bean-definition-dsl.adoc

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:

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:

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:

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

In order to allow a more declarative approach and cleaner syntax, Spring Framework provides a {spring-framework-api-kdoc}/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

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:

val context = GenericApplicationContext().apply {
	myBeans.initialize(this)
	refresh()
}

Note

Spring Boot is based on JavaConfig and {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 {stackoverflow-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 {spring-github-org}-experimental/spring-fu[Spring Fu incubator].