[GROOVY-5929] Support meta-annotations for AST transformation annotations

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Duplicate
• Affects Version/s: 2.0.6
• Assignee: Guillaume Sauthier
• Reporter: Daniel Gredler
• Created: 17/Jan/13 22:41
• Updated: 19/Jan/13 06:01
• Resolved: 18/Jan/13 15:47

Description

I've been using Spring's meta-annotation support lately [1], and I think a similar feature would be a good fit for Groovy's AST transformation annotations: @Bindable, @Canonical, @ToString, @EqualsAndHashCode, @TupleConstructor, @Immutable, etc.

The idea is that in order to avoid repetition and boilerplate code, you can annotate a custom annotation, and the custom annotation carries its metadata around with it.

So for example if I have 20 DTOs, and they all look like this:

@Bindable
@Canonical(includes = "some,list,of,fields")
@ToString(includeNames = true, includePackage = false)
class MyDto {
  ...
}

With meta-annotation support, I can instead write a custom annotation:

@Bindable
@ToString(includeNames = true, includePackage = false)
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Dto {
}

And then use that annotation in my DTOs:

@Dto
@Canonical(includes = "some,list,of,fields")
class MyDto {
  ...
}

In the example above, we weren't able to move the @Canonical annotation into the meta-annotation because the list of "includes" fields varies by DTO. However, you could theoretically tell the meta-annotation to pull the @Canonical "includes" value from a parameter in your custom annotation, essentially allowing for parameterized meta-annotations:

@Bindable
@ToString(includeNames = true, includePackage = false)
@Canonical(includes = "businessKey")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Dto {
    String[] businessKey() default {};
}

Which would leave you with a beautifully terse, parameterized meta-annotation in those 20 DTOs:

@Dto(businessKey = "some,list,of,fields")
class MyDto {
  ...
}

[1] http://static.springsource.org/spring/docs/current/spring-framework-reference/htmlsingle/#tx-custom-attributes

Issue Links

• duplicates: GROOVY-5821 Provide a way to assemble an annoation consisting of several other