从 Java 访问 Scala 嵌套类

问题描述

假设我们在 Scala 中有以下类结构.

Let's say we have the following class structure in Scala.

object Foo {
  class Bar
}

我们可以使用 new Foo.Bar() 在 Java 中轻松构建 Bar.但是当我们添加额外级别的嵌套类时，一切都会改变.

We can easily construct Bar in Java with new Foo.Bar(). But everything changes when we add an extra level of nested classes.

object Foo {
  object Bar {
    class Baz
  }
}

不知何故，不再可能在 Java 中构造最内部的类 Baz.查看 javap 输出，我看不出第一(2 个级别)和第二个案例(3 个级别)之间有任何显着差异.生成的代码对我来说看起来很合理.

Somehow, it's no longer possible to construct the most inner class Baz in Java. Looking at the javap output, I can't see any significant difference between first (2 levels) and second cases (3 levels). Generated code looks pretty reasonable to me.

2 级:

public class Foo$Bar { ... }

3 级

public class Foo$Bar$Baz { ... }

话虽如此，当从 Java 访问时，2 级和 3 级嵌套 Scala 类之间有什么区别?

With that said, what's the difference between 2-level vs. 3-level nested Scala classes when they accessed from Java?

推荐答案

让我们给这两个版本起不同的名字，让他们更容易讨论:

Let's give the two versions different names to make them a little easier to talk about:

object Foo1 {
  class Bar1
}

object Foo2 {
  object Bar2 {
    class Baz2
  }
}

现在，如果您查看类文件，您将看到 Scala 编译器创建了一个 Foo1 类.当您在 Foo1$Bar1 上运行 javap -v 时，您会看到该类被列为封闭类:

Now if you look at the class files, you'll see that the Scala compiler has created a Foo1 class. When you run javap -v on Foo1$Bar1, you'll see that that class is listed as the enclosing class:

InnerClasses:
     public static #14= #2 of #13; //Bar1=class Foo1$Bar1 of class Foo1

这正是 Java 中的静态嵌套类会发生的情况，因此 Java 编译器非常乐意为您编译 new Foo1.Bar1().

This is exactly what would happen with a static nested class in Java, so the Java compiler is perfectly happy to compile new Foo1.Bar1() for you.

现在查看 Foo2$Bar2$Baz2 的 javap -v 输出:

Now look at the javap -v output for Foo2$Bar2$Baz2:

InnerClasses:
     public static #16= #13 of #15; //Bar2$=class Foo2$Bar2$ of class Foo2
     public static #17= #2 of #13; //Baz2=class Foo2$Bar2$Baz2 of class Foo2$Bar2$

现在封闭类是 Foo2$Bar2$，而不是 Foo2$Bar2(实际上 Scala 编译器甚至不生成 Foo2$Bar2 除非您为 object Bar2 添加伴随类).Java 编译器期望封闭类 Foo2$Bar2$ 的静态内部类 Baz2 被命名为 Foo2$Bar2$$Baz2，其中有两个美元符号.这与它实际得到的不匹配 (Foo2$Bar2$Baz2)，所以它对 new Foo2.Bar2.Baz2() 说不.

Now the enclosing class is Foo2$Bar2$, not Foo2$Bar2 (in fact the Scala compiler doesn't even generate a Foo2$Bar2 unless you add a companion class for object Bar2). The Java compiler expects a static inner class Baz2 of a enclosing class Foo2$Bar2$ to be named Foo2$Bar2$$Baz2, with two dollar signs. This doesn't match what it's actually got (Foo2$Bar2$Baz2), so it says no to new Foo2.Bar2.Baz2().

Java 非常乐意在类名中接受美元符号，在这种情况下，由于它无法弄清楚如何将 Foo2$Bar2$Baz2 解释为某种内部类，它将让您使用 new Foo2$Bar2$Baz2() 创建一个实例.所以这是一种解决方法，只是不是很漂亮.

Java is perfectly happy to accept dollar signs in class names, and in this case since it can't figure out how to interpret Foo2$Bar2$Baz2 as an inner class of some kind, it'll let you create an instance with new Foo2$Bar2$Baz2(). So that's a workaround, just not a very pretty one.

为什么 Scala 编译器对 Foo1 和 Bar2 的处理方式不同(在这个意义上，Bar2 没有得到 Bar2 类)，以及为什么 Baz2 的 InnerClasses 属性中列出的封闭类在末尾有一个美元符号，而 Bar1 没有?我真的没有任何想法.但这就是区别——你只需要更详细一点就可以用 javap 看到它.

Why does the Scala compiler treat Foo1 and Bar2 differently (in the sense that Bar2 doesn't get a Bar2 class), and why does the enclosing class listed in the InnerClasses attribute for Baz2 have a dollar sign on the end, while the one for Bar1 doesn't? I don't really have any idea. But that's the difference—you just need a little more verbosity to see it with javap.

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