[[aop-introductions]]
= Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare
that advised objects implement a given interface, and to provide an implementation of
that interface on behalf of those objects.

You can make an introduction by using the `@DeclareParents` annotation. This annotation
is used to declare that matching types have a new parent (hence the name). For example,
given an interface named `UsageTracked` and an implementation of that interface named
`DefaultUsageTracked`, the following aspect declares that all implementors of service
interfaces also implement the `UsageTracked` interface (e.g. for statistics via JMX):

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim",role="primary"]
----
	@Aspect
	public class UsageTracking {

		@DeclareParents(value="com.xyz.service.*+", defaultImpl=DefaultUsageTracked.class)
		public static UsageTracked mixin;

		@Before("execution(* com.xyz..service.*.*(..)) && this(usageTracked)")
		public void recordUsage(UsageTracked usageTracked) {
			usageTracked.incrementUseCount();
		}

	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
----
	@Aspect
	class UsageTracking {

		companion object {
			@DeclareParents(value = "com.xyz.service.*+",
				defaultImpl = DefaultUsageTracked::class)
			lateinit var mixin: UsageTracked
		}

		@Before("execution(* com.xyz..service.*.*(..)) && this(usageTracked)")
		fun recordUsage(usageTracked: UsageTracked) {
			usageTracked.incrementUseCount()
		}
	}
----
======

The interface to be implemented is determined by the type of the annotated field. The
`value` attribute of the `@DeclareParents` annotation is an AspectJ type pattern. Any
bean of a matching type implements the `UsageTracked` interface. Note that, in the
before advice of the preceding example, service beans can be directly used as
implementations of the `UsageTracked` interface. If accessing a bean programmatically,
you would write the following:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim",role="primary"]
----
	UsageTracked usageTracked = context.getBean("myService", UsageTracked.class);
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
----
	val usageTracked = context.getBean("myService", UsageTracked.class)
----
======


