如何创建注释并在 Scala 中获取它们 [英] How to create annotations and get them in scala

问题描述

我想定义一些注解并在 Scala 中使用它们.

I want to define some annotations and use them in Scala.

我查看了 Scala 的源代码，在 scala.annotation 包中找到了一些注释，例如 tailrec、switch、elidable，等等.所以我像他们一样定义了一些注释:

I looked into the source of Scala, found in scala.annotation package, there are some annotations like tailrec, switch, elidable, and so on. So I defined some annotations as them do:

class A extends StaticAnnotation

@A
class X {
    @A
    def aa() {}
}

然后我写一个测试:

object Main {
    def main(args: Array[String]) {
        val x = new X
        println(x.getClass.getAnnotations.length)
        x.getClass.getAnnotations map { println }
    }
}

它打印了一些奇怪的消息:

It prints some strange messages:

1
@scala.reflect.ScalaSignature(bytes=u1" !1* 1!AbCaE
9"a!Q!! 1gn!!.<b    iBPE*,7
    Ii#)1oY1mC&1'G.Y(cUGCa#=S:LGO/AA!A  1mI!)

似乎我无法获得注释aaa.A.

如何在 Scala 中正确创建注释?以及如何使用和获取它们?

How can I create annotations in Scala correctly? And how to use and get them?

推荐答案

是否与保留有关?我敢打赌 @tailrec 不包含在生成的字节码中.

Could it have something to do with retention? I bet @tailrec is not included in the bytecode getting generated.

如果我尝试扩展 ClassfileAnnotation(为了有运行时保留)，Scala 告诉我它不能完成，必须在 Java 中完成:

If I try to extend ClassfileAnnotation (in order to have runtime retention), Scala tells me that it can't be done, and it has to be done in Java:

./test.scala:1: warning: implementation restriction: subclassing Classfile does not
make your annotation visible at runtime.  If that is what
you want, you must write the annotation class in Java.
class A extends ClassfileAnnotation
      ^

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