spring-kotlin: A Kotlin repository from dbacinski

Spring Framework 5 will provide Kotlin extensions and support builtin, so this specific library won't be needed for most common Spring + Kotlin features. you can use this link to see Spring Framework Kotlin related JIRA issues.

spring-kotlin is a repository that contains Kotlin extensions, documentation, and any helpful assets that can help to build Spring + Kotlin applications. This project has been created after this discussion on Spring Boot issue tracker.

You will find more information about using Spring Boot + Kotlin in these 3 blog posts:

Using Spring Kotlin

To use Spring Kotlin extensions, add the following dependency in your build.gradle file, with x.y.z replaced by the latest release available here:

repositories {
  maven { url 'https://dl.bintray.com/sdeleuze/maven' }
}

dependencies {
  compile 'org.springframework.kotlin:spring-kotlin:x.y.z'
}

Spring + Kotlin FAQ

###What is the simplest way to start a Spring Boot + Kotlin application?

Go to http://start.spring.io/#!language=kotlin, add your dependencies, choose Gradle or Maven and click on "Generate Project" ;-)

###Where this "Configuration problem: @Configuration class 'Foo' may not be final" error message come from?

Spring applications are using proxies to deal with most @Component beans like @Configuration, @Serviceor @Repository. JDK dynamic proxies are used when possible (your bean should implement an interface, and proxyTargetClass should be set to false, which is the default) while CGLIB proxies are used with classes that do not expose methods with interfaces, and/or when proxyTargetClass is set to true.

Unlike Java, Kotlin have made the choice to have a final by default behavior, with open keyword used at class or method level to allow extending or overriding them. As a consequence, with CGLIB proxies you need to explicitely specify open at class and method level since they require to extend/override the class/methods for technical reasons.

But Kotlin now provides a solution to that issue with the kotlin-spring plugin provided with Kotlin 1.0.6+, see this blog post for more information or spring-boot-kotlin-demo for a sample project. start.spring.io provide you projects pre-configured with the spring-kotlin plugin.

###Why annotation array attributes require arrayOf() for single value unlike in Java?

That's a known Kotlin 1.0 issue quite annoying with Spring annotations like @RequestMapping(method = arrayOf(RequestMethod.GET)), that will be fixed in Kotlin 1.1, see issue KT-11235 for more details. As a workaround, as of Spring Framework 4.3 / Spring Boot 1.4 you can use annotations aliases like @GetMapping, @PostMapping, etc.

###Why @Value("${thing}") does not work with Kotlin?

That's because $ is used for string interpolation in Kotlin. Workarounds are:

Escaping $: @Value("\${some.property}")
Configure PropertySourcesPlaceholderConfigurer to use another PlaceholderPrefix
Use @ConfigurationProperties instead

See this Stackoverflow response for more details.

###What is the recommanded way to inject dependencies in a Spring Kotlin application

Try to favor contructor injection. As of Spring Framework 4.3 / Spring Boot 1.4 you just have to write class MessageController(val repository: MessageService) and Spring will automatically autowire the constructor (there should be a single constructor with no default values). For previous versions, you need to write class MessageController @Autowired constructor(val repository: MessageService).

If you really need to use field injection, use lateinit var:

@Component
class YourBean {

    @Autowired
    lateinit var mongoTemplate: MongoTemplate

    @Autowired
    lateinit var solrClient: SolrClient
}

###How can I get Jackson working correctly with Kotlin classes? You should register Jackson Kotlin module. As of Spring Framework 4.3 / Spring Boot 1.4, you just have to add Maven or Gradle dependency and it will be registered automatically. With previous version, you will have to register it manually in one of your @Configuration classes:

@SpringBootApplication
open class Application {

    @Bean
    open fun objectMapperBuilder(): Jackson2ObjectMapperBuilder =
        Jackson2ObjectMapperBuilder().modulesToInstall(KotlinModule())
    
    // ...

How can I get Jackson working correctly with Kotlin Data Classes?

Once you have your Jackson Kotlin module registered you can use Jackson annotations. It is pretty straightforward for annotations like com.fasterxml.jackson.annotation.JsonIgnoreProperties but can get more complex when trying to use Jackson field annotations. Assuming you have a Data Class - Person, that you would serialize to JSON but with snake case convention for field names. In Java you would just use @JsonProperty("field_name"). It will not work in Kotlin. Here you have to tell Kotlin compiler that annotation has to be placed on generated getter method (as there are no backing fields in Kotlin Data Classes). You do that in the following way:

data class Person(
    @get:JsonProperty("name")
    val name: String,
    
    @get:JsonProperty("last_name")
    val lastName: String,
    
    @get:JsonProperty("year_of_birth")
    val yearOfBirth: Int
)

###How can I make my JPA or Spring Data entities immutable?

Use kotlin-jpa and/or kotlin-noarg compiler plugins as described in this blog post.

What are other Kotlin features or issues to follow?

Contribute

Before we accept a non-trivial patch or pull request we will need you to sign the contributor's agreement. Signing the contributor's agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests. Use Spring Framework in the project field and Sébastien Deleuze in the project lead field when you complete the form.

License