How do you call a Scala singleton method from Java

问题:

I'm trying to inject some Scala code into my existing Java app. (So, being said, I want some more fun).

I create a singleton stuff in Scala

ScalaPower.scala

    package org.fun
    class ScalaPower
    object ScalaPower{
      def showMyPower(time:Int) = {
        (0 to time-1).mkString(", ")
      }
    }

Now, inside OldJava.java

class OldJava {
  public void demo(){
    System.out.println(?)
  }
}

What should I fill in ? so that Java will call the showMyPower method? I tried both org.fun.ScalaPower.showMyPower(10) and org.fun.ScalaPower.getInstance().showMyPower(10) but none work.

(Decompile the class file using Jad show me nothing but nonsense code.)

Edit I remove the class ScalaPower declaration and scala produce the static method as expected. (call to org.fun.ScalaPower.showMyPower(10) just works).

Wonder if it's a bug in scala compiler or not

回答1:

I think this indirectly covers it:

Companion Objects and Java Static Methods

There is one more thing to know about companion objects. Whenever you define a main method to use as the entry point for an application, Scala requires you to put it in an object. However, at the time of this writing, main methods cannot be defined in a companion object. Because of implementation details in the generated code, the JVM won’t find the main method. This issue may be resolved in a future release. For now, you must define any main method in a singleton object (i.e., a “non-companion” object) [ScalaTips]. Consider the following example of a simple Person class and companion object that attempts to define main.

As found here: http://programming-scala.labs.oreilly.com/ch06.html

In short because your Object is a companion object (has a companion class) you can't call it like you expect. As you found if you get rid of the class it will work.

回答2:

It's usually better to access the singleton directly from its own class.

In this case:

org.fun.ScalaPower$.MODULE$.showMyPower(10);

Without going too much into the implementation details, Scala differentiates namespaces between Object and Class/Trait. This means they can use the same name. However, an object has a class, and therefore needs a mangled name on the JVM. The current Scala conventions is to add a $ at the end of the module name (for top-level modules). If the object is defined in a class, I believe the convention is OuterClass$ModuleName$. To enforce the singleton property of the ScalaPower module, there is also a static MODULE$ member of the ModuleName$ class. This is initialised at class-load time, ensuring that there is only one instance. A side effect of this is that you should not do any sort of locking in a module's constructor.

In any case, Scala also has built into it a "make things nicer for Java" static-forwarders mechanism. This is where it writes static methods on the ScalaPower class that just call ScalaPower$.MODULE$.someMethod(). If you also define a companion class, the forwarders that could be generated are limited, as you are not allowed to have naming conflicts with static and instance-level methods on the JVM. I think in 2.8.0 this means if you have a companion object, you lose your static forwarders.

In this case a "best practice" would be to always use the ScalaPower$.MODULE$ reference instead of a static forwarder, as the forwarder could disappear with modifications to the ScalaPower class.

EDIT: Typo

回答3:

What were the errors you were getting? Using your Scala sample and the following Java class:

cat test.java:

import org.fun.*;

public class test {
    public static void main(String args[]) {
       System.out.println("show my power: " + ScalaPower.showMyPower(3));       
    }
}

And running it as follows:

java -cp .:<path-to/scala/install-dir>/lib/scala-library.jar test

gives my the output:

show my power: 0, 1, 2

回答4:

Keep in mind that the stock javap tool can be used to see what the Scala compiler produces. It doesn't directly answer your question, of course, but when what you need is just to be reminded of the code generation patterns, it's sufficient.

回答5:

After doing

class ScalaPower 
object ScalaPower{
  def showMyPower(time:Int) = {
    (0 to time-1).mkString(", ")
  }
}

ScalaPower.showMyPower(10) works as expected.