormat = new SimpleDateFormat("yyyymmdd")

  private def getDaysAgo(n: Int) = {
    val today = Calendar.getInstance()
    today.add(Calendar.DAY_OF_MONTH, -n)
    //println(dateNumFormat.format(today.getTime))
    today.getTime
  }
}

当我取消注释 println 语句时，我得到（确切的）以下错误：

$ sbt run
[info] Loading project definition from C:\Work\metaswitch\RandomProject\project
[info] Set current project to RandomProject (in build file:/C:/Work/metaswitch/RandomProject/)
[info] Compiling 1 Scala source to C:\Work\metaswitch\RandomProject\target\scala-2.11\classes...
[info] Running MainClass
[error] (run-main-0) java.lang.ExceptionInInitializerError
java.lang.ExceptionInInitializerError
    at MyObject.<init>(MyObject.scala:21)
    at MainClass$.main(MainClass.scala:6)
    at MainClass.main(MainClass.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
Caused by: java.lang.NullPointerException
    at MyObject$.getDaysAgo(MyObject.scala:13)
    at MyObject$.<init>(MyObject.scala:6)
    at MyObject$.<clinit>(MyObject.scala)
    at MyObject.<init>(MyObject.scala:21)
    at MainClass$.main(MainClass.scala:6)
    at MainClass.main(MainClass.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
[trace] Stack trace suppressed: run last compile:run for the full output.
java.lang.RuntimeException: Nonzero exit code: 1
    at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last compile:run for the full output.
[error] (compile:run) Nonzero exit code: 1
[error] Total time: 2 s, completed 06-Jan-2016 11:21:16

Answer 1

字段的初始化按文本顺序进行（即，按顺序，字段在 class/object 中声明）。

鉴于代码中的声明顺序，首先声明 yesterday，它将首先被初始化，从而调用 getDaysAgo。该方法依次尝试读取（尚未初始化的）字段 dateNumFormat，它产生 null，导致表达式 dateNumFormat.format(today.getTime) 抛出 NullPointerException。

最简单的解决方法是，将 dateNumFormat 的声明（和初始化）移到 yesterday 之前——正如您已经发现的那样。

Scala 对象的初始化是如何工作的？

How does initialisation of Scala objects work?

jvm

scala

sbt