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@antonarhipov Spring extensions for Kotlin

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@antonarhipov

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import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication … @SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) }

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import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication … @SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) }

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import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication … @SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) }

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import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication … @SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) } @RestController class MessageResource(val service: MessageService) { @GetMapping fun index(): List = service.findMessages() @PostMapping fun post(@RequestBody message: Message) { service.post(message) } }

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@Service class MessageService(val db: MessageRepository) { fun findMessages(): List = db.findMessages() fun post(message: Message){ db.save(message) } } interface MessageRepository : CrudRepository{ @Query("select * from messages") fun findMessages(): List } @Table("MESSAGES") data class Message(@Id val id: String?, val text: String)

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@Service class MessageService(val db: MessageRepository) { fun findMessages(): List = db.findMessages() fun post(message: Message){ db.save(message) } } interface MessageRepository : CrudRepository { @Query("select * from messages") fun findMessages(): List } @Table("MESSAGES") data class Message(@Id val id: String?, val text: String)

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@Service class MessageService(val db: MessageRepository) { fun findMessages(): List = db.findMessages() fun post(message: Message){ db.save(message) } } interface MessageRepository : CrudRepository{ @Query("select * from messages") fun findMessages(): List } @Table("MESSAGES") data class Message(@Id val id: String?, val text: String)

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@Service class MessageService(val db: MessageRepository) { fun findMessages(): List = db.findMessages() fun post(message: Message){ db.save(message) } } interface MessageRepository : CrudRepository{ @Query("select * from messages") fun findMessages(): List } @Table("MESSAGES") data class Message(@Id val id: String?, val text: String) Not to be used with JPA!

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https://stackover fl ow.com/questions/58127353/should-i-use-kotlin-data-class-as-jpa-entity Resources: https://kotlinexpertise.com/hibernate-with-kotlin-spring-boot/ https://dzone.com/articles/kotlin-data-classes-and-jpa https://blog.codecentric.de/en/2017/06/kotlin-spring-working-jpa-data-classes/ https://spring.io/guides/tutorials/spring-boot-kotlin/

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Important for starters

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Important for starters * Kotlin classes are fi nal by default

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Important for starters * Kotlin classes are fi nal by default * Compiler plugins to the rescue!

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Important for starters * Kotlin classes are fi nal by default * Compiler plugins to the rescue! * Data classes don’t play well with JPA

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Important for starters * Kotlin classes are fi nal by default * Compiler plugins to the rescue! * Data classes don’t play well with JPA * Use normal classes with JPA

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* Kotlin classes are fi nal by default * Compiler plugins to the rescue! * Data classes don’t play well with JPA * Use normal classes with JPA Important for starters * Use start.spring.io to generate con fi gs

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kotlin extensions

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println("hello".uuid())

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fun String.uuid(): String = UUID.nameUUIDFromBytes(this.encodeToByteArray()).toString() println("hello".uuid())

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fun String.uuid(): String = UUID.nameUUIDFromBytes(encodeToByteArray()).toString() println("hello".uuid())

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fun String.uuid() = UUID.nameUUIDFromBytes(encodeToByteArray()).toString() println("hello".uuid())

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } }

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } }

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id)

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id)

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id)

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id) RowMapper is a functional interface

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id) “SAM conversion” is performed by the compiler

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id)

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id, { rs, _ -> Message(rs.getString("id"), rs.getString("text")) })

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id, { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }) vararg is not at the last position

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id, { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }) Lambda expression is at the last position

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } }

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } Passing trailing lambda as a parameter to the function

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } }

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } } This is an extension function For the existing Java class in the Spring framework

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@Service class MessageService(val db: JdbcTemplate) { fun findMessages(): List = db.query("select * from messages") { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun findMessageById(id: String): List = db.query("select * from messages where id = ?", id) { rs, _ - > Message(rs.getString("id"), rs.getString("text")) } fun post(message: Message){ db.update("insert into messages values ( ?, ? )", message.id ?: message.text.uuid(), message.text) } }

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extensions & Trailing lambdas

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fun String.uuid() = UUID.nameUUIDFromBytes(encodeToByteArray()).toString()

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fun String.uuid() = UUID.nameUUIDFromBytes(encodeToByteArray()).toString() fun applyAction(vararg s: String, action: (String) -> Unit) { s.forEach(action) }

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fun String.uuid() = UUID.nameUUIDFromBytes(encodeToByteArray()).toString() fun applyAction(vararg s: String, action: (String) -> Unit) { s.forEach(action) } fun main() { applyAction("hello", "bye") { s: String -> println(s.uuid()) } }

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fun String.uuid() = UUID.nameUUIDFromBytes(encodeToByteArray()).toString() fun applyAction(vararg s: String, action: (String) -> Unit) { s.forEach(action) } fun main() { applyAction("hello", "bye") { s: String -> println(s.uuid()) } } Note to self: open the IDE

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more kotlin extensions In spring

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class HtmlController(val messageService: MessageService) { @GetMapping("/") fun index(model: Model): String { val messages = messageService.latest() model.asMap()["messages"] = messages model.asMap()["lastMessageId"] = messages.lastOrNull() ? . id ? : "" return "chat" } }

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class HtmlController(val messageService: MessageService) { @GetMapping("/") fun index(model: Model): String { val messages = messageService.latest() model.asMap()["messages"] = messages model.asMap()["lastMessageId"] = messages.lastOrNull() ? . id ? : "" return "chat" } }

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class HtmlController(val messageService: MessageService) { @GetMapping("/") fun index(model: Model): String { val messages = messageService.latest() model.asMap()["messages"] = messages model.asMap()["lastMessageId"] = messages.lastOrNull() ? . id ? : "" return "chat" } }

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class HtmlController(val messageService: MessageService) { @GetMapping("/") fun index(model: Model): String { val messages = messageService.latest() model["messages"] = messages model["lastMessageId"] = messages.lastOrNull() ?. id ?: "" return "chat" } }

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Type-safe builders A.k.a. dsl

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@SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) { addInitializers(beans) } }

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@SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) { this.addInitializers(beans) } } “this” refers to SpringApplication instance

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val beans = beans { bean { CommandLineRunner { println("start data initialization .. . ") val repository = ref() repository.save(Message(text = "this is the first message!")) repository.save(Message(text = "this is the second } } } @SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) { this.addInitializers(beans) } }

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@SpringBootApplication class DemoApplication fun main(args: Array) { runApplication(*args) { this.addInitializers(beans) } } val beans = beans { bean { CommandLineRunner { println("start data initialization .. . ") val repository = ref() repository.save(Message(text = "this is the first message!")) repository.save(Message(text = "this is the second } } }

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Trailing lambdas + (extension) functions + named parameters + lambda w/ receiver = DSL fun foo(f: () -> Unit) {} fun bar(f: () -> Unit) {} fun baz(f: () -> Unit) {} fun blah(id: Int, f: Klazz.() -> Unit) {} class Klazz(var text: String) fun Klazz.hello() : String = "Hello"

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Null safety

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val person: Person? = getPerson() println(person.firstName) fun getPerson(): Person? {…} data class Person( var firstName: String, // … )

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val person: Person? = getPerson() println(person.firstName) fun getPerson(): Person? {…} data class Person( var firstName: String, // … ) Can return null

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val person: Person? = getPerson() println(person.firstName) fun getPerson(): Person? {…} data class Person( var firstName: String, // … ) Can be null

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val person: Person? = getPerson() println(person ?. firstName) fun getPerson(): Person? {…} data class Person( var firstName: String, // … ) Null-safe dereferencing

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val person: Person? = getPerson() println(person !! .firstName) fun getPerson(): Person? {…} data class Person( var firstName: String, // … ) “I know what I’m doing”

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Can be null Should not be null

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build.gradle.kts

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Kotlin features you have seen now Type inference Inline functions Data classes Rei fi ed generics Top-level functions Named arguments Operator overloading Trailing lambdas Extension functions Functional literal with receiver Nullable types Support for JSR-305

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