Kotlin serialization is a plug-in officially provided by kotlin, which can serialize and deserialize kotlin objects. The supported serialization formats include JSON, Protobuf, CBOR, Hocon and Properties  

serialization is easy to use:

    //serialize
    val data = Project("kotlinx.serialization", "Kotlin")
    val json = Json.encodeToString(data)  

    //Deserialization
    val obj = Json.decodeFromString<Project>(json)

1, Add serialization dependency

In module   Add in build.gradle

plugins {
    id 'kotlin-android'
    id 'kotlinx-serialization'
    id 'kotlin-kapt'
}
dependencies {
    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.3.0"
}

In project   Add in build.gradle

buildscript {
    ext.kotlin_version = '1.5.31'
    repositories { mavenCentral() }

    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-serialization:$kotlin_version"
    }
}

2, Kotlin serialization Guide

1. Serialization operation         

@Serializable
class Project(val name: String, val language: String)

fun main() {
    val data = Project("kotlinx.serialization", "Kotlin")
    println(Json.encodeToString(data))
}

    1. Fields that can be serialized

          Only properties of classes with supported fields will be serialized, so properties with getters / setters and without supported fields and delegate properties will not be serialized, as shown in the following example.

@Serializable 
class Project(
    // name has supported field properties and can be serialized
    var name: String
) {
    var stars: Int = 0 // With supported field properties, it can be serialized
    val path: String //Without field, it will not be serialized
        get() = "kotlin/$name"                                         

    var id by ::name //Proxy properties, not serialized
}

fun main() {
    val data = Project("kotlinx.serialization").apply { stars = 9000 }
    println(Json.encodeToString(data))
}

//print:{"name":"kotlinx.serialization","stars":9000}

    2. Default action

        By default, the default value does not participate in encoding:        

@Serializable 
data class Project(val name: String, val language: String = "Kotlin")

fun main() {
    val data = Project("kotlinx.serialization")
    println(Json.encodeToString(data))
}

print:{"name":"kotlinx.serialization"} 

        If you need the default value to participate in serialization, you can set the encodeDefaults property:

@Serializable 
class Project(
    val name: String, val language: String = "Kotlin", val website: String? = null)           

fun main() {
    val data = Project("kotlinx.serialization")
    println(Json { encodeDefaults = true }.encodeToString(data))
}
print:{"name":"kotlinx.serialization","language":"Kotlin","website":null}

        Explicit nulls can be set to false if you do not want null to participate in serialization

3. Mandatory security type

serialization implements the type safety of Kotlin language. If there is null in json and the corresponding attribute is not marked as nullable, an exception will be reported

@Serializable 
data class Project(val name: String, val language: String = "Kotlin")

fun main() {
    val data = Json.decodeFromString<Project>("""
        {"name":"kotlinx.serialization","language":null}""")
    println(data)
}

Error: Unexpected JSON token at offset 52: Expected string literal but 'null' literal was found
Use 'coerceInputValues = true' in 'Json {}` builder to coerce nulls to default values.

If you want to use the default value when json is null, set coerceInputValues = true

@Serializable 
data class Project(val name: String, val language: String = "Kotlin")

fun main() {
    val data = Json { coerceInputValues = true }.decodeFromString<Project>("""
        {"name":"kotlinx.serialization","language":null}
    """)
    println(data)
}

print: Project(name=kotlinx.serialization, language=Kotlin)

4. Sequence field name

By default, the attribute names used in the encoded representation (JSON in our example) are the same as their names in the source code. The name used for serialization is called the sequence name and can be changed using the @ SerialName annotation
 

@Serializable
class Project(val name: String, @SerialName("lang") val language: String)

fun main() {
    val data = Project("kotlinx.serialization", "Kotlin")
    println(Json.encodeToString(data))
}
print: {"name":"kotlinx.serialization","lang":"Kotlin"}

2. Deserialization operation

@Serializable 
class Project(val name: String, val language: String)

fun main() {
    val data = Project("kotlinx.serialization", "Kotlin")
    println(Json.encodeToString(data))
}

1. Optional properties

An object can be deserialized only if all the properties of the object exist in the sequence. For example:

@Serializable 
data class Project(val name: String, val language: String)

fun main() {
    val data = Json.decodeFromString<Project>("""{"name":"kotlinx.serialization"}""")
    println(data)
}

Because there is no language attribute in json, an exception will be reported: Exception in thread "main" kotlinx.serialization.MissingFieldException: Field 'language' is required for type with serial name 'example.exampleClasses04.Project', but it was missing

You can fix this problem by adding default values to properties, such as:

@Serializable 
data class Project(val name: String, val language: String = "Kotlin")

fun main() {
    val data = Json.decodeFromString<Project>("""{"name":"kotlinx.serialization"}""")
    println(data)
}

Print results: Project(name=kotlinx.serialization, language=Kotlin)

2. Required properties

In deserialization, if you want json to contain the specified attribute name, you can use the @ Required annotation

@Serializable 
data class Project(val name: String, @Required val language: String = "Kotlin")

fun main() {
    val data = Json.decodeFromString<Project>("""
        {"name":"kotlinx.serialization"}
    """)
    println(data)
}

In the example, because json does not contain the language attribute marked @ Required, an exception will be reported. Field 'language' is Required for type with serial name 'example. Exampleclasss07. Project', but it was missing

  3. JSON property settings

        Json {
            prettyPrint = true //json formatting
            isLenient = true //Loose parsing, json format exceptions can also be parsed, such as: {name: "Xiaohong", age:"18"}
            ignoreUnknownKeys = true //Ignore unknown keys, such as {"name": "little red", "age": "18"} - > person (Val Name: String)
            coerceInputValues =  true //Forced input value. If the json attribute does not match the object format, the object default value is used, such as: {name ":" little red "," age":null} + Person(val name:String =" little green ", Val age: int = 18) - > person (" little red ", 18)
            encodeDefaults =  true //Encode the default value. By default, the attribute of the default value will not participate in serialization. By setting encodeDefaults = true, the default attribute can participate in serialization (refer to the above example)
            explicitNulls =  true //Ignore null when serializing
            allowStructuredMapKeys =  true //Allow structured mapping (map key s can use objects)
            allowSpecialFloatingPointValues =  true //Special floating point value: Double is allowed to be NaN or infinity
        }