diff --git a/spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc b/spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc index ffbc173c035..2faecdeea55 100644 --- a/spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc +++ b/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[] diff --git a/spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc b/spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc index e7eb5e981f9..e4cc59c4264 100644 --- a/spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc +++ b/spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc @@ -7534,6 +7534,225 @@ Boot's core features will be honoured by the presence of the core starter. +[[boot-features-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 +{kotlin-documentation}java-interop.html[interoperability] with existing libraries written +in Java. + +Spring Boot provides Kotlin support by leveraging the support in other Spring projects +such as Spring Framework, Spring Data, and Reactor. See the +{spring-reference}languages.html#kotlin[Spring Framework Kotlin support documentation] +for more information. + +The easiest 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[Stack +Overflow] if you need support. + + + +[[boot-features-kotlin-requirements]] +=== Requirements +Spring Boot supports Kotlin 1.2.x. To use Kotlin, `org.jetbrains.kotlin:kotlin-stdlib` and +`org.jetbrains.kotlin:kotlin-reflect` must be present on the classpath. The +`kotlin-stdlib` variants `kotlin-stdlib-jdk7` and `kotlin-stdlib-jdk8` can also be used. + +Since https://discuss.kotlinlang.org/t/classes-final-by-default/166[Kotlin classes are +final by default], 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 so that they can be +proxied. + +https://github.com/FasterXML/jackson-module-kotlin[Jackson's Kotlin module] is required +for serializing / deserializing JSON data in Kotlin. It is automatically registered when +found on the classpath. A warning message is logged if Jackson and Kotlin are present but +the Jackson Kotlin module is not. + +TIP: These dependencies and plugins are provided by default if one bootstraps a Kotlin +project on https://start.spring.io/#!language=kotlin[start.spring.io]. + + + +[[boot-features-kotlin-null-safety]] +=== Null-safety +One of Kotlin's key features is {kotlin-documentation}null-safety.html[null-safety]. It +deals with `null` values at compile time rather than deferring the problem to runtime and +encountering a `NullPointerException`. This helps to eliminate a common source of bugs +without paying the cost of wrappers like `Optional`. Kotlin also allows using functional +constructs with nullable values as described in this +http://www.baeldung.com/kotlin-null-safety[comprehensive guide to null-safety in Kotlin]. + +Although Java does not allow one to express null-safety in its type system, Spring +Framework, Spring Data, and Reactor now provide 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's support for JSR 305 +annotations] combined with nullability annotations provide null-safety for the related +Spring API in Kotlin. + +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 as +`-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). + +WARN: Generic type arguments, varargs and array elements nullability are not yet +supported. See https://jira.spring.io/browse/SPR-15942[SPR-15942] for up-to-date +information. Also be aware that Spring Boot's own API is {github-issues}10712[not yet +annotated]. + + + +[[boot-features-kotlin-api]] +=== Kotlin API + + + +[[boot-features-kotlin-api-runapplication]] +==== runApplication +Spring Boot provides an idiomatic way to run an application with +`runApplication(*args)` as shown in the following example: + +[source,kotlin,indent=0] +---- +import org.springframework.boot.autoconfigure.SpringBootApplication +import org.springframework.boot.runApplication + +@SpringBootApplication +class FooApplication + +fun main(args: Array) { + runApplication(*args) +} +---- + +This is a drop-in replacement for +`SpringApplication.run(FooApplication::class.java, *args)`. It also allows customization +of the application as shown in the following example: + +[source,kotlin,indent=0] +---- +runApplication(*args) { + setBannerMode(OFF) +} +---- + + + +[[boot-features-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 APIs. + +`TestRestTemplate` extensions, similar to those provided by Spring Framework for +`RestOperations` in Spring Framework, are provided. Among other things, the extensions +make it possible to take advantage of Kotlin reified type parameters. + + + +[[boot-features-kotlin-dependency-management]] +=== Dependency management +In order to avoid mixing different version of Kotlin dependencies on the classpath, +dependency management of the following Kotlin dependencies is provided: + + - `kotlin-reflect` + - `kotlin-runtime` + - `kotlin-stdlib` + - `kotlin-stdlib-jdk7` + - `kotlin-stdlib-jdk8` + - `kotlin-stdlib-jre7` + - `kotlin-stdlib-jre8` + +With Maven, the Kotlin version can be customized via the `kotlin.version` property and +plugin management is provided for `kotlin-maven-plugin`. With Gradle, the Spring Boot +plugin automatically aligns the `kotlin.version` with the version of the Kotlin plugin. + + + +[[boot-features-kotlin-configuration-properties]] +=== `@ConfigurationProperties` +`@ConfigurationProperties` currently only works with `lateinit` or nullable `var` +properties (the former is recommended), since immutable classes initialized by +constructors are {github-issues}8762[not yet supported]. + +[source,kotlin,indent=0] +---- +@ConfigurationProperties("example.kotlin") +class KotlinExampleProperties { + + lateinit var foo1: String + + lateinit var foo2: String + + lateinit val bar = Bar() + + class Bar { + + lateinit var bar1: String + + lateinit var bar2: String + + } + +} +---- + + +[[boot-features-kotlin-testing]] +=== Testing +While it is possible to use JUnit 4 (the default provided by `spring-boot-starter-test`) +to test Kotlin code, JUnit 5 is recommended. JUnit 5 enables a test class to be +instantiated once and reused for all of the class's tests. This makes it possible to use +`@BeforeAll` and `@AfterAll` annotations on non-static methods, which is a good fit for +Kotlin. + +To use JUnit 5, exclude `junit:junit` dependency from `spring-boot-starter-test`, add +JUnit 5 dependencies, and configure the Maven or Gradle plugin accordingly. See the +{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"]. + + + +[[boot-features-kotlin-resources]] +=== Resources + + + +[[boot-features-kotlin-resources-further-reading]] +==== Further reading +* {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] +* 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] + + + +[[boot-features-kotlin-resources-examples]] +==== 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 If you want to learn more about any of the classes discussed in this section, you can