Serializers are represented at runtime as KSerializer<T>, which in turn, implements interfaces SerializationStrategy<T> and DeserializationStrategy<T>, where T is class you serialize.
You don't need to call them by yourself; you just have to pass them properly to serialization format. You can write them on your own (see custom serializers) or let the compiler plugin do the dirty work by marking class @Serializable.
To retrieve the generated serializer, plugin emits special function on companion object called .serializer().
If your class has generic type arguments, this function will have arguments for specifying serializers on type parameters, because it's impossible to serialize generic class statically in general case:
@Serializable
data class Data(val a: Int)
@Serializable
data class Box<T>(val boxed: T)
val dataSerializer: KSerializer<Data> = Data.serializer()
val boxedDataSerializer: KSerializer<Box<Data>> = Box.serializer(dataSerial)Built-in types, like Int, and standard collections doesn't have that method and instead are available via
factories and companion extensions:
val intSerializer: KSerializer<Int> = Int.serializer()
val intListSerializer: KSerializer<List<Int>> = ListSerializer(Int.serializer()) // generic, requires instantiationFor convenience, serializers have extension properties:
val dataListSerializer: KSerializer<List<Data>> = Data.serializer().list
val mapSerializer: KSerializer<Map<String, Int>> = MapSerializer(String.serializer(), Int.serializer())To convert from serializer for type T to serializer for nullable type T?, you can use extension factory method nullable:
val nullableIntSerializer: KSerializer<Int?> = IntSerializer.nullableAll external serializers (defined by user) are instantiated in a user-specific way. To learn how to write them, see docs.
In following special case:
- Class explicitly marked
@Serializable - Class does not have generic type arguments
You can obtain serializer from KClass instance: val d: KSerializer<MyData> = MyData::class.serializer().
This approach is discouraged in general because it is implicit and uses reflection (and therefore not working on Kotlin/Native),
but may be useful shorthand in some cases.
Functions which uses this or similar functionality are annotated
with experimental
annotation kotlinx.serialization.ImplicitReflectionSerializer.
Consult annotation documentation
to learn about restrictions of this approach.
To learn how to use experimental annotations, look at theirs KEEP
or use this guide.
There are two special serializers which are turned on using corresponding annotations:
@Contextual for ContextSerializer and @Polymorphic for PolymorphicSerializer.
The former allows to switch to the run-time resolving of serializers instead of compile-time. This can be useful when you want to use some custom external serializer or to define different serializers for different formats. The latter allows polymorphic serialization and deserialization using runtime class information and recorded name of a class. Consult their documentation for details. Polymorphic serialization is explained in details here.
Both use serial modules system, which is explained here.
Runtime library provides three ready-to use formats: JSON, CBOR and ProtoBuf.
JSON format represented by Json class from kotlinx.serialization.json package.
It is configurable via JsonConfiguration class, which has following parameters:
- encodeDefaults - set this to false to omit writing optional properties if they are equal to their default values.
- strictMode - Prohibits unknown keys when parsing JSON. Prohibits NaN and Infinity float values when serializing JSON. Enabled by default.
- unquoted - means that all field names and other objects (where it's possible) would not be wrapped in quotes. Useful for debugging.
- prettyPrint - classic pretty-printed multiline JSON.
- indent - size of indent, applicable if parameter above is true.
- useArrayPolymorphism – switches to writing polymorphic values in
[className, object]format. Disabled by default. - classDiscriminator – name of the class descriptor property in polymorphic serialization.
It also has two pre-defined sets of parameters: Default and Stable.
Default provides recommended and sane configuration, however, due to a library evolution,
it can be tweaked and changed between library releases.
Stable provides configuration which is guaranteed to be unchanged between library releases.
Since JsonConfiguration is a data class, you can copy any configuration you like to tweak it.
All unstable constructors and configurations are annotated with experimental annotation kotlinx.serialization.UnstableDefault.
You can also specify desired behaviour for duplicating keys.
By default it is UpdateMode.OVERWRITE.
You can use UpdateMode.UPDATE, and by doing that you'll be able to merge two lists or maps with same key into one; but be aware that serializers for non-collection types are throwing UpdateNotSupportedException by default.
To prohibit duplicated keys, you can use UpdateMode.BANNED.
JSON API:
fun <T> stringify(serializer: SerializationStrategy<T>, obj: T): String
inline fun <reified T : Any> stringify(obj: T): String = stringify(T::class.serializer(), obj)
fun <T> parse(loader: DeserializationStrategy<T>, str: String): T
inline fun <reified T : Any> parse(str: String): T = parse(T::class.serializer(), str)stringify transforms object to string, parse parses. No surprises.
Besides this, functions toJson and fromJson allow converting @Serializable Kotlin object to and from abstract JSON syntax tree. To build JSON AST from String, use parseJson.
You can also use one of predefined instances, like Json.plain, Json.indented, Json.nonstrict or Json.unquoted. API is duplicated in companion object, so Json.parse(...) equals to Json.plain.parse(...).
Note: because JSON doesn't support maps with keys other than strings (and primitives), Kotlin maps with non-trivial key types are serialized as JSON lists.
Caveat: T::class.serializer() assumes that you use it on class defined as @Serializable,
so it wouldn't work with root-level collections or external serializers out of the box. It's always better to specify serializer explicitly.
Cbor class provides following functions:
fun <T : Any> dump(serializer: SerializationStrategy<T>, obj: T): ByteArray // saves object to bytes
inline fun <reified T : Any> dump(obj: T): ByteArray // same as above, resolves serializer by itself
inline fun <reified T : Any> dumps(obj: T): String // dump object and then pretty-print bytes to string
fun <T : Any> load(loader: DeserializationStrategy<T>, raw: ByteArray): T // load object from bytes
inline fun <reified T : Any> load(raw: ByteArray): T // save as above
inline fun <reified T : Any> loads(hex: String): T // inverse operation for dumpsIt has UpdateMode.BANNED by default. As Json, Cbor supports omitting default values.
Note: CBOR, unlike JSON, supports maps with non-trivial keys,
and Kotlin maps are serialized as CBOR maps, but some parsers (like jackson-dataformat-cbor) don't support this.
Because protobuf relies on serial ids of fields, called 'tags', you have to provide this information,
using serial annotation @SerialId:
@Serializable
data class KTestInt32(@SerialId(1) val a: Int)This class is equivalent to the following proto definition:
message Int32 {
required int32 a = 1;
}Note that we are using proto2 semantics, where all fields are explicitly required or optional.
Number format is set via @ProtoType annotation. ProtoNumberType.DEFAULT is default varint encoding (intXX), SIGNED
is signed ZigZag representation (sintXX), and FIXED is fixedXX type. uintXX and sfixedXX are not supported yet.
Repeated fields represented as lists. Because format spec says that if the list is empty, there will be no elements in the stream with such tag,
you must explicitly mark any field of list type with default = emptyList(). Same for maps. Update mode for Protobuf is set to UPDATE and can't be changed, thus allowing merging several scattered lists into one.
Other known issues and limitations:
- Packed repeated fields are not supported
More examples of mappings from proto definitions to Koltin classes can be found in test data: here and here
Mapper allows you to serialize/deserialize object to/from map:
@Serializable
data class Data(val first: Int, val second: String)
val map: Map<String, Any> = Mapper.map(Data(42, "foo")) // mapOf("first" to 42, "second" to "foo")To get your object back, use unmap function. To support objects with nullable values, use mapNullable and unmapNullable.
Allows you to convert JS objects of dynamic types into fair correct Kotlin objects.
@Serializable
data class Data(val a: Int)
@Serializable
data class DataWrapper(val s: String, val d: Data?)
val dyn = js("""{s:"foo", d:{a:42}}""")
val parsed = DynamicObjectParser().parse<DataWrapper>(dyn)
parsed == DataWrapper("foo", Data(42)) // trueParser does not support kotlin maps with keys other than
String.