Merge pull request #12073 from Sébastien Deleuze
* gh-12073: Polish "Add Kotlin support reference documentation" Add Kotlin support reference documentation
This commit is contained in:
Andy Wilkinson
commit
87953b32b1
|
|
@ -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[]
|
||||
|
|
|
|||
|
|
@ -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<FooApplication>(*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<String>) {
|
||||
runApplication<FooApplication>(*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<FooApplication>(*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
|
||||
|
|
|
|||
Loading…
Reference in New Issue