18.6 Adding Methods at Compile Time

Grails 3.0 makes it easy to add new traits to existing artefact types from a plugin. For example say you wanted to add methods for manipulating dates to controllers. This can be done by defining a trait in src/main/groovy:

package myplugin
@Enhances("Controller")
trait DateTrait {
  Date currentDate() {
    return new Date()
  }
}

The @Enhances annotation defines the types of artefacts that the trait should be applied to.

As an alternative to using the @Enhances annotation above, you can implement a TraitInjector to tell Grails which artefacts you want to inject the trait into at compile time:

package myplugin
@CompileStatic
class ControllerTraitInjector implements TraitInjector {
    @Override
    Class getTrait() {
        SomeTrait
    }
    @Override
    String[] getArtefactTypes() {
        ['Controller'] as String[]
    }
}

The above TraitInjector will add the SomeTrait to all controllers. The getArtefactTypes method defines the types of artefacts that the trait should be applied to.

Applying traits conditionally

A TraitInjector implementation can also implement the SupportsClassNode interface to apply traits to only those artefacts which satisfy a custom requirement.For example, if a trait should only be applied if the target artefact class has a specific annotation, it can be done as below

package myplugin
@CompileStatic
class AnnotationBasedTraitInjector implements TraitInjector, SupportsClassNode {
    @Override
    Class getTrait() {
        SomeTrait
    }
    @Override
    String[] getArtefactTypes() {
        ['Controller'] as String[]
    }
    boolean supports(ClassNode classNode) {
      return GrailsASTUtils.hasAnnotation(classNode, SomeAnnotation)
    }
}

Above TraitInjector will add the SomeTrait to only those controllers which has the SomeAnnotation declared.

The framework discovers trait injectors by way of a META-INF/grails.factories descriptor that is in the .jar file. This descriptor is automatically generated. The descriptor generated for the code shown above would look like this:

#Grails Factories File
grails.compiler.traits.TraitInjector=
myplugin.ControllerTraitInjector,myplugin.DateTraitTraitInjector

That file is generated automatically and added to the .jar file at build time. If for any reason the application defines its own grails.factories file at src/main/resources/META-INF/grails.factories, it is important that the trait injectors be explicitly defined in that file. The auto-generated metadata is only reliable if the application does not define its own src/main/resources/META-INF/grails.factores file.