Question

它像Scala那样将相伴物体的方法编纂成静态方法,使得从java法典中援引这些方法稍加容易。例如,你可以撰写CompanionObject.method(而不是CompanionObject$)。教育部然而,有时似乎不相干的法典变化将打破这一行为。我举这个例子来说明问题。

$ cat TestCompanion.scala 
class TestCompanion

object TestCompanion {
  def init2 {}
}

@SerialVersionUID(1L)
class TestCompanion2

object TestCompanion2 {
  def init2 {}
}


$ scalac -version
Scala compiler version 2.9.0.1 -- Copyright 2002-2011, LAMP/EPFL

$ scalac TestCompanion.scala
$ javap TestCompanion
Compiled from "TestCompanion.scala"
public class TestCompanion extends java.lang.Object implements scala.ScalaObject{
    public static final void init2();
    public TestCompanion();
}

$ javap TestCompanion2
Compiled from "TestCompanion.scala"
public class TestCompanion2 extends java.lang.Object implements scala.ScalaObject{
    public static final long serialVersionUID;
    public static {};
    public TestCompanion2();
}

So the only difference between TestCompanion and TestCompanion2 is that the latter is annotated with @SerialVersionUID, and init2 is compiled into a static method in TestCompanion but not in TestCompanion2.

谁能解释为什么这些班级的加油处理方式不同? 我看不出“SerialVersionUID”说明如何影响静态方法的提供。

Answer 1

这是一个众所周知的ug:。添加了SerialVersionUID 后,便失踪了Stat Cough。根据ug的描述:

In the following code, adding @SerialVersionUID to the class results in the static forwarders missing from the byte code.

object WithoutUID {
 val instance = new WithoutUID
}
class WithoutUID extends scala.Serializable

object WithUID {
 val instance = new WithUID
}
@SerialVersionUID(0) 
class WithUID extends scala.Serializable

Here is the relevant decompiled byte code:

public class WithoutUID implements Serializable, ScalaObject {
  public static final WithoutUID instance(){
    return WithoutUID.MODULE$.instance();
  }
}

public class WithUID implements Serializable, ScalaObject {
  public static final long serialVersionUID = 0L;
}

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