# Tutorial 4 - Concerns

In this tutorial we refactor the mixin from the previous steps so that the result of the say() method is modified to be prefixed with "Simon says:". To do this we need to implement a Concern for the say() method. Concerns are a type of Modifier which modify the behaviour of the methods in Mixins. They do this by intercepting the invocation of the TransientComposite. This allows them to change the invocation parameters, return their own values or throw their own exceptions, and do other things which directly affect the invocation of the method.

Concerns should not perform any side-effects, such as updating state in another TransientComposite, Mixin or similar. Any side-effects are done in SideEffects, which is another type of Modifier, which are allowed to perform side-effects but, in contrast to Concerns, cannot change the parameters or in any other way change the result of the invocation.

Concerns are implemented in one of two ways: either create a class which directly implements the interface whose methods should be modified, or create a generic Modifier by implementing the InvocationHandler interface (or subclass GenericConcern which does this for you). Add an @ConcernFor dependency injection, as a field, constructor parameter or method parameter, which has the same type as the interface the Concern implements. When the TransientComposite is invoked the Concern will be called, allowing it to perform it's work. If the call should proceed, then invoke the method again on the injected object. The preferred way to do all of this is to subclass ConcernOf which does all of this for you.

Concerns are applied by adding an @Concerns annotation on the TransientComposite, the domain interface, or the Mixin implementation. Any of these works, and where to put it is a matter of design choice.

Steps for this tutorial:

1. Create a typed concern, implement the HelloWorldBehaviour and let it modify the result of the base method by prefix the result with "Simon says:".
2. Add an @Concerns annotation on the HelloWorldBehaviourMixin which references the Concern class.