Skip to content

S
spring-boot
Commit d96c5706 authored by Andy Wilkinson's avatar Andy Wilkinson
Browse files
Options

Polish "Add Kotlin support reference documentation" 

Closes gh-12073
parent 889a06b6
Hide whitespace changes
Inline Side-by-side
Showing with 103 additions and 90 deletions
spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc
View file @ d96c5706
...@@ -7533,93 +7533,91 @@ relies on another starter). If a project is created with only your custom starte ...@@ -7533,93 +7533,91 @@ 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. Boot's core features will be honoured by the presence of the core starter.
[[kotlin]]
== Kotlin support
[[boot-features-kotlin]]
== Kotlin support
https://kotlinlang.org[Kotlin] is a statically-typed language targeting the JVM (and other 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 platforms) which allows writing concise and elegant code while providing
{kotlin-documentation}java-interop.html[interoperability] with {kotlin-documentation}java-interop.html[interoperability] with existing libraries written
existing libraries written in Java. in Java.
As of version 2.0, Spring Boot provides Kotlin support leveraging other Spring projects Spring Boot provides Kotlin support by leveraging the support in other Spring projects
Kotlin support like Spring Framework, Spring Data and Reactor. See the such as Spring Framework, Spring Data, and Reactor. See the
{spring-reference}languages.html#kotlin[Spring Framework Kotlin support] documentation {spring-reference}languages.html#kotlin[Spring Framework Kotlin support documentation]
for more information. for more information.
The easy way to start with Spring Boot and Kotlin is to create a project via 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 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` 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] and `kotlin` tags on https://stackoverflow.com/questions/tagged/spring+kotlin[Stack
if you need support. Overflow] if you need support.
[[kotlin-requirements]]
=== Requirements
Spring Boot supports Kotlin 1.2+ and requires [[boot-features-kotlin-requirements]]
https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib[`kotlin-stdlib`] === Requirements
(or one of its https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jdk7[`kotlin-stdlib-jdk7`] Spring Boot supports Kotlin 1.2.x. To use Kotlin, `org.jetbrains.kotlin:kotlin-stdlib` and
/ https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jdk8[`kotlin-stdlib-jdk8`] variants) `org.jetbrains.kotlin:kotlin-reflect` must be present on the classpath. The
and https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-reflect[`kotlin-reflect`] `kotlin-stdlib` variants `kotlin-stdlib-jdk7` and `kotlin-stdlib-jdk8` can also be used.
to be present on the classpath.
Since https://discuss.kotlinlang.org/t/classes-final-by-default/166[classes are final by default in Kotlin], Since https://discuss.kotlinlang.org/t/classes-final-by-default/166[Kotlin classes are
you are likely to want to configure final by default], you are likely to want to configure
{kotlin-documentation}compiler-plugins.html#spring-support[kotlin-spring] {kotlin-documentation}compiler-plugins.html#spring-support[kotlin-spring]
plugin in order to automatically open Spring-annotated classes. plugin in order to automatically open Spring-annotated classes so that they can be
proxied.
https://github.com/FasterXML/jackson-module-kotlin[Jackson Kotlin module] which is required https://github.com/FasterXML/jackson-module-kotlin[Jackson's Kotlin module] is required
for serializing / deserializing JSON data is automatically registered when for serializing / deserializing JSON data in Kotlin. It is automatically registered when
found in the classpath and a warning message will be logged if Jackson and Kotlin are found on the classpath. A warning message is logged if Jackson and Kotlin are present but
detected without the Jackson Kotlin module present. the Jackson Kotlin module is not.
These dependencies and plugins are provided by default if one bootstraps a Kotlin project on TIP: These dependencies and plugins are provided by default if one bootstraps a Kotlin
https://start.spring.io/#!language=kotlin[start.spring.io]. 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] [[boot-features-kotlin-null-safety]]
- which cleanly deals with `null` values at compile time rather than bumping into the === Null-safety
famous `NullPointerException` at runtime. This makes applications safer through One of Kotlin's key features is {kotlin-documentation}null-safety.html[null-safety]. It
nullability declarations and expressing "value or no value" semantics without paying the deals with `null` values at compile time rather than deferring the problem to runtime and
cost of wrappers like `Optional`. (Kotlin allows using functional constructs with nullable encountering a `NullPointerException`. This helps to eliminate a common source of bugs
values; check out this without paying the cost of wrappers like `Optional`. Kotlin also allows using functional
http://www.baeldung.com/kotlin-null-safety[comprehensive guide to Kotlin null-safety].) 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 provides null-safety of their API via 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 tooling-friendly annotations. By default, types from Java APIs used in Kotlin are
recognized as recognized as
{kotlin-documentation}java-interop.html#null-safety-and-platform-types[platform types] {kotlin-documentation}java-interop.html#null-safety-and-platform-types[platform types]
for which null-checks are relaxed. for which null-checks are relaxed.
{kotlin-documentation}java-interop.html#jsr-305-support[Kotlin support for JSR 305 annotations] {kotlin-documentation}java-interop.html#jsr-305-support[Kotlin's support for JSR 305
+ Spring nullability annotations provide null-safety for the related Spring API to Kotlin annotations] combined with nullability annotations provide null-safety for the related
developers, with the advantage of dealing with `null` related issues at compile time. Spring API in Kotlin.
The JSR 305 checks can be configured by adding the `-Xjsr305` compiler flag with the 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 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 `-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 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 API nullability declaration could evolve even between minor releases and more checks may
be added in the future). be added in the future).
Note: Generic type arguments, varargs and array elements nullability are not supported yet, WARN: Generic type arguments, varargs and array elements nullability are not yet
but should be in an upcoming release, see https://jira.spring.io/browse/SPR-15942[SPR-15942] supported. See https://jira.spring.io/browse/SPR-15942[SPR-15942] for up-to-date
for up-to-date information. Also be aware that Spring Boot own API is information. Also be aware that Spring Boot's own API is {github-issues}10712[not yet
{github-issues}10712[not annotated yet]. annotated].
[[kotlin-api]]
[[boot-features-kotlin-api]]
=== 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 [[boot-features-kotlin-api-runapplication]]
`SpringApplication.run(FooApplication::class.java, *args)`. ==== runApplication
Spring Boot provides an idiomatic way to run an application with
`runApplication<FooApplication>(*args)` as shown in the following example:
[source,kotlin,indent=0] [source,kotlin,indent=0]
---- ----
...@@ -7630,35 +7628,39 @@ import org.springframework.boot.runApplication ...@@ -7630,35 +7628,39 @@ import org.springframework.boot.runApplication
class FooApplication class FooApplication
fun main(args: Array<String>) { fun main(args: Array<String>) {
runApplication<FooApplication>(*args) runApplication<FooApplication>(*args)
} }
---- ----
It also allows to customize the application thanks to the following syntax: 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] [source,kotlin,indent=0]
---- ----
runApplication<FooApplication>(*args) { runApplication<FooApplication>(*args) {
setBannerMode(OFF) setBannerMode(OFF)
} }
---- ----
[[kotlin-api-extensions]]
==== Extensions
[[boot-features-kotlin-api-extensions]]
==== Extensions
Kotlin {kotlin-documentation}extensions.html[extensions] provide the ability Kotlin {kotlin-documentation}extensions.html[extensions] provide the ability
to extend existing classes with additional functionality. The Spring Boot Kotlin API makes 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. use of these extensions to add new Kotlin specific conveniences to existing APIs.
`TestRestTemplate` extensions are provided similar to the `RestOperations` ones in order `TestRestTemplate` extensions, similar to those provided by Spring Framework for
to be able to take advantage of Kotlin reified type parameters for example. `RestOperations` in Spring Framework, are provided. Among other things, the extensions
make it possible to take advantage of Kotlin reified type parameters.
[[kotlin-dependency-management]]
=== Dependency management
Dependency management of the following Kotlin dependencies is provided in order to avoid [[boot-features-kotlin-dependency-management]]
mixing different version of Kotlin dependencies in the classpath: === 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-reflect`
- `kotlin-runtime` - `kotlin-runtime`
...@@ -7668,69 +7670,80 @@ mixing different version of Kotlin dependencies in the classpath: ...@@ -7668,69 +7670,80 @@ mixing different version of Kotlin dependencies in the classpath:
- `kotlin-stdlib-jre7` - `kotlin-stdlib-jre7`
- `kotlin-stdlib-jre8` - `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 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 plugin management is provided for `kotlin-maven-plugin`. With Gradle, the Spring Boot
automatically defines `kotlin.version` based on the version plugin automatically aligns the `kotlin.version` with the version of the Kotlin plugin.
specified for the Kotlin plugin.
=== @ConfigurationProperties
`@ConfigurationProperties` currently only works with `lateinit` or nullable `var` properties [[boot-features-kotlin-configuration-properties]]
(the former is recommended) since immutable classes initialized by constructors are not === `@ConfigurationProperties`
supported yet until {github-issues}8762[this issue] is fixed. `@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] [source,kotlin,indent=0]
---- ----
@ConfigurationProperties("foo") @ConfigurationProperties("example.kotlin")
class FooProperties { class KotlinExampleProperties {
lateinit var foo1: String lateinit var foo1: String
lateinit var foo2: String lateinit var foo2: String
lateinit val bar = Bar() lateinit val bar = Bar()
class Bar { class Bar {
lateinit var bar1: String lateinit var bar1: String
lateinit var bar2: String lateinit var bar2: String
} }
} }
---- ----
[[kotlin-testing]] [[boot-features-kotlin-testing]]
=== Testing === Testing
While it is possible to use JUnit 4 (the default provided by `spring-boot-starter-test`)
While using JUnit 4 (used by default in `spring-boot-starter-test`) is possible, JUnit 5 to test Kotlin code, JUnit 5 is recommended. JUnit 5 enables a test class to be
enables a single instantiation of test classes which makes it possible to use `@BeforeAll` instantiated once and reused for all of the class's tests. This makes it possible to use
and `@AfterAll` annotations on non-static methods, which is a good fit for Kotlin. `@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 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 {junit5-documentation}/#dependency-metadata-junit-jupiter-samples[JUnit 5
documentation] for more details. You also need to documentation] for more details. You also need to
{junit5-documentation}/#writing-tests-test-instance-lifecycle-changing-default[switch test {junit5-documentation}/#writing-tests-test-instance-lifecycle-changing-default[switch test
instance lifecycle to "per-class"]. instance lifecycle to "per-class"].
[[kotlin-resources]]
[[boot-features-kotlin-resources]]
=== Resources === Resources
[[boot-features-kotlin-resources-further-reading]]
==== Further reading
* {kotlin-documentation}[Kotlin language reference] * {kotlin-documentation}[Kotlin language reference]
* http://slack.kotlinlang.org/[Kotlin Slack] (with a dedicated #spring channel) * 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://stackoverflow.com/questions/tagged/spring+kotlin[Stackoverflow with `spring` and `kotlin` tags]
* https://try.kotlinlang.org/[Try Kotlin in your browser] * https://try.kotlinlang.org/[Try Kotlin in your browser]
* https://blog.jetbrains.com/kotlin/[Kotlin blog] * https://blog.jetbrains.com/kotlin/[Kotlin blog]
* https://kotlin.link/[Awesome Kotlin] * 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/2016/02/15/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/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/2017/01/04/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] * https://spring.io/blog/2017/08/01/spring-framework-5-kotlin-apis-the-functional-way[Spring Framework 5 Kotlin APIs, the functional way]
=== Examples
[[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/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/mixitconf/mixit[mixit]: Spring Boot 2 + WebFlux + Reactive Spring Data MongoDB
......
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment