Kotlin – your 2017 Java replacement

Do you dream about life without unexpected nulls? Do you fancy a little bit of functional programming? Are you tired of Java ceremony?

Then maybe 2017 is The Right Moment to choose Kotlin! This JVM language brings you stuff like null-safety and immutability by default. Less boilerplate, more safety. And you can introduce it in your project class after class, module after module, without risky revolution.

I will show you pros and cons of Kotlin, based on last 8 months of my experience. In November 2016 I started a new backend project built on top of Gradle and Spring. Today I still do not regret the decision to write it in Kotlin. Let's see if I convince you! :-)

Transcript

1. Kotlin your 2017 Java replacement
2. ?
3. goo.gl/maps/j4braT8Ucgv
4. goo.gl/maps/4aSgyZRdnKq ⬅ Java Kotlin ➡
6. contribution over time github.com/JetBrains/kotlin/graphs/contributors
7. www.tiobe.com/tiobe-index
8. ❤
9. ❤
10. my story
11. Vavr - Functional Java Done Right
 Grzegorz Piwowarek @ Confitura 2016 more „classic” projects greenfield project
 solo on backend
 head full of ideas April
 2016 October
 2016 Kotlin here, Kotlin there
12. start.spring.io/#!type=gradle-project&language=kotlin
13. stack Kotlin 1.0 –> 1.1 JUnit 4
 - AssertJ
 - JUnitParams Spring Boot 1.3 –> 1.5
 - web
 - security
 - jdbc
 - test
14. 8 months later… Kotlin in the whole service team of 2 devs 3 months on (real) production still no regret
15. first impressions
16. syntax
17. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random())
18. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) name –> type
19. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) fields declared as
 constructor arguments
20. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) defaults
21. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) named parameters
22. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) String interpolation
23. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) no type, because…
 type inference
24. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) smart `when` compiles
 if all options are covered
25. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) expressions everywhere
 `if`, `when`, `try`…
26. class Task( val id: TaskId, val name: String, private val priority: TaskPriority = NORMAL ) { fun isImportant() = when (priority) { HIGH -> true NORMAL, LOW -> false } fun duplicate(): Task { val newName = "$name (copy)" /* ... */ } } Task(name = "prepare a talk for Devoxx PL", id = TaskId.random()) function as a single expression
27. immutability
28. var map1: Map<String, Int> = mapOf("A" to 1) val map2: MutableMap<String, Int> = mutableMapOf("B" to 2) fun whatever() { map1 += ("A" to 111) map2["B"] = 222 } modification
 mutable collections only shorter = immutable
29. var map1: Map<String, Int> = mapOf("A" to 1) val map2: MutableMap<String, Int> = mutableMapOf("B" to 2) fun whatever() { map1 += ("A" to 111) map2["B"] = 222 } reassignment
 `var` only var ≠ val
30. data classes
31. data class Color( val red: Pigment, val green: Pigment, val blue: Pigment, val alpha: Alpha = Alpha.none() ) { fun transparent() = copy(alpha = Alpha.medium()) } out of the box: - copy
32. data class Color( val red: Pigment, val green: Pigment, val blue: Pigment, val alpha: Alpha = Alpha.none() ) { fun transparent() = copy(alpha = Alpha.medium()) } val (red, green, blue, alpha) = myColor out of the box: - copy - destructuring
33. data class Color( val red: Pigment, val green: Pigment, val blue: Pigment, val alpha: Alpha = Alpha.none() ) { fun transparent() = copy(alpha = Alpha.medium()) } out of the box: - copy - destructuring - equal & hashCode - toString
34. Kotlin + Spring
35. Kotlin + Spring works OK, but…
36. everything is final • Java –> „final” on demand
 Kotlin –> „open” on demand • Spring, Mockito, … how to proxy? • kotlin-allopen / kotlin-spring
 blog.jetbrains.com/kotlin/2016/12/kotlin-1-0-6-is-here
37. buildscript { dependencies { classpath "org.jetbrains.kotlin:kotlin-allopen:1.1.2-5" } } apply plugin: "kotlin-allopen" allOpen { annotation("com.your.Annotation") } everything is final
38. buildscript { dependencies { classpath "org.jetbrains.kotlin:kotlin-allopen:1.1.2-5" } } apply plugin: "kotlin-spring" everything is final
39. killer feature
40. (definitely not) William Shakespeare „can be null,
 or
 cannot be null”
41. null-safety
42. null-safety
43. interface ProjectRepository { fun createProject(name: ProjectName): Project fun findProjectBy(id: ProjectId): Project? } null? nope can be null
44. val importantTasks = projectRepository .findProjectBy(projectId) ?.tasks ?.filter { it.isImportant() } ?: emptyList() ?
45. val importantTasks = projectRepository .findProjectBy(projectId) ?.tasks ?.filter { it.isImportant() } ?: emptyList() ? Project
 or null
46. val importantTasks = projectRepository .findProjectBy(projectId) ?.tasks ?.filter { it.isImportant() } ?: emptyList() ? Project's Tasks
 or null
47. val importantTasks = projectRepository .findProjectBy(projectId) ?.tasks ?.filter { it.isImportant() } ?: emptyList() ? Project's important Tasks
 or null
48. val importantTasks = projectRepository .findProjectBy(projectId) ?.tasks ?.filter { it.isImportant() } ?: emptyList() ? fallback for null
49. val project: Project? = projectRepository.findProjectBy(projectId) return project?.tasks compiler is smart
50. val project: Project? = projectRepository.findProjectBy(projectId) if (project == null) { throw ProjectNotFound() } return project.tasks compiler is smart
51. @Autowired var projectRepository: ProjectRepository? = null @Autowired nullable ☹
52. @Autowired var projectRepository: ProjectRepository? = null @Autowired lateinit var projectRepository: ProjectRepository @Autowired there is no null
 to handle
53. @Override public void configure(WebSecurity webSecurity) { webSecurity .ignoring() .mvcMatchers(/* ... */); } when null comes from Java
54. @Override public void configure(WebSecurity webSecurity) { webSecurity .ignoring() .mvcMatchers(/* ... */); } when null comes from Java
55. override fun configure(webSecurity: WebSecurity) { webSecurity .ignoring() .mvcMatchers(/* ... */) } override fun configure(webSecurity: WebSecurity?) { webSecurity!! .ignoring() .mvcMatchers(/* ... */) } override fun configure4(webSecurity: WebSecurity?) { webSecurity ?.ignoring() ?.mvcMatchers(/* ... */) } when null comes from Java
56. override fun configure(webSecurity: WebSecurity) { webSecurity .ignoring() .mvcMatchers(/* ... */) } override fun configure(webSecurity: WebSecurity?) { webSecurity!! .ignoring() .mvcMatchers(/* ... */) } override fun configure4(webSecurity: WebSecurity?) { webSecurity ?.ignoring() ?.mvcMatchers(/* ... */) } when null comes from Java
57. override fun configure(webSecurity: WebSecurity) { webSecurity .ignoring() .mvcMatchers(/* ... */) } override fun configure(webSecurity: WebSecurity?) { webSecurity!! .ignoring() .mvcMatchers(/* ... */) } override fun configure4(webSecurity: WebSecurity?) { webSecurity ?.ignoring() ?.mvcMatchers(/* ... */) } when null comes from Java
58. override fun configure(webSecurity: WebSecurity) { webSecurity .ignoring() .mvcMatchers(/* ... */) } override fun configure(webSecurity: WebSecurity?) { webSecurity!! .ignoring() .mvcMatchers(/* ... */) } override fun configure4(webSecurity: WebSecurity?) { webSecurity ?.ignoring() ?.mvcMatchers(/* ... */) } when null comes from Java
59. we need to go deeper
60. what about…
61. …request/response bodies? • if you want JSON structure declared explicitly
62. class ActiveProjectsJson( val projects: List<ProjectJson> ) class ArchivedProjectsJson( val archivedProjects: List<ProjectJson> ) class ProjectJson( val id: String, val name: String, val color: String ) class ProjectUpdateJson( val name: String?, val color: String? ) …request/response bodies?
63. class ActiveProjectsJson( val projects: List<ProjectJson> ) class ArchivedProjectsJson( val archivedProjects: List<ProjectJson> ) class ProjectJson( val id: String, val name: String, val color: String ) class ProjectUpdateJson( val name: String?, val color: String? ) …request/response bodies?
64. class ActiveProjectsJson( val projects: List<ProjectJson> ) class ArchivedProjectsJson( val archivedProjects: List<ProjectJson> ) class ProjectJson( val id: String, val name: String, val color: String ) class ProjectUpdateJson( val name: String?, val color: String? ) …request/response bodies? in one file
 if you prefer
65. compile 'com.fasterxml.jackson.module:jackson-module-kotlin' …request/response bodies?
66. what about…
67. …working with JSONs? • when you don't want strict mapping into types • eg. in tests
68. Kotson • github.com/SalomonBrys/Kotson • Gson in a Kotlin way
69. jsonObject( "name" to "John", "age" to 29, "phoneNumbers" to jsonArray( jsonObject( "type" to "personal", "number" to "123 456 789" ), jsonObject( "type" to "office", "number" to "987 654 321" ) ) ).toString() Kotson
70. personJson["phoneNumbers"].asJsonArray .first { it["type"].asString == "office" } .let { it["number"].asString } Kotson
71. what about…
72. …SQL? • me & Hibernate = • me & tables coupled with domain entities = • me & types = ❤
73. Exposed • github.com/JetBrains/Exposed • simple SQL typed abstraction
 good enough for simple cases
74. object ProjectsTable : Table("projects") { val id = long("id").autoIncrement().primaryKey() val ownerId = (long("owner") references UsersTable.id) val name = varchar("name", 50) val isArchived = bool("is_archived").default(false) } Exposed
75. SchemaUtils.create(ProjectsTable) Exposed
76. ProjectsTable .select { ProjectsTable.id eq projectId } .map { row -> Project( id = row[ProjectsTable.id], name = row[ProjectsTable.name], isArchived = row[ProjectsTable.isArchived] ) } .firstOrNull() Exposed
77. compile 'org.jetbrains.exposed:spring-transaction' //... 
 import org.jetbrains.exposed.spring.SpringTransactionManager @Configuration @EnableTransactionManagement class TransactionConfiguration { @Bean fun transactionManager(dataSource: DataSource) = SpringTransactionManager(dataSource) } Exposed
78. what about…
79. …functional constructs? • currying • function composition • Either • memoization • partial application • Try
80. funKTionale • github.com/MarioAriasC/funKTionale • functional constructs and patterns for Kotlin
81. return Try { fetchProjectsOverHttp() // may throw an exception }.handle { throw ConnectionFailed() }.onEach { response -> logger.info { response } }.flatMap { response -> Try { parse(response) } .handle { throw ResponseParsingFailed() } }.fold({ result -> result.projects },
 { exception -> throw exception })
82. return Try { fetchProjectsOverHttp() // may throw an exception }.handle { throw ConnectionFailed() }.onEach { response -> logger.info { response } }.flatMap { response -> Try { parse(response) } .handle { throw ResponseParsingFailed() } }.fold({ result -> result.projects },
 { exception -> throw exception }) wrap value in Try.Sucess
 wrap exception in Try.Failure
83. return Try { fetchProjectsOverHttp() // may throw an exception }.handle { throw ConnectionFailed() }.onEach { response -> logger.info { response } }.flatMap { response -> Try { parse(response) } .handle { throw ResponseParsingFailed() } }.fold({ result -> result.projects },
 { exception -> throw exception }) do something
 with Try.Failure
84. return Try { fetchProjectsOverHttp() // may throw an exception }.handle { throw ConnectionFailed() }.onEach { response -> logger.info { response } }.flatMap { response -> Try { parse(response) } .handle { throw ResponseParsingFailed() } }.fold({ result -> result.projects },
 { exception -> throw exception }) do something
 with Try.Success
85. return Try { fetchProjectsOverHttp() // may throw an exception }.handle { throw ConnectionFailed() }.onEach { response -> logger.info { response } }.flatMap { response -> Try { parse(response) } .handle { throw ResponseParsingFailed() } }.fold({ result -> result.projects },
 { exception -> throw exception }) nested Try logic
 inside `flatMap`
86. return Try { fetchProjectsOverHttp() // may throw an exception }.handle { throw ConnectionFailed() }.onEach { response -> logger.info { response } }.flatMap { response -> Try { parse(response) } .handle { throw ResponseParsingFailed() } }.fold({ result -> result.projects },
 { exception -> throw exception }) unwrap value or exception
87. and if I want modularization…
88. Kotlin + Gradle internal scope multi-project build
89. each sub-project is a separate module with one common
 parent project
90. private (default) package-private protected public private protected internal public (default)
91. private (default) package-private protected public private protected internal public (default) visible within
 module
92. one more minor thing…
93. Mockito.`when`(…) `backquotes`
94. @Test fun `User cannot archive Project owned by another User`() { // ... } `backquotes in tests ✌`
95. final thoughts
96. • maintained by JetBrains • easy to start with • nice syntax, immutability, data classes • works well with Spring • Kotson, Exposed, funKTionale • null-safety • low risk, pragmatic approach
97. • maintained by JetBrains • easy to start with • nice syntax, immutability, data classes • works well with Spring • Kotson, Exposed, funKTionale • null-safety • low risk, pragmatic approach
98. interoperability • using Java inside Kotlin
 kotlinlang.org/docs/reference/java-interop.html • using Kotlin inside Java
 kotlinlang.org/docs/reference/java-to-kotlin-interop.html
100. want more? • type aliases • couroutines (async/await/yield) • lazy sequences • sealed classes • gradle-script-kotlin • …
101. • Spring Boot 1.5 + Kotlin + customized Exposed (blog post + git repo)
 spring.io/blog/2016/03/20/a-geospatial-messenger-with-kotlin-spring-boot-and-postgresql • functional Spring 5 + Kotlin (blog post + git repo)
 spring.io/blog/2017/01/04/introducing-kotlin-support-in-spring-framework-5-0 • reference 
 kotlinlang.org/docs/reference • links, libs, blogs…
 kotlin.link • community 
 slack.kotlinlang.org • these slides 
 speakerdeck.com/nkoder/kotlin-your-2017-java-replacement
102. try Kotlin be productive have fun
103. Paweł Barszcz @nkoder pawelbarszcz@gmail.com timbercode.pl