访问Scala中的注释值

(MyAnnotation) Thing.getClass().getAnnotations()[0]

尽管我可以愉快地发现基于其类型的注释和查询,但我似乎无法从scala.reflect.runtime.universe.Annotation获取到我的实际类型.

scala> // Declare an annotation (it seems StaticAnnotation means runtime
scala> // retention)
scala> case class MyAnnotation(x: Int,y: String) extends scala.annotation.StaticAnnotation
defined class MyAnnotation

scala> // Make a thing decorated with MyAnnotation
scala> @MyAnnotation(x=5,y="cool") case class Thing()
defined class Thing

scala> // Look at the annotation on the Thing...the runtime clearly
scala> // understands the values on it
scala> val annotation = scala.reflect.runtime.universe.typeOf[Thing].typeSymbol.asClass.annotations(0)
annotation: reflect.runtime.universe.Annotation = MyAnnotation(5,"cool")

scala> // I can sort of get at the values by index,which isn't terribly
scala> // safe
scala> annotation.scalaArgs(0)
res0: reflect.runtime.universe.Tree = 5

scala> // And what is a Tree here anyway? It certainly isn't a String (or
scala> // Int). I just want the value!
scala> annotation.scalaArgs(1)
res1: reflect.runtime.universe.Tree = "cool"

scala> // But how do I get at those values programatically?
scala> annotation.asInstanceOf[MyAnnotation]
java.lang.ClassCastException: scala.reflect.internal.AnnotationInfos$CompleteAnnotationInfo cannot be cast to MyAnnotation
        at .<init>(<console>:13)
        at .<clinit>(<console>)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at $print(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)
        at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:983)
        at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:573)
        at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:604)
        at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:568)
        at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:760)
        at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:805)
        at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:717)
        at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:581)
        at scala.tools.nsc.interpreter.ILoop.innerLoop$1(ILoop.scala:588)
        at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:591)
        at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:882)
        at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:837)
        at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:837)
        at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135)
        at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:837)
        at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:83)
        at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96)
        at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:105)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

关于这一切的欢乐部分是我甚至不能使用传统的Java方法,因为Scala不再费心从getAnnotations()填充数组了.

scala> Thing.getClass.getAnnotations.length
res2: Int = 0

有关

我认为我想要的是在“Reflection Overview”的“在运行时实例化类型”一节中,但我不明白为什么我必须跳过这么多的箍只是为了获得注释中的值.在Java中,价值只是一个抛弃.

问题“See annotations in Scala reflection”似乎有关,但问题是从2011年开始,适用于Scala 2.9.我正在使用2.10,据我所知,从那时起,大量的反射工作方式发生了变化.