Spring ❤️ Kotlin #jjug

‹#›© 2016 Pivotal Software, Inc. All rights reserved. ‹#›© 2016 Pivotal Software, Inc. All rights reserved.
Spring ❤ Kotlin
Toshiaki Maki (@making)
tmaki@pivotal.io
JJUG Night Seminar 2017 Feb
2017-02-20

© 2016 Pivotal Software, Inc. All rights reserved.
Who am I ?
• Toshiaki Maki (@making) https://ik.am
• Sr. Solutions Architect @Pivotal
• Spring ☘ / Cloud Foundry ☁ / Concourse ✈ / BOSH 🐚
bit.ly/hajiboot2

Agenda
•Spring Boot with Kotlin
•Kotlin support in Spring 5

‹#›© 2016 Pivotal Software, Inc. All rights reserved.
Spring Boot with Kotlin

Spring Initializr

package com.example 
 
import org.springframework.boot.SpringApplication 
import org.springframework.boot.autoconfigure.SpringBootApplication 
 
@SpringBootApplication 
class DemoApplication 
 
fun main(args: Array<String>) { 
SpringApplication.run(DemoApplication::class.java, *args) 
}

package com.example 
 
import org.springframework.web.bind.annotation.GetMapping 
import org.springframework.web.bind.annotation.RestController 
 
@RestController 
class HelloController { 
@GetMapping("/") 
fun hello() = "Hello World!" 
}

// Kotlin
open class AppConfig { 
@Bean 
open fun restTemplate() = RestTemplate() 
}

// Kotlin
@Bean 
}
👇
👇

// Kotlin
@Bean 
}
👇
👇
😩

Cglib and Spring
// Java
class AppConfig { 
@Bean 
RestTemplate restTemplate() {
return new RestTemplate();
} 
}

Cglib and Spring
// Java
@Bean 
return new RestTemplate();
} 
}
Singleton

Cglib and Spring (Pseudo Code)
class AppConfig$$ extends AppConfig {
@Override 
if (context.contains("restTemplate")) {
return context.get("restTemplate");
}
return super.restTemplate();
} 
}

Cglib and Spring (Pseudo Code)
class AppConfig$$ extends AppConfig {
@Override 
if (context.contains("restTemplate")) {
return context.get("restTemplate");
}
return super.restTemplate();
} 
}
Methods in Kotlin
are final by default !!
open removes final

<plugin> 
<artifactId>kotlin-maven-plugin</artifactId> 
<groupId>org.jetbrains.kotlin</groupId> 
<version>${kotlin.version}</version> 
<configuration> 
<compilerPlugins> 
<plugin>spring</plugin> 
</compilerPlugins> 
</configuration>
</plugin>

@Bean 
}

@Bean 
fun restTemplate() = RestTemplate() 
}

@Bean 
fun restTemplate() = RestTemplate() 
}
😍

http://start.spring.io/#!language=kotlin

‹#›© 2016 Pivotal Software, Inc. All rights reserved.
Kotlin support in Spring 5

Spring Framework 5
•Java 8 baseline, Java 9 compatibility
•Reactive Support & Spring WebFlux
•Router Functions
•Performance improvements
•HTTP/2 support
•Kotlin support

Kotlin Support

Kotlin Support
•Extension Functions
•Reified type parameters

Kotlin Support
•Extension Functions
•Reified type parameters
🤔

package com.example;
public class Foo {
public <T> T create(Class<T> clazz) {
try {
return clazz.newInstance();
}
catch (Exception e) {
throw new IllegalStateException(e);
}
}
}

// Java
Foo foo = new Foo();
Bar bar = foo.create(Bar.class);

// Java
// Kotlin
val foo = Foo()
val bar = foo.create(Bar::class.java)

// Java
// Kotlin
val foo = Foo()
👇

KClass
Bar::class // kotlin.reflect.KClass
Bar::class.java // java.lang.Class

Extension Functions
•Extends a class with new functionality
without having to inherit from the class
•https://kotlinlang.org/docs/reference/
extensions.html#extension-functions

// Kotlin
val foo = Foo()

// Kotlin
val foo = Foo()
val bar = foo.create(Bar::class)

Extension Functions
// Kotlin
val foo = Foo()

Extension Functions
package com.example
import kotlin.reflect.KClass
fun <T : Any> Foo.create(kclass: KClass<T>)
= create(kclass.java) 
FooExtensions.kt

Extension Functions
package com.example
import kotlin.reflect.KClass
fun <T : Any> Foo.create(kclass: KClass<T>)
= create(kclass.java) 
👇
FooExtensions.kt

Extension Functions
// Kotlin
val foo = Foo()
😍

Reified type parameters
•Access a type passed to us as a parameter
•https://kotlinlang.org/docs/reference/inline-
functions.html#reified-type-parameters

inline fun <reified T : Any> Foo.create()
= create(T::class.java)

inline fun <reified T : Any> Foo.create()
= create(T::class.java) 👇
👇

val foo = Foo()

val foo = Foo()
val bar = foo.create<Bar>() 
😍

val foo = Foo()
val bar: Bar = foo.create() 
😍

val foo = Foo()
val bar: Bar = foo.create() 
😍
"idiomatic Kotlin code"

"idiomatic Kotlin code" with Spring 5

Application Context
// Java
ApplicationContext context = ...;
Bar bar = context.getBean(Bar.class);

Application Context (Extension)
// Kotlin
val context = ...
val bar = context.getBean(Bar::class)

Application Context (Reified Type)
// Kotlin
val context = ...
val bar = context.getBean<Bar>()

Application Context (Reified Type)
// Kotlin
val context = ...
val bar: Bar = context.getBean()

JdbcTemplate
// Java
Long count = jdbcTemplate
.queryForObject("SELECT count(*) FROM foo"
, Long.class);

JdbcTemplate (Extension)
// Kotlin
val count = jdbcTemplate
.queryForObject("SELECT count(*) FROM foo"
, Long::class)

JdbcTemplate (Reified Type)
// Kotlin
val count = jdbcTemplate
.queryForObject<Long>(
"SELECT count(*) FROM foo")

JdbcTemplate (Reified Type)
// Kotlin
val count: Long = jdbcTemplate
.queryForObject("SELECT count(*) FROM foo")

RestTemplate
// Java
String foo = restTemplate
.getForObject("http://abc.io",String.class);

RestTemplate (Reified Type)
// Kotlin
val foo = restTemplate
.getForObject<String>("http://abc.io");

// Kotlin
val foo: String = restTemplate
.getForObject("http://abc.io");

RestTemplate
// Java
List<Foo> foos = restTemplate
.getForObject("http://abc.io",
List<Foo>.class);

RestTemplate
// Java
.getForObject("http://abc.io",
List<Foo>.class);
Compile Error!

RestTemplate
// Java
.exchange("http://abc.io", HttpMethod.GET,
null, new
ParameterizedTypeReference<List<Foo>>()
{}).getBody();

RestTemplate
// Java
.exchange("http://abc.io", HttpMethod.GET,
null, new
ParameterizedTypeReference<List<Foo>>()
{}).getBody();
💩

// Kotlin
val foos: List<Foo> = restTemplate

// Kotlin
val foos: List<Foo> = restTemplate
😍

Spring ❤ Kotlin

Spring Framework 5
•Router Functions
•HTTP/2 support
•Kotlin support
👇
👇

Spring Framework 5
•Router Functions
•HTTP/2 support
•Kotlin support
👇
👇
🤔

WebMVC + @Controller in Java
@RestController
public class UserController {
private final UserRepository repo;
UserController(UserRepository repo) {/.../}
@GetMapping
List<User> users() {
return repo.findAll();
}
}

WebFlux + @Controller in Java
@RestController
private final ReactiveUserRepository repo;
UserController(ReactiveUserRepository repo) {/.../}
@GetMapping
Flux<User> users() {
}
}

WebFlux + @Controller in Java
@RestController
private final ReactiveUserRepository repo;
UserController(ReactiveUserRepository repo) {/.../}
@GetMapping
Flux<User> users() {
}
}
Non-Blocking!!

WebFlux + RouterFunction in Java

WebFlux + RouterFunction in Java
RouterFunction<?> routes =
route(GET("/users"), req -> ok()
.body(repo.findAll(), User.class));

WebFlux + RouterFunction in Kotlin
{
accept(APPLICATION_JSON).apply {
GET("/users",{
ok().body(fromPublisher(repo.findAll())
})
}
}

Check source code!!
• https://github.com/mix-it/mixit
• https://github.com/making/demo-router-functions

Other Kotlin Support
•Functional Bean Registration with Kotlin
•WebFlux functional API, the Kotlin way
•Leveraging Kotlin nullable information
•Kotlin Script based templates
•...
•https://speakerdeck.com/sdeleuze/functional-
web-applications-with-kotlin-and-spring-5?

Release date
https://jira.spring.io/browse/SPR

Resources
• https://spring.io/blog/2017/01/04/introducing-kotlin-support-
in-spring-framework-5-0
• https://speakerdeck.com/sdeleuze
• https://blog.ik.am/entries/407

CfP for JJUG CCC 2017 Spring
•Submit Call for Paper!!! 🙇
•http://www.java-users.jp/?p=2830

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