Add Kotlin support reference documentation

See gh-12073

spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc

@@ -15,6 +15,7 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson;
 :github-repo: spring-projects/spring-boot
 :github-raw: https://raw.github.com/{github-repo}/{github-tag}
 :github-code: https://github.com/{github-repo}/tree/{github-tag}
+:github-issues: https://github.com/{github-repo}/issues/
 :github-wiki: https://github.com/{github-repo}/wiki
 :github-master-code: https://github.com/{github-repo}/tree/master
 :sc-ext: java
@@ -68,6 +69,8 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson;
 :jetty-documentation: https://www.eclipse.org/jetty/documentation/9.4.x
 :micrometer-concepts-documentation: https://micrometer.io/docs/concepts
 :tomcat-documentation: https://tomcat.apache.org/tomcat-8.5-doc
+:kotlin-documentation: https://kotlinlang.org/docs/reference/
+:junit5-documentation: https://junit.org/junit5/docs/current/user-guide
 // ======================================================================================
 
 include::documentation-overview.adoc[]

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -7533,6 +7533,212 @@ relies on another starter). If a project is created with only your custom starte
 Boot's core features will be honoured by the presence of the core starter.
 
 
+[[kotlin]]
+== Kotlin support
+
+https://kotlinlang.org[Kotlin] is a statically-typed language targeting the JVM (and other
+platforms) which allows writing concise and elegant code while providing very good
+{kotlin-documentation}java-interop.html[interoperability] with
+existing libraries written in Java.
+
+As of version 2.0, Spring Boot provides Kotlin support leveraging other Spring projects
+Kotlin support like Spring Framework, Spring Data and Reactor. See the
+{spring-reference}languages.html#kotlin[Spring Framework Kotlin support] documentation
+for more information.
+
+The easy way to start with Spring Boot and Kotlin is to create a project via
+https://start.spring.io/#!language=kotlin[start.spring.io]. Feel free to join the #spring
+channel of http://slack.kotlinlang.org/[Kotlin Slack] or ask a question with `spring`
+and `kotlin` tags on https://stackoverflow.com/questions/tagged/spring+kotlin[Stackoverflow]
+if you need support.
+
+
+[[kotlin-requirements]]
+=== Requirements
+
+Spring Boot supports Kotlin 1.2+ and requires
+https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib[`kotlin-stdlib`]
+(or one of its https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jdk7[`kotlin-stdlib-jdk7`]
+/ https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jdk8[`kotlin-stdlib-jdk8`] variants)
+and https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-reflect[`kotlin-reflect`]
+to be present on the classpath.
+
+Since https://discuss.kotlinlang.org/t/classes-final-by-default/166[classes are final by default in Kotlin],
+you are likely to want to configure
+{kotlin-documentation}compiler-plugins.html#spring-support[kotlin-spring]
+plugin in order to automatically open Spring-annotated classes.
+
+https://github.com/FasterXML/jackson-module-kotlin[Jackson Kotlin module] which is required
+for serializing / deserializing JSON data is automatically registered when
+found in the classpath and a warning message will be logged if Jackson and Kotlin are
+detected without the Jackson Kotlin module present.
+
+These dependencies and plugins are provided by default if one bootstraps a Kotlin project on
+https://start.spring.io/#!language=kotlin[start.spring.io].
+
+
+[[kotlin-null-safety]]
+=== Null-safety
+
+One of Kotlin's key features is {kotlin-documentation}null-safety.html[null-safety]
+- which cleanly deals with `null` values at compile time rather than bumping into the
+famous `NullPointerException` at runtime. This makes applications safer through
+nullability declarations and expressing "value or no value" semantics without paying the
+cost of wrappers like `Optional`. (Kotlin allows using functional constructs with nullable
+values; check out this
+http://www.baeldung.com/kotlin-null-safety[comprehensive guide to Kotlin null-safety].)
+
+Although Java does not allow one to express null-safety in its type-system, Spring
+Framework, Spring Data and Reactor now provides null-safety of their API via
+tooling-friendly annotations. By default, types from Java APIs used in Kotlin are
+recognized as
+{kotlin-documentation}java-interop.html#null-safety-and-platform-types[platform types]
+for which null-checks are relaxed.
+{kotlin-documentation}java-interop.html#jsr-305-support[Kotlin support for JSR 305 annotations]
++ Spring nullability annotations provide null-safety for the related Spring API to Kotlin
+developers, with the advantage of dealing with `null` related issues at compile time.
+
+The JSR 305 checks can be configured by adding the `-Xjsr305` compiler flag with the
+following options: `-Xjsr305={strict|warn|ignore}`. The default behavior is the same to
+`-Xjsr305=warn`. The `strict` value is required to have null-safety taken in account in
+Kotlin types inferred from Spring API but should be used with the knowledge that Spring
+API nullability declaration could evolve even between minor releases and more checks may
+be added in the future).
+
+Note: Generic type arguments, varargs and array elements nullability are not supported yet,
+but should be in an upcoming release, see https://jira.spring.io/browse/SPR-15942[SPR-15942]
+for up-to-date information. Also be aware that Spring Boot own API is
+{github-issues}10712[not annotated yet].
+
+
+[[kotlin-api]]
+=== Kotlin API
+
+[[kotlin-api-runapplication]]
+==== runApplication
+
+Spring Boot provides a Kotlin idiomatic way to run an application with
+`runApplication<FooApplication>(*args)` which is a drop in replacement for
+`SpringApplication.run(FooApplication::class.java, *args)`.
+
+[source,kotlin,indent=0]
+----
+import org.springframework.boot.autoconfigure.SpringBootApplication
+import org.springframework.boot.runApplication
+
+@SpringBootApplication
+class FooApplication
+
+fun main(args: Array<String>) {
+    runApplication<FooApplication>(*args)
+}
+----
+
+It also allows to customize the application thanks to the following syntax:
+
+[source,kotlin,indent=0]
+----
+runApplication<FooApplication>(*args) {
+   setBannerMode(OFF)
+}
+----
+
+[[kotlin-api-extensions]]
+==== Extensions
+
+Kotlin {kotlin-documentation}extensions.html[extensions] provide the ability
+to extend existing classes with additional functionality. The Spring Boot Kotlin API makes
+use of these extensions to add new Kotlin specific conveniences to existing Spring APIs.
+
+`TestRestTemplate` extensions are provided similar to the `RestOperations` ones in order
+to be able to take advantage of Kotlin reified type parameters for example.
+
+
+[[kotlin-dependency-management]]
+=== Dependency management
+
+Dependency management of the following Kotlin dependencies is provided in order to avoid
+mixing different version of Kotlin dependencies in the classpath:
+
+ - `kotlin-reflect`
+ - `kotlin-runtime`
+ - `kotlin-stdlib`
+ - `kotlin-stdlib-jdk7`
+ - `kotlin-stdlib-jdk8`
+ - `kotlin-stdlib-jre7`
+ - `kotlin-stdlib-jre8`
+
+More dependencies should be managed {github-issues}9486[when a Kotlin BOM] will be available.
+
+With Maven, the Kotlin version can be customized via the `kotlin.version` property and
+plugin management is provided for `kotlin-maven-plugin`. With Gradle, Spring Boot plugin
+automatically defines `kotlin.version` based on the version
+specified for the Kotlin plugin.
+
+
+=== @ConfigurationProperties
+
+`@ConfigurationProperties` currently only works with `lateinit` or nullable `var` properties
+(the former is recommended) since immutable classes initialized by constructors are not
+supported yet until {github-issues}8762[this issue] is fixed.
+
+[source,kotlin,indent=0]
+----
+@ConfigurationProperties("foo")
+class FooProperties {
+	lateinit var foo1: String
+	lateinit var foo2: String
+	lateinit val bar = Bar()
+
+	class Bar {
+		lateinit var bar1: String
+		lateinit var bar2: String
+	}
+}
+----
+
+
+[[kotlin-testing]]
+=== Testing
+
+While using JUnit 4 (used by default in `spring-boot-starter-test`) is possible, JUnit 5
+enables a single instantiation of test classes which makes it possible to use `@BeforeAll`
+and `@AfterAll` annotations on non-static methods, which is a good fit for Kotlin.
+
+To use it, you need to exclude `junit:junit` dependency from `spring-boot-starter-test`
+and add JUnit 5 dependencies and configure Maven/Gradle plugin accordingly, see
+{junit5-documentation}/#dependency-metadata-junit-jupiter-samples[JUnit 5
+documentation] for more details. You also need to
+{junit5-documentation}/#writing-tests-test-instance-lifecycle-changing-default[switch test
+instance lifecycle to "per-class"].
+
+
+[[kotlin-resources]]
+=== Resources
+
+* {kotlin-documentation}[Kotlin language reference]
+* http://slack.kotlinlang.org/[Kotlin Slack] (with a dedicated #spring channel)
+* https://stackoverflow.com/questions/tagged/spring+kotlin[Stackoverflow with `spring` and `kotlin` tags]
+* https://try.kotlinlang.org/[Try Kotlin in your browser]
+* https://blog.jetbrains.com/kotlin/[Kotlin blog]
+* https://kotlin.link/[Awesome Kotlin]
+
+=== Blog posts
+
+* https://spring.io/blog/2016/02/15/developing-spring-boot-applications-with-kotlin[Developing Spring Boot applications with Kotlin]
+* https://spring.io/blog/2016/03/20/a-geospatial-messenger-with-kotlin-spring-boot-and-postgresql[A Geospatial Messenger with Kotlin, Spring Boot and PostgreSQL]
+* https://spring.io/blog/2017/01/04/introducing-kotlin-support-in-spring-framework-5-0[Introducing Kotlin support in Spring Framework 5.0]
+* https://spring.io/blog/2017/08/01/spring-framework-5-kotlin-apis-the-functional-way[Spring Framework 5 Kotlin APIs, the functional way]
+
+=== Examples
+
+* https://github.com/sdeleuze/spring-boot-kotlin-demo[spring-boot-kotlin-demo]: regular Spring Boot + Spring Data JPA project
+* https://github.com/mixitconf/mixit[mixit]: Spring Boot 2 + WebFlux + Reactive Spring Data MongoDB
+* https://github.com/sdeleuze/spring-kotlin-fullstack[spring-kotlin-fullstack]: WebFlux Kotlin fullstack example with Kotlin2js for frontend instead of JavaScript or TypeScript
+* https://github.com/spring-petclinic/spring-petclinic-kotlin[spring-petclinic-kotlin]: Kotlin version of the Spring PetClinic Sample Application
+* https://github.com/sdeleuze/spring-kotlin-deepdive[spring-kotlin-deepdive]: a step by step migration for Boot 1.0 + Java to Boot 2.0 + Kotlin
+
+
 
 [[boot-features-whats-next]]
 == What to Read Next