Method in companion compiled into static methods in scala?

Question

It seems that scala compiles methods in companion objects into static methods, which makes it easier to access them from java code. For example, you can write CompanionObject.method () instead of CompanionObject $ .MODULE $ .method (). However, sometimes a seemingly inappropriate code change violates this behavior. I came up with this example to illustrate the problem.

$ 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 using @SerialVersionUID, and init2 is compiled into a static method in TestCompanion, but not in TestCompanion2.

Can someone explain why scalac treats these two classes differently? I do not see how the @SerialVersionUID annotation should affect the availability of static methods.

Answer 1

This is a known bug: Static forwarders are missing after adding @SerialVersionUID raised by Josh Cash . From the error description:

In the following code, adding @SerialVersionUID to the class results in static forwarders that are not in 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 corresponding decompiled bytecode:

 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; }