Kotlin In Spreadshirt. JUG Saxony Day, Spreadshirt

Transcription

1 Kotlin In JUG Saxony Day,

2 2

3 Hands Up!

4 Kotlin Features and Usage in Practice

5 References Immutable and mutable References val id = 1 id = 2 var id2 = 1 id2 = 2 5

6 Data Classes Immutability made easy: data class DesignData( val id: Int, val filename: String, val uploaderid: Int, val width: Int = 0, val height: Int = 0 ) Constructor (assign args to props) Getter tostring() hashcode(), equals() copy() final Default Arguments (no chaining) val design = DesignData(id = 1, filename = "cat.jpg", uploaderid = 2) val filename = design.filename design.filename = "dog.jpg" val design2 = design.copy(filename = "dog.jpg") 6

7 Value Objects with Data Classes //without value object: fun send(target: String){ //expressive, readable, safe fun send(target: Address){ //with value object: data class Address(val value: String) 7

8 Putting Classes Together Java Kotlin 8

9 Null-Safety and Means for Null Handling String? String "Clean" null "Clean" val value: String = "Clean Code" val value: String = null val nullablevalue: String? = "Clean Code" val nullablevalue: String? = null val v: String = nullablevalue val v: String = if (nullablevalue == null) "default" else nullablevalue val v: String = nullablevalue?: "default" smart-cast! 9

10 Null-Safety and Means for Null Handling val city = order.customer.address.city val city = order!!.customer!!.address!!.city avoid this! if (order == null order.customer == null order.customer.address == null){ throw IllegalArgumentException("Invalid Order") val city = order.customer.address.city smart-cast val city = order?.customer?.address?.city val city = order?.customer?.address?.city?: throw IllegalArgumentException("Invalid Order") 10

11 Expressions Flow control structures are expressions! val json = """{"message": "HELLO"""" val message = try { JSONObject(json).getString("message") catch (ex: JSONException) { json 11

12 Expressions Single Expression Functions fun getmessage(json: String): String { val message = try { JSONObject(json).getString("message") catch (ex: JSONException) { json return message fun getmessage(json: String) = try { JSONObject(json).getString("message") catch (ex: JSONException) { json 12

13 Concise Mapping between Model Classes data class SnippetDTO( val code: String, val author: String, val date: Instant ) data class SnippetEntity( val code: String, val author: AuthorEntity, val date: Instant ) data class AuthorEntity( val firstname: String, val lastname: String ) Icon made by Freepik from is licensed by CC 3.0 BY 13

14 Concise Mapping between Model Classes fun maptodto(entity: SnippetEntity) = SnippetDTO( code = entity.code, date = entity.date, author = "${entity.author.firstname ${entity.author.lastname" ) Icon made by Freepik from is licensed by CC 3.0 BY 14

15 Processing an HTTP Response in Java public Product parseproduct(response response){ if (response == null){ throw new ClientException("Response is null"); int code = response.code(); if (code == 200 code == 201){ return maptodto(response.body()); if (code >= 400 && code <= 499){ throw new ClientException("Sent an invalid request"); if (code >= 500 && code <= 599){ throw new ClientException("Server error"); throw new ClientException("Error. Code " + code); 15

16 Processing an HTTP Response in Kotlin: when fun parseproduct(response: Response?) = when (response?.code()){ null -> throw ClientException("Response is null") 200, 201 -> maptodto(response.body()) in > throw ClientException("Sent an invalid request") in > throw ClientException("Server error") else -> throw ClientException("Error. Code ${response.code()") 16

17 Do-It-Yourself class UserDAO(private val template: JdbcTemplate) { fun findallusers() = template.query("select * FROM users;", this::maptouser) fun finduser(id: Int) = try { template.queryforobject("select * FROM users WHERE id = $id;", this::maptouser) catch (e: EmptyResultDataAccessException) { null private fun maptouser(rs: ResultSet, rownum: Int) = User( id = rs.getint("id"), = rs.getstring(" "), name = mergenames(rs), role = if (rs.getboolean("guest")) Role.GUEST else Role.USER, datecreated = rs.gettimestamp("date_created").toinstant(), state = State.valueOf(rs.getString("state")) ) 17

18 Spring: Easy Constructor Injection // Java public class CustomerResource { private CustomerRepository repo; private CRMClient client; public CustomerResource(CustomerRepository repo, CRMClient client) { this.repo = repo; this.client = client; // Kotlin class CustomerResource(private val repo: CustomerRepository, private val client: CRMClient){ 18

19 Concise Lambda Expressions & Vaadin val button = Button("Delete") button.addclicklistener( { event -> println(event) ) button.addclicklistener { event -> println(event) button.addclicklistener { println(it) 19

20 Collection API Read-only Collections val list = listof(1,2,3,4) list.add(1) Collections API val evenlist = list.filter { it % 2 == 0 val dayslist = list.filter { it % 2 == 0.map { DayOfWeek.of(it) println(dayslist) //[TUESDAY, THURSDAY] 20

21 Structuring with apply() //Java: Table mytable = new Table("MyTable", container); mytable.setsizefull(); mytable.setcolumnheader("code", "Code"); mytable.setcolumnheader("date", "Date"); mytable.addgeneratedcolumn("code", ShortValueColumnGenerator); mytable.setconverter("date", StringToInstantConverter); 21

22 Structuring with apply() //Kotlin: val mytable = Table("MyTable", container).apply { setsizefull() setcolumnheader("code", "Code") setcolumnheader("date", "Date") addgeneratedcolumn("code", ShortenedValueColumnGenerator) setconverter("date", StringToInstantConverter) 22

23 Structuring with apply() val layout = FormLayout().apply { setmargin(true) isspacing = true val codelabel = Label().apply { caption = "Code" value = "Select * From dual;" val statelabel = Label().apply { caption = "State" value = "${icon Successfully executed" val closebutton = Button("Close").apply { addclicklistener { close() addcomponents(codelabel, statelabel, closebutton) 23

24 Testing with JUnit (5): Backticks class AnalyserTest fun `valid user data`() { val inconsistencies = Analyser.find(createValidData()) inner class `inconsistent s` fun `different auth mail`() { //... 24

25 Testing with JUnit (5): No class DAOTest { private lateinit var mysql: KMySQLContainer private lateinit var jdbctemplate: fun init() { mysql = KMySQLContainer("mysql:5.7.18") mysql.start() val datasource = DataSourceBuilder.create().url(mysql.jdbcUrl).username(mysql.username).password(mysql.password).driverClassName("com.mysql.cj.jdbc.Driver").build() jdbctemplate = fun destroy(){ mysql.stop() 25

26 Top-Level and Extension Functions Top-Level Functions // HelloWorld.kt fun main(args: Array<String>) { println("hello World") // HelloWorld.java public class HelloWorld { public static void main(string[] args) { System.out.println("Hello World"); Extension functions // definition fun String.wrap(wrapWith: String) = wrapwith + this + wrapwith // usage val wrapped = "hello".wrap("*") // as opposed to: val wrapped = StringUtils.wrap("hello", "*") 26

27 Vaadin: Extension Functions to Add UI Logic enum class SnippetState {EXECUTED, NOT_EXECUTED fun SnippetState.toIcon() = when (this){ SnippetState.EXECUTED -> FontAwesome.THUMBS_O_UP SnippetState.NOT_EXECUTED -> FontAwesome.THUMBS_O_DOWN //usage: val icon = state.toicon() 27

28 Kotlin at

29 Ecosystem vs. Language 29

30 Evaluation of Kotlin Pros Reuse of the powerful and well-known Java ecosystem Interoperability with Java. Productivity Less error-prone Easy to learn. No paradigm shift. Stepwise migration possible. Brilliant IDE support with IntelliJ IDEA. Cons Training required Further development depends on JetBrains. Poor Support for other IDEs (like Eclipse) Less documentation available compared with Java. Low Risks 30

31 Kotlin Usage at 1 Java service enriched with Kotlin 11 new services and tools purely written in Kotlin 2 Test Projects 31

32 Adoption of Kotlin Today (Outside of )

33 Google Search Trends Peak: Google I/O '17: "Kotlin is an official language for Android development" 33

34 Further Support and Integrations for Kotlin start.spring.io Kotlin Compiler Plugin Kotlin Support in Spring 5.0 Kotlin Gradle Lifecycle.PER_CLASS) Kotlin Android Extensions 34

35 Drawbacks and Pitfalls

36 Missing Default Constructor for Data Classes data class Snippet(val code: String, val author: String) val snippet = Snippet() Issues with Object Mapping: JAXB requires default constructor Jackson: jackson-module-kotlin allows parameterized constructors Hibernate: kotlin-noarg compiler plugin for JPA Synthetic default constructor apply plugin: "kotlin-jpa" Spring Data or kotlin-noarg plugin 36

37 Final by Default class CustomerService { fun findcustomer(id: Int){ //... Can t be extended by subclasses! Some frameworks rely on extension of classes Spring Mockito Solutions: Open classes and methods explicitly Open-all-plugin for Kotlin compiler buildscript { dependencies { classpath("org.jetbrains.kotlin:kotlin-allopen:${kotlinversion") apply plugin: 'kotlin-spring' 37

38 Hammer and Nails Icon made by Freepik from is licensed by CC 3.0 BY 38

39 Be aware of Train Wrecks! fun map(dto: OrderDTO, authdata: RequestAuthData) = OrderEntity( id = dto.id, shopid = try { extractitemids(dto.orderitems[0].element.href).shopid catch (e: BatchOrderProcessingException) { restexc("couldn't retrieve shop id from first order item: ${e.msg"), batchstate = BatchState.RECEIVED, orderdata = OrderDataEntity( orderitems = dto.orderitems.map { dto -> maptoentity(dto), shippingtype = dto.shipping.shippingtype.id, address = maptoentity(dto.shipping.address), correlationorderid = dto.correlation?.partner?.orderid, externalinvoicedata = dto.externalinvoicedata?.let { ExternalInvoiceDataEntity( url = it.url, total = it.total, currencyid = it.currency.id ) ), partneruserid = authdata.sessionownerid?: restexc("no sessionid supplied", 401), apikey = authdata.apikey, datecreated = if (dto.datecreated!= null) dto.datecreated else Instant.now(), )

40 Hammer and Nails Be careful with: Unreadable monster expressions Complicated null-safe-calls and elvis structures //Don't value?.emptytonull()?.let { map.put("bla", it) fun String.emptyToNull() = if (this.isempty()) null else this // KISS! if (!value.isnullorempty()){ map.put("key", value!!) Icon made by Freepik from is licensed by CC 3.0 BY 40

41 Kotlin Compiler - slow? Basically, Kotlin Compiler is slower than Java's Solution: Gradle daemon Gradle incremental builds Ten consecutive clean builds without the Gradle daemon Ten consecutive incremental builds with one core file changed Source: "Kotlin vs Java: Compilation speed" by AJ Alt 41

42 Conclusion

43 Kotlin at : A Success Story!

44 Questions?