This is a release candidate for the next version with many new features to try. It uses Kotlin 1.8.0 by default.

Json naming strategies

A long-awaited feature (#33) is available in this release. A new interface, JsonNamingStrategy and Json configuration property namingStrategy allow defining a transformation that is applied to all properties' names serialized by a Json instance. There's also a predefined implementation for the most common use case: Json { namingStrategy = JsonNamingStrategy.SnakeCase }. Check out the PR for more details and documentation.

Json unquoted literals

kotlinx-serialization-json has an API for manipulating raw Json values: functions and classes JsonObject, JsonPrimitive, etc. In this release, there is a new addition to this API: JsonUnquotedLiteral constructor function. It allows to produce a string that is not quoted in the Json output. This function has a lot of valuable applications: from writing unsigned or large numbers to embedding whole Json documents without the need for re-parsing. For an example, read the Encoding literal Json content docs. This huge feature was contributed to us by aSemy: #2041.

Stabilization of serializer(java.lang.Type) function family

Functions serializer, serializerOrNull and extensions SerializersModule.serializer, SerializersModule.serializerOrNull have JVM-only overloads that accept java.lang.Type. These overloads are crucial for interoperability: with them, third-party Java frameworks like Spring, which usually rely on Java's reflection and type tokens, can retrieve KSerializer instance and use kotlinx.serialization properly. We've removed @ExperimentalSerializationApi from these functions, and starting from 1.5.0-RC they're considered stable with all backward compatibility guarantees. This change should improve third-party support for kotlinx.serialization in various frameworks. See the PR for details.

Deprecations in module builders for polymorphism

Some time ago, in 1.3.2, new functions SerializersModuleBuilder.polymorphicDefaultSerializer/polymorphicDefaultDeserializer and PolymorphicModuleBuilder.defaultDeserializer were introduced — better names allow an easier understanding of which serializers affect what part of the process. In 1.5.0-RC, we finish the migration path: these functions are no longer experimental. And old functions, namely SerializersModuleCollector.polymorphicDefault and PolymorphicModuleBuilder.default, are now deprecated. See the PR for details.

Bundled Proguard rules

The kotlinx-serialization-core-jvm JAR file now includes consumer Proguard rules, so manual Proguard configuration is no longer necessary for most of the setups. See updated Android setup section and corresponding PRs: #2092, #2123.

Support for kotlin.Duration in HOCON format

HOCON specifies its own formatting for duration values. Starting with this release, kotlinx-serialization-hocon is able to serialize and deserialize kotlin.Duration using proper representation instead of the default one. Big thanks to Alexander Mikhailov and his PRs: #2080, #2073.

Functional and performance improvements

• Make DeserializationStrategy covariant at declaration-site (#1897) (thanks to Lukellmann)
• Added support for the kotlin.Nothing class as built-in (#1991, #2150)
• Further improve stream decoding performance (#2101)
• Introduce CharArray pooling for InputStream decoding (#2100)
• Consolidate exception messages and improve them (#2068)

Bugfixes

• Add stable hashCode()/equals() calculation to PrimitiveSerialDescriptor (#2136) (thanks to Vasily Vasilkov)
• Added a factory that creates an enum serializer with annotations on the class (#2125)
• Correctly handle situation where different serializers can be provided for the same KClass in SealedClassSerializer (#2113)
• Fixed serializers caching for parametrized types from different class loaders (#2070)