[[kotlin-bean-definition-dsl]]
= Bean Definition DSL

Spring Framework supports registering beans in a functional way by using lambdas
as an alternative to XML or Java configuration (`@Configuration` and `@Bean`). In a nutshell,
it lets you register beans with a lambda that acts as a `FactoryBean`.
This mechanism is very efficient, as it does not require any reflection or CGLIB proxies.

In Java, you can, for example, write the following:

[source,java,indent=0]
----
	class Foo {}

	class Bar {
		private final Foo foo;
		public Bar(Foo foo) {
			this.foo = foo;
		}
	}

	GenericApplicationContext context = new GenericApplicationContext();
	context.registerBean(Foo.class);
	context.registerBean(Bar.class, () -> new Bar(context.getBean(Foo.class)));
----

In Kotlin, with reified type parameters and `GenericApplicationContext` Kotlin extensions,
you can instead write the following:

[source,kotlin,indent=0]
----
	class Foo

	class Bar(private val foo: Foo)

	val context = GenericApplicationContext().apply {
		registerBean<Foo>()
		registerBean { Bar(it.getBean()) }
	}
----

When the class `Bar` has a single constructor, you can even just specify the bean class,
the constructor parameters will be autowired by type:

[source,kotlin,indent=0]
----
	val context = GenericApplicationContext().apply {
		registerBean<Foo>()
		registerBean<Bar>()
	}
----

In order to allow a more declarative approach and cleaner syntax, Spring Framework provides
a {docs-spring-framework}/kdoc-api/spring-context/org.springframework.context.support/-bean-definition-dsl/index.html[Kotlin bean definition DSL]
It declares an `ApplicationContextInitializer` through a clean declarative API,
which lets you deal with profiles and `Environment` for customizing
how beans are registered.

In the following example notice that:

* Type inference usually allows to avoid specifying the type for bean references like `ref("bazBean")`
* It is possible to use Kotlin top level functions to declare beans using callable references like `bean(::myRouter)` in this example
* When specifying `bean<Bar>()` or `bean(::myRouter)`, parameters are autowired by type
* The `FooBar` bean will be registered only if the `foobar` profile is active

[source,kotlin,indent=0]
----
	class Foo
	class Bar(private val foo: Foo)
	class Baz(var message: String = "")
	class FooBar(private val baz: Baz)

	val myBeans = beans {
		bean<Foo>()
		bean<Bar>()
		bean("bazBean") {
			Baz().apply {
				message = "Hello world"
			}
		}
		profile("foobar") {
			bean { FooBar(ref("bazBean")) }
		}
		bean(::myRouter)
	}

	fun myRouter(foo: Foo, bar: Bar, baz: Baz) = router {
		// ...
	}
----

NOTE: This DSL is programmatic, meaning it allows custom registration logic of beans
through an `if` expression, a `for` loop, or any other Kotlin constructs.

You can then use this `beans()` function to register beans on the application context,
as the following example shows:

[source,kotlin,indent=0]
----
	val context = GenericApplicationContext().apply {
		myBeans.initialize(this)
		refresh()
	}
----

NOTE: Spring Boot is based on JavaConfig and
https://github.com/spring-projects/spring-boot/issues/8115[does not yet provide specific support for functional bean definition],
but you can experimentally use functional bean definitions through Spring Boot's `ApplicationContextInitializer` support.
See https://stackoverflow.com/questions/45935931/how-to-use-functional-bean-definition-kotlin-dsl-with-spring-boot-and-spring-w/46033685#46033685[this Stack Overflow answer]
for more details and up-to-date information. See also the experimental Kofu DSL developed in https://github.com/spring-projects/spring-fu[Spring Fu incubator].