How to wrap @Column annotation with my own annotation in Java or Kotlin

Question

I simply want to have my own annotation to clean up the annotation mass and to be able to change them easily when I want;

import javax.persistence.Column
import javax.validation.constraints.Size
class Foo(){
    @Column(name="bar_", nullable = false, length = 32)
    @Size(min = 32, max = 32)
    String bar;

    @Column(nullable = false, length = 32)
    @Size(min = 32, max = 32)
    String bas;

    @Column(nullable = false, length = 32, unique=true)
    @Size(min = 32, max = 32)
    String baq;
}

Wish I could

class Foo(){
    @MyColumn(name="bar_")
    String bar;

    @MyColumn
    String bas;

    @MyColumn(unique=true)
    String baq;
}

nullable = false, length = 32 are the default params.

Java or Kotlin solutions are welcome.

Answer 1

Since you're using 3-rd party annotations imported from javax the best option is to introduce a composite annotation. (Kotlin doesn't support annotation inheritance.

@Column(name = "bar_", nullable = false, length = 32)
@Size(min = 32, max = 32)
annotation class Anno

Spring boot is doing a pretty good job combining tons of config annotations all together - check it out.

There is a problem with composite annotation Anno, tho. You have to supply annotation parameters with constant values.

If you're sure, you need a parametrised annotation like

@Column(...)
@Size(min = Anno.max, max = Anno.min)
annotation class Anno(val min: Int, val max: Int)

have a look at Kapt or Kotlin Compiler plugins, you will need a piece of code generation.

With Kapt or Kotlin compiler plugin you will need just to override a newField method of your custom ClassBuilder:

  override fun newField(
      origin: JvmDeclarationOrigin,
      access: Int,
      name: String,
      desc: String,
      signature: String?,
      value: Any?
  ): FieldVisitor {
    // if field is annotated with Anno -- add two custom annotation with parameters of your choice
    // otherwise perform a standard field init
  }

And then register it with

class AnnoRegister : ComponentRegistrar {
  override fun registerProjectComponents(
      project: MockProject,
      configuration: CompilerConfiguration
  ) {
    ...
  }

It should be relatively easy to integrate this processing into an existing gradle or maven project, or just pass to kotlinc.