(Quick Reference) # 18.6 Adding Methods at Compile Time Version: 3.2.8 ## 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. | Quick Reference (hide)