如何在Scala代码中访问Scala批注

如何解决如何在Scala代码中访问Scala批注

我已经定义了一个简单的scala注释，并使用它来注释我的案例类Person：

package com.my

import  scala.annotation.StaticAnnotation

@scala.annotation.meta.field
class Description(value: String) extends StaticAnnotation{

}

然后在我的Person类中使用它：

package com.my

import scala.beans.BeanProperty

case class Person(

                   @Description(value = "name00")
                   name: String,@Description(value = "age00")
                   age: Int,@BeanProperty
                   xyz: String = "xyz"
                 )

object Person {
  def main(args: Array[String]): Unit = {
    val p = Person("abc",21)
    classOf[Person].getDeclaredFields.foreach {
      field =>
        field.setAccessible(true)
        val name = field.getName
        val value = field.get(p)
        //annotations always return empty array
        val annotations = field.getDeclaredAnnotations
        annotations.foreach {
          annotation =>
            val tpe = annotation.annotationType()
            println(tpe)
        }
        println(s"name is $name,value is: $value")
    }
  }
}

在Person对象的main方法中，注释数组始终为空， 我想问一下如何获取在字段上定义的注释信息。

解决方法

首先，可以在源文件中（如果注释扩展了scala.annotation.Annotation和类文件（如果注释扩展了scala.annotation.StaticAnnotation）访问用Scala编写的注释。为了在运行时可访问，注释必须用Java编写

import java.lang.annotation.*;

@Target({ElementType.PARAMETER,ElementType.METHOD,ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Description {
    String value();
}

How to use Scala annotations in Java code

Why annotations written in Scala are not accessible at runtime?

https://www.reddit.com/r/scala/comments/81qzs2/how_to_write_annotations_in_scala/

或者，您可以使用原始的@Description（用Scala编写并扩展StaticAnnotation），但是随后您必须在运行时通过Scala reflection而不是Java反射来访问它。

其次，您滥用了元注释（@scala.annotation.meta.field）。它们不应注释@Description的定义，而应注释其应用。

import scala.annotation.meta.field

case class Person(
                   @(Description @field)(value = "name00")
                   name: String,@(Description @field)(value = "age00")
                   age: Int,@BeanProperty
                   xyz: String = "xyz"
                 )

Annotating case class parameters

How can I reflect on a field annotation (Java) in a Scala program?

Calling a method from Annotation using reflection

输出：

interface Description // appears
name is name,value is: abc
interface Description // appears
name is age,value is: 21
name is xyz,value is: xyz

---

借助Scala反射，您可以做到

import scala.annotation.StaticAnnotation
import scala.beans.BeanProperty
import scala.annotation.meta.field
import scala.reflect.runtime.universe._

class Description(value: String) extends StaticAnnotation

case class Person(
                   @(Description @field)(value = "name00")
                   name: String,@BeanProperty
                   xyz: String = "xyz"
                 )

def main(args: Array[String]): Unit = {
  val p = Person("abc",21)
  typeOf[Person].decls
    .collect { case t: TermSymbol if t.isVal => t.annotations }
    .foreach(println)
}

//List(Description @scala.annotation.meta.field("name00"))
//List(Description @scala.annotation.meta.field("age00"))
//List(scala.beans.BeanProperty)