代码之家 › 专栏 › 技术社区 › Tom

如何在scala代码中访问scala注释

scala

0

Tom · 技术社区 · 5 年前

我定义了一个简单的scala注释,并用它来注释我的case类Person:

package com.my

import  scala.annotation.StaticAnnotation

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

}

然后我在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,annotations数组总是空的,

0 回复 | 直到 5 年前

1

2

Dmytro Mitin 5 年前

首先, 在源代码中可以访问用Scala编写的注释(如果注释扩展 scala.annotation.Annotation )和类文件(如果注释扩展 scala.annotation.StaticAnnotation ). 为了在运行时可以访问,必须用Java编写注释

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/

或者你可以使用原版 @Description (用Scala编写并扩展) StaticAnnotation Scala reflection 而不是Java反射。

其次, 你误用了元注释( @scala.annotation.meta.field ). 他们不应该注释 @说明 但它的应用。

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

使用Scala反射,您可以

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,

                   @(Description @field)(value = "age00")
                   age: Int,

                   @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)