This tutorial shows you how to use Jackson 2.x to convert Kotlin object to/from JSON.
Related Post: Kotlin List & Mutable List tutorial with examples
>>> Refer to: JavaSampleApproach.com
I. Technology
– Java 1.8
– Kotlin 1.1.2
– Maven 3.5.1
II. Overview
1. Goal
Convert JSON string/JSON file/JSON url to Person(name:String,age:Int,messages:List) Kotlin Object, then convert Person object to JSON string/JSON file.
2. Steps to do
– add Dependency:
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<dependency> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-stdlib</artifactId> <version>1.1.2</version> </dependency> <dependency> <groupId>com.fasterxml.jackson.module</groupId> <artifactId>jackson-module-kotlin</artifactId> <version>2.8.8</version> </dependency> |
– import com.fasterxml.jackson.module.kotlin.*
– use ObjectMapper instance:
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val mapper = jacksonObjectMapper() // Json String/URL/File to Object var person: Person = -> mapper.readValue<Person>(String) -> mapper.readValue<Person>(URL) -> mapper.readValue<Person>(File) // Object to Json String/File var jsonStr = -> mapper.writeValueAsString(Person) -> mapper.writerWithDefaultPrettyPrinter().writeValueAsString(Person) -> mapper.writeValue(File, Person) -> mapper.writerWithDefaultPrettyPrinter().writeValue(File, Person) |
III. Practice
0. Person Class
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package com.kotlination.jsonstring data class Person(val name: String, val age: Int, val messages: List<String>) { } |
1. Json String/URL/File to Object
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package com.kotlination.jsonstring import com.fasterxml.jackson.module.kotlin.* import java.net.URL import java.io.File fun main(args: Array<String>) { val json = """{"name": "Kolineer", "age": "26", "messages" : ["message a","message b"]}""" val mapper = jacksonObjectMapper() println("=== JSON to Kotlin Object ===") println("1- read String") var person: Person = mapper.readValue<Person>(json) println(person) println("2- read URL") person = mapper.readValue<Person>(URL("https://kotlination.com/wp-content/uploads/2017/05/person.json")) println(person) println("3- read File") /* content of person.json { "name" : "Kolineer", "age" : 28, "messages" : [ "message AA", "message BB" ] } */ person = mapper.readValue<Person>(File("person.json")) println(person) } |
Check Result:
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=== JSON to Kotlin Object === 1- read String Person(name=Kolineer, age=26, messages=[message a, message b]) 2- read URL Person(name=Kolineer, age=27, messages=[message A, message B]) 3- read File Person(name=Kolineer, age=28, messages=[message AA, message BB]) |
2. Object to Json String/File
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package com.kotlination.jsonstring import com.fasterxml.jackson.module.kotlin.* import java.io.File fun main(args: Array<String>) { val mapper = jacksonObjectMapper() println("=== Kotlin Object to JSON ===") person = Person("Kolineer Master", 30, listOf("I am Kotlin Master", "still learning Kotlin")) println("1- String") var jsonStr = mapper.writeValueAsString(person) println(jsonStr) println("2- Formatted String") jsonStr = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(person) println(jsonStr) println("3- File -> manually check file for result") mapper.writerWithDefaultPrettyPrinter().writeValue(File("newPerson.json"), person) } |
Check Result:
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=== Kotlin Object to JSON === 1- String {"name":"Kolineer Master","age":30,"messages":["I am Kotlin Master","still learning Kotlin"]} 2- Formatted String { "name" : "Kolineer Master", "age" : 30, "messages" : [ "I am Kotlin Master", "still learning Kotlin" ] } 3- File -> manually check file for result |
IV. More Practice
1. Json array format to List
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val jsonList = """[{"name": "Kolineer", "age": "26", "messages" : ["message a","message b"]}, {"name":"Kolineer Master","age":30,"messages":["I am Kotlin Master","still learning Kotlin"]}]""" var personList: List<Person> = mapper.readValue(jsonList) personList.forEach { println(it) } |
Result:
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Person(name=Kolineer, age=26, messages=[message a, message b]) Person(name=Kolineer Master, age=30, messages=[I am Kotlin Master, still learning Kotlin]) |
2. Json to Map
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val json = """{"name": "Kolineer", "age": "26", "messages" : ["message a","message b"]}""" var personMap: Map<String, Any> = mapper.readValue(json) personMap.forEach { println(it) } |
Result:
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name=Kolineer age=26 messages=[message a, message b] |