target

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.

For instance in the following class definition

class C(@myAnnot @BeanProperty var c: Int)

there are six entities which can carry the annotation @myAnnot: the constructor parameter, the generated field and the four accessors.

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 meta-annotations in package scala.annotation.target are used to control where annotations on fields and class parameters are copied. This is done by annotating either the annotation type or the annotation class with one or several of the meta-annotations in this package.

Annotating the annotation type

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
}

In order to annotate the field as well, the meta-annotation @field would need to be added.

The syntax can be improved using a type alias:

object ScalaJPA {
  type Id = javax.persistence.Id @beanGetter
}
import ScalaJPA.Id
class A {
  @Id @BeanProperty val x = 0
}

Annotating the annotation class

For annotations defined in Scala, a default target can be specified in the annotation class itself, for example

@getter
class myAnnotation extends Annotation

This only changes the default target for the annotation myAnnotation. When instantiating the annotation, the target can still be specified as described in the last section.