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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`:

[source,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 {apiDocs}grails/compiler/traits/TraitInjector.html[TraitInjector] to tell Grails which artefacts you want to inject the trait into at compile time:

[source,groovy]
----
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 {apiDocs}grails/compiler/ast/SupportsClassNode.html[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


[source,groovy]
----
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:

[source]
----
#Grails Factories File
grails.compiler.traits.TraitInjector=
myplugin.ControllerTraitInjector,myplugin.DateTraitTraitInjector
----

NOTE: Due to formatting issues, above code snippet includes a line break after equal sign.

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.
