groovy.transform

Annotation Type SelfType

@Documented
@Retention(value=RUNTIME)
@Target(value=TYPE)
public @interface SelfType

This annotation can be added on a trait to declare the list of types that a class implementing that trait is supposed to extend. This is useful when you want to be able to call methods from the class implementing the trait without having to declare all of them as members of the trait. Self types are particularly useful in combination with CompileStatic, if you know that a trait can only be applied to a specific type but that the trait cannot extend that type itself. For example, imagine the following code:

     class Component { void methodInComponent() }
     trait ComponentDecorator {
         void logAndCall() {
             println "Calling method in component"
             methodInComponent()
         }
         // other useful methods
     }
     class DecoratedComponent extends Component implements ComponentDecorator {}
 

This will work because the trait uses the dynamic backend, so there is no check at compile time that the methodInComponent call in logAndCall is actually defined. If you annotate the trait with CompileStatic, compilation will fail because the trait does not define the method. To declare that the trait can be applied on something that will extend Component, you need to add the SelfType annotation like this:

     class Component { void methodInComponent() }

     @CompileStatic
     @SelfType(Component)
     trait ComponentDecorator {
         void logAndCall() {
             println "Calling method in component"
             methodInComponent()
         }
         // other useful methods
     }
     class DecoratedComponent extends Component implements ComponentDecorator {}
 

This pattern can therefore be used to avoid explicit casts everywhere you need to call a method that you know is defined in the class that will implement the trait but normally don't have access to, which is often the case where a trait needs to be applied on a class provided by a third-party library.

Since:

2.4.0

Author:

Cédric Champeau

Required Element Summary

Required Elements

Modifier and Type	Required Element and Description
Class[]	value

Element Detail

value

public abstract Class[] value