blocks|key|767294|text|这会不会与留存有关？我敢打赌，生成的字节码中没有包含@tailrec。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|767295|如果我试图扩展ClassfileAnnotation+(为了拥有运行时保留)，Scala告诉我这是不可能的，必须用Java语言完成：|767296|./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
++++++%5E|code-block|syntax|javascript|767297|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

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

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:

<pre><code>./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
 ^
</code></pre>

blocks|key|767338|text|我认为你现在只能在Java中定义注解。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|767339|http://www.scala-lang.org/node/106|offset|length|767340|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|Y|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

I think you can only define annotations in Java now.

<a href="http://www.scala-lang.org/node/106" rel="nofollow">http://www.scala-lang.org/node/106</a>

blocks|key|767391|text|您可以在Programming+Scala中找到关于如何在Scala中使用注释的详细描述。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|767392|因此，您可以在scala中定义或使用注释。然而，至少有一个限制：|767393|运行时保留是不太可能的。从理论上讲，您应该子类化ClassFileAnnotation来实现这一点，但目前scalac报告说，如果您这样做，会出现以下警告：|style|CODE|767394|实现限制:子类化Classfile不会使您的批注在运行时可见。如果这是您想要的，则必须用Java编写批注类。|767395|这也意味着您的代码没有问题(至少和目前在Scala中一样好)，但是注解只在编译时存在于类上。所以你可以在编译器插件中使用它，但是你不能在运行时访问它。|767396|entityMap|0|LINK|mutability|MUTABLE|url|http://programming-scala.labs.oreilly.com/ch13.html^0|4|H|0|0|0|O|J|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|6|7|Z|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|10|8|@$A|11|B|12|H|I]]|9|@]|C|$]]|$1|J|3|K|5|6|7|13|8|@]|9|@]|C|$]]|$1|L|3|M|5|6|7|14|8|@]|9|@]|C|$]]|$1|N|3|-4|5|6|7|15|8|@]|9|@]|C|$]]]|O|$P|$5|Q|R|S|C|$T|U]]]]

You can find a nice description of how annotations are to be used in Scala in <a href="http://programming-scala.labs.oreilly.com/ch13.html" rel="nofollow">Programming Scala</a>.

So you can define or use annotations in scala. However there is at least one limitation:

Runtime retention is not quite possible. In theory you should subclass <code>ClassFileAnnotation</code> to achieve this, but currently scalac reports the following warning if you do it:

"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."

It also means that your code is fine as it is (at least as fine as it is currently possible in Scala), but the annotation is on the class only during compile time. So you could use it e.g. in compiler plugins, but you will not be able to access it runtime.

blocks|key|767424|text|FWIW，你现在可以在Scala2.10中定义scala注解，并使用反射来读回它们。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|767425|以下是一些示例：Reflecting+Annotations+in+Scala+2.10|offset|length|767426|entityMap|0|LINK|mutability|MUTABLE|url|http://web.archive.org/web/20131014161251/http://www.veebsbraindump.com/2013/01/reflecting-annotations-in-scala-2-10/^0|0|8|10|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

FWIW, you can now define scala annotation in scala 2.10 and use reflection to read them back.

Here are some examples:
<a href="http://web.archive.org/web/20131014161251/http://www.veebsbraindump.com/2013/01/reflecting-annotations-in-scala-2-10/" rel="nofollow noreferrer">Reflecting Annotations in Scala 2.10</a>

blocks|key|982559|text|在scala+2.11.6中，这可以提取注解的值：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|982560|case+class+MyAnnotationClass(id:+String)+extends+scala.annotation.StaticAnnotation

val+myAnnotatedClass:+ClassSymbol+=+u.runtimeMirror(Thread.currentThread().getContextClassLoader).staticClass("MyAnnotatedClass")
val+annotation:+Option[Annotation]+=+myAnnotatedClass.annotations.find(_.tree.tpe+=:=+u.typeOf[MyAnnotationClass])
val+result+=+annotation.flatMap+{+a+=>
++a.tree.children.tail.collect({+case+Literal(Constant(id:+String))+=>+doSomething(id)+}).headOption
}|code-block|syntax|javascript|982561|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

With scala 2.11.6, this works to extract values of a annotation:

<pre class="lang-scala prettyprint-override"><code>case class MyAnnotationClass(id: String) extends scala.annotation.StaticAnnotation

val myAnnotatedClass: ClassSymbol = u.runtimeMirror(Thread.currentThread().getContextClassLoader).staticClass("MyAnnotatedClass")
val annotation: Option[Annotation] = myAnnotatedClass.annotations.find(_.tree.tpe =:= u.typeOf[MyAnnotationClass])
val result = annotation.flatMap { a =&gt;
 a.tree.children.tail.collect({ case Literal(Constant(id: String)) =&gt; doSomething(id) }).headOption
}
</code></pre>

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

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

<pre><code>class A extends StaticAnnotation

@A
class X {
 @A
 def aa() {}
}
</code></pre>

Then I write a test:

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

It prints some strange messages: 

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

Seems I can't get the annotation <code>aaa.A</code>.

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

How to create annotations and get them in scala

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我想定义一些注释并在Scala中使用它们。我查看了Scala的源代码，在scala.annotation包中找到了一些注释，比如tailrec、switch、elidable等等。因此，我定义了一些注释：class A extends StaticAnnotation@Aclass X {    @A    def a...

问如何在scala中创建批注和获取批注
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