Scala annotation target issue

README.md

Scala annotation behaviour change

The target of Java annotations used in Scala code has changed in some scenarios. It looks like the behaviour in Scala 2.11 was slightly incorrect if the containing case class was defined within a trait. From 2.12 onwards the behaviour has changed significantly.

Scala Library Docs

Taken from the scala.annotation.meta API reference:

When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with @BeanProperty, a bean getter and a bean setter.

...

By default, annotations on (val-, var- or plain) constructor parameters end up on the parameter, not on any other entity. Annotations on fields by default only end up on the field.

...

The target meta-annotations can be put on the annotation type when instantiating the annotation. In the following example, the annotation @Id will be added only to the bean getter getX.

import javax.persistence.Id
class A {
  @(Id @beanGetter) @BeanProperty val x = 0
}

Test Harness

The test harness applies the Java annotation @Info to the fields of two case classes - one defined in a separate compilation unit (TestBean) and one within a trait (WrappedBean). It then uses the Java reflection API to test where the annotation has been applied.

The test harness can be run from sbt using:

sbt +run

Results

Test Case	Expected	2.11	2.12	2.13
TestBean.defaultTarget	C	C	C	C
TestBean.fieldTarget	F	F	F	F
TestBean.getterTarget	M	M	M	M
TestBean.beanPropertyDefault	C	C	C	C
TestBean.beanPropertyField	F	F	F	F
TestBean.beanPropertyGetter	M	M	M	M
TestBean.beanPropertyBeanGetter	B	B	B	B
WrappedBean.defaultTarget	C	-	M	M
WrappedBean.fieldTarget	F	F	FMC	FMC
WrappedBean.getterTarget	M	M	M	M
WrappedBean.beanPropertyDefault	C	-	MB	MB
WrappedBean.beanPropertyField	F	FC	FMCB	FMCB
WrappedBean.beanPropertyGetter	M	MC	MC	MC
WrappedBean.beanPropertyBeanGetter	B	B	B	B

Key:

• C: constructor
• F: field
• M: getter method
• B: bean getter method

Raw Output

Scala 2.11.12

=== TestBean ===
defaultTarget:
  field:       false
  method:      false
  constructor: true
fieldTarget:
  field:       true
  method:      false
  constructor: false
getterTarget:
  field:       false
  method:      true
  constructor: false
beanPropertyDefault:
  field:       false
  method:      false
  getter:      false
  constructor: true
beanPropertyField:
  field:       true
  method:      false
  getter:      false
  constructor: false
beanPropertyGetter:
  field:       false
  method:      true
  getter:      false
  constructor: false
beanPropertyBeanGetter:
  field:       false
  method:      false
  getter:      true
  constructor: false
=== WrappedBean ===
defaultTarget:
  field:       false
  method:      false
  constructor: false
fieldTarget:
  field:       true
  method:      false
  constructor: true
getterTarget:
  field:       false
  method:      true
  constructor: false
beanPropertyDefault:
  field:       false
  method:      false
  getter:      false
  constructor: false
beanPropertyField:
  field:       true
  method:      false
  getter:      false
  constructor: true
beanPropertyGetter:
  field:       false
  method:      true
  getter:      false
  constructor: true
beanPropertyBeanGetter:
  field:       false
  method:      false
  getter:      true
  constructor: false

Scala 2.12.10

=== TestBean ===
defaultTarget:
  field:       false
  method:      false
  constructor: true
fieldTarget:
  field:       true
  method:      false
  constructor: false
getterTarget:
  field:       false
  method:      true
  constructor: false
beanPropertyDefault:
  field:       false
  method:      false
  getter:      false
  constructor: true
beanPropertyField:
  field:       true
  method:      false
  getter:      false
  constructor: false
beanPropertyGetter:
  field:       false
  method:      true
  getter:      false
  constructor: false
beanPropertyBeanGetter:
  field:       false
  method:      false
  getter:      true
  constructor: false
=== WrappedBean ===
defaultTarget:
  field:       false
  method:      true
  constructor: false
fieldTarget:
  field:       true
  method:      true
  constructor: true
getterTarget:
  field:       false
  method:      true
  constructor: false
beanPropertyDefault:
  field:       false
  method:      true
  getter:      true
  constructor: false
beanPropertyField:
  field:       true
  method:      true
  getter:      true
  constructor: true
beanPropertyGetter:
  field:       false
  method:      true
  getter:      false
  constructor: true
beanPropertyBeanGetter:
  field:       false
  method:      false
  getter:      true
  constructor: false

Scala 2.13.1

=== TestBean ===
defaultTarget:
  field:       false
  method:      false
  constructor: true
fieldTarget:
  field:       true
  method:      false
  constructor: false
getterTarget:
  field:       false
  method:      true
  constructor: false
beanPropertyDefault:
  field:       false
  method:      false
  getter:      false
  constructor: true
beanPropertyField:
  field:       true
  method:      false
  getter:      false
  constructor: false
beanPropertyGetter:
  field:       false
  method:      true
  getter:      false
  constructor: false
beanPropertyBeanGetter:
  field:       false
  method:      false
  getter:      true
  constructor: false
=== WrappedBean ===
defaultTarget:
  field:       false
  method:      true
  constructor: false
fieldTarget:
  field:       true
  method:      true
  constructor: true
getterTarget:
  field:       false
  method:      true
  constructor: false
beanPropertyDefault:
  field:       false
  method:      true
  getter:      true
  constructor: false
beanPropertyField:
  field:       true
  method:      true
  getter:      true
  constructor: true
beanPropertyGetter:
  field:       false
  method:      true
  getter:      false
  constructor: true
beanPropertyBeanGetter:
  field:       false
  method:      false
  getter:      true
  constructor: false

build.sbt

name := "annotation-behaviour"

scalaVersion := "2.12.10"
crossScalaVersions := Seq(scalaVersion.value, "2.11.12", "2.13.1")

Info.java

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface Info { }

Main.scala

import scala.util.Properties

object Main extends App with Wrapper {
  val annotation = classOf[Info]

  def checkAnnotations(clazz: Class[_]): Unit = {
    println(s"=== ${clazz.getSimpleName} ===")

    val constructor = clazz.getConstructors.head
    val params = constructor.getParameters

    println("defaultTarget:")
    println(s"  field:       ${clazz.getDeclaredField("defaultTarget").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("defaultTarget").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(0).isAnnotationPresent(annotation)}")

    println("fieldTarget:")
    println(s"  field:       ${clazz.getDeclaredField("fieldTarget").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("fieldTarget").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(1).isAnnotationPresent(annotation)}")

    println("getterTarget:")
    println(s"  field:       ${clazz.getDeclaredField("getterTarget").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("getterTarget").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(2).isAnnotationPresent(annotation)}")

    println("beanPropertyDefault:")
    println(s"  field:       ${clazz.getDeclaredField("beanPropertyDefault").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("beanPropertyDefault").isAnnotationPresent(annotation)}")
    println(s"  getter:      ${clazz.getDeclaredMethod("getBeanPropertyDefault").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(3).isAnnotationPresent(annotation)}")

    println("beanPropertyField:")
    println(s"  field:       ${clazz.getDeclaredField("beanPropertyField").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("beanPropertyField").isAnnotationPresent(annotation)}")
    println(s"  getter:      ${clazz.getDeclaredMethod("getBeanPropertyField").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(4).isAnnotationPresent(annotation)}")

    println("beanPropertyGetter:")
    println(s"  field:       ${clazz.getDeclaredField("beanPropertyGetter").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("beanPropertyGetter").isAnnotationPresent(annotation)}")
    println(s"  getter:      ${clazz.getDeclaredMethod("getBeanPropertyGetter").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(4).isAnnotationPresent(annotation)}")

    println("beanPropertyBeanGetter:")
    println(s"  field:       ${clazz.getDeclaredField("beanPropertyBeanGetter").isAnnotationPresent(annotation)}")
    println(s"  method:      ${clazz.getDeclaredMethod("beanPropertyBeanGetter").isAnnotationPresent(annotation)}")
    println(s"  getter:      ${clazz.getDeclaredMethod("getBeanPropertyBeanGetter").isAnnotationPresent(annotation)}")
    println(s"  constructor: ${params(5).isAnnotationPresent(annotation)}")
  }

  println(s"Scala ${Properties.versionNumberString}")
  checkAnnotations(classOf[TestBean])
  checkAnnotations(classOf[WrappedBean])
}

TestBean.scala

import scala.annotation.meta.{beanGetter, field, getter}
import scala.beans.BeanProperty

case class TestBean(
    @Info defaultTarget: String,
    @(Info @field) fieldTarget: String,
    @(Info @getter) getterTarget: String,
    @BeanProperty @Info beanPropertyDefault: String,
    @BeanProperty @(Info @field) beanPropertyField: String,
    @BeanProperty @(Info @getter) beanPropertyGetter: String,
    @BeanProperty @(Info @beanGetter) beanPropertyBeanGetter: String)

Wrapper.scala

import scala.annotation.meta.{beanGetter, field, getter}
import scala.beans.BeanProperty

trait Wrapper {
  case class WrappedBean(
      @Info defaultTarget: String,
      @(Info @field) fieldTarget: String,
      @(Info @getter) getterTarget: String,
      @BeanProperty @Info beanPropertyDefault: String,
      @BeanProperty @(Info @field) beanPropertyField: String,
      @BeanProperty @(Info @getter) beanPropertyGetter: String,
      @BeanProperty @(Info @beanGetter) beanPropertyBeanGetter: String)
}