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  • v1.4.1 Changes

    October 14, 2022

    πŸ›  This is patch release contains several bugfixes and improvements. 0️⃣ Kotlin 1.7.20 is used by default.

    πŸ‘Œ Improvements

    • Add @MustBeDocumented to certain annotations (#2059)
    • Deprecate .isNullable in SerialDescriptor builder (#2040)
    • Unsigned primitives and unsigned arrays serializers can be retrieved as built-ins (#1992)
    • Serializers are now cached inside reflective lookup, leading to faster serializer retrieval (#2015)
    • Compiler plugin can create enum serializers using static factories for better speed (#1851) (Kotlin 1.7.20 required)
    • Provide foundation for compiler plugin intrinsics available in Kotlin 1.8.0 (#2031)

    πŸ›  Bugfixes

    • Support polymorphism in Properties format (#2052) (thanks to Rodrigo Vedovato)
    • Added support of UTF-16 surrogate pairs to okio streams (#2033)
    • Fix dependency on core module from HOCON module (#2020) (thanks to Osip Fatkullin)
  • v1.4.0 Changes

    August 18, 2022

    πŸ›  This release contains all features and bugfixes from 1.4.0-RC plus some bugfixes on its own (see below). 0️⃣ Kotlin 1.7.10 is used as a default.

    πŸ›  Bugfixes

    • Fixed decoding of huge JSON data for okio streams (#2006)
  • v1.4.0-RC Changes

    July 20, 2022

    πŸš€ This is a candidate for the next big release with many new exciting features to try. 0️⃣ It uses Kotlin 1.7.10 by default.

    Integration with Okio's BufferedSource and BufferedSink

    Okio library by Square is a popular solution for fast and efficient IO operations on JVM, K/N and K/JS. πŸ“œ In this version, we have added functions that parse/write JSON directly to Okio's input/output classes, saving you the overhead of copying data to String beforehand. These functions are called Json.decodeFromBufferedSource and Json.encodeToBufferedSink, respectively. There's also decodeBufferedSourceToSequence that behaves similarly to decodeToSequence from Java streams integration, so you can lazily decode multiple objects the same way as before.

    Note that these functions are located in a separate new artifact, so users who don't need them wouldn't find themselves dependent on Okio. To include this artifact in your project, use the same group id org.jetbrains.kotlinx and artifact id kotlinx-serialization-json-okio. πŸ“š To find out more about this integration, check new functions' documentation and corresponding pull requests: #1901 and #1982.

    Inline classes and unsigned numbers do not require experimental annotations anymore

    Inline classes and unsigned number types have been promoted to a Stable feature in Kotlin 1.5, πŸ‘ and now we are promoting support for them in kotlinx.serialization to Stable status, too. 🚚 To be precise, we've removed all @ExperimentalSerializationApi annotations from functions related to inline classes encoding and decoding, πŸ“š namely SerialDescriptor.isInline, Encoder.encodeInline, and some others. We've also updated related [documentation article](docs/

    βž• Additionally, all @ExperimentalUnsignedTypes annotations were removed completely, so you can freely use types such as UInt and their respective serializers as a stable feature without opt-in requirement.

    Part of SerializationException's hierarchy is public now

    πŸš€ When kotlinx.serialization 1.0 was released, all subclasses of SerializationException were made internal, since they didn't provide helpful information besides the standard message. Since then, we've received a lot of feature requests with compelling use-cases for exposing some of these internal types to the public. πŸš€ In this release, we are starting to fulfilling these requests by making MissingFieldException public. πŸ‘ One can use it in the catch clause to better understand the reasons of failure β€” for example, to return 400 instead of 500 from an HTTP server β€” πŸ‘ and then use its fields property to communicate the message better. πŸ‘€ See the details in the corresponding PR.

    πŸš€ In future releases, we'll continue work in this direction, and we aim to provide more useful public exception types & properties. πŸ‘» In the meantime, we've revamped KDoc for some methods regarding the exceptions β€” all of them now properly declare which exception types are allowed to be thrown. For example, KSerializer.deserialize is documented to throw IllegalStateException to indicate problems unrelated to serialization, such as data validation in classes' constructors.

    @MetaSerializable annotation

    πŸš€ This release introduces a new @MetaSerializable annotation that adds @Serializable behavior to user-defined annotations β€” i.e., those annotations would also instruct the compiler plugin to generate a serializer for class. In addition, all annotations marked with @MetaSerializable are saved in the generated @SerialDescriptor as if they are annotated with @SerialInfo.

    πŸ“‡ This annotation will be particularly useful for various format authors who require adding some metadata to the serializable class β€” this can now be done using a single annotation instead of two, and without the risk of forgetting @Serializable. Check out details & examples in the KDoc and corresponding PR.

    Note: Kotlin 1.7.0 or higher is required for this feature to work.

    πŸ“š Moving documentation from GitHub pages to

    πŸ“š As a part of a coordinated effort to unify kotlinx libraries users' experience, Dokka-generated documentation pages (KDoc) were moved from to No action from you is required β€” there are proper redirects at the former address, so there is no need to worry about links in your blogpost getting obsolete or broken.

    🚚 Note that this move does not affect guides written in Markdown in the docs folder. We aim to move them later, enriching text with runnable examples as in the Kotlin language guides.

    Other improvements

    • Allow Kotlin's null literal in JSON DSL (#1907) (thanks to Lukellmann)
    • Stabilize EmptySerializersModule (#1921)
    • Boost performance of polymorphic deserialization in optimistic scenario (#1919)
    • Added serializer for the kotlin.time.Duration class (plugin support comes in Kotlin 1.7.20) (#1960)
    • Support tagged not null marks in TaggedEncoder/Decoder (#1954) (thanks to EdwarDDay)

    πŸ›  Bugfixes

    • Support quoting unsigned integers when used as map keys (#1969)
    • Fix protocol buffer enum schema generation (#1967) (thanks to mogud)
    • Support diamond inheritance of sealed interfaces in SealedClassSerializer (#1958)
    • Support retrieving serializer for sealed interface (#1968)
    • Fix misleading token description in JSON errors (#1941) (thanks to TheMrMilchmann)
  • v1.3.3 Changes

    May 11, 2022

    πŸ›  This release contains support for Protocol Buffers packed fields, as well as several bugfixes. 0️⃣ It uses Kotlin 1.6.21 by default.

    Protobuf packed fields

    πŸ“„ It is now possible to encode and decode Kotlin classes to/from Protobuf messages with packed repeated fields. To mark the field as packed, use @ProtoPacked annotation on it. Note it affects only List and primitive collection such as IntArray types. 0️⃣ With this feature, it is now possible to decode Proto3 messages, where all repeated fields are packed by default. πŸ‘ Protobuf schema generator also supports new @ProtoPacked annotation.

    Many thanks to Paul de Vrieze for his valuable contribution!

    Other improvements & small features

    • Incorporate JsonPath into exception messages (#1841)
    • Mark block in corresponding encodeStructure/decodeStructure extensions as crossinline to reduce amount of bytecode (#1917)
    • Support serialization of compile-time Collection<E> properties that are not lists at the runtime (#1821)
    • Best-effort kotlin reflect avoidance in serializer(Type) (#1819)

    πŸ›  Bugfixes

    • Iterate over element indices in ObjectSerializer in order to let the format skip unknown keys (#1916)
    • Correctly support registering both default polymorphic serializer & deserializer (#1849)
    • Make error message for captured generic type parameters much more straightforward (#1863)
  • v1.3.2 Changes

    December 23, 2021

    πŸ›  This release contains several features and bugfixes for core API as well as for HOCON format. 0️⃣ It uses Kotlin 1.6.10 by default.

    Serializing objects to HOCON

    It's now possible to encode Kotlin objects to Config values with new Hocon.encodeToConfig function. This feature may help edit existing configs inside Kotlin program or generate new ones.

    Big thanks to Osip Fatkullin for implementing this.

    0️⃣ Polymorphic default serializers

    πŸ— As of now, polymorphicDefault clause inside SerializersModule { } builder specifies a fallback serializer to be used only during deserialization process. A new function has been introduced to allow setting 0️⃣ fallback serializer for serialization: polymorphicDefaultSerializer. This function should ease serializing vast hierarchies of third-party or Java classes.

    0️⃣ Note that there are two new experimental functions, polymorphicDefaultSerializer and polymorphicDefaultDeserializer. πŸš€ To avoid naming confusion, we are going to deprecate polymorphicDefault in favor of polymorphicDefaultDeserializer in the next minor release (1.4.0).

    πŸ’» Credit for the PR goes to our contributor Joseph Burton.

    Other improvements

    • HOCON: parse strings into integers and booleans if possible (#1795) (thanks to tobiaslieber)
    • Add an encodeCollection extensions (#1749) (thanks to Nicklas Ansman Giertz)

    πŸ›  Bugfixes

    • Properly handle top-level value classes in encodeToJsonElement (#1777)
    • Fix incorrect handling of object end when JsonTreeReader (JsonElement) is used with decodeToSequence (#1782)
  • v1.3.1 Changes

    November 11, 2021

    πŸ›  This release mainly contains bugfixes for 1.3.0 and provides new experimental Json.decodeToSequence function.

    πŸ‘Œ Improvements

    • Provide decodeToSequence to read multiple objects from stream lazily (#1691)

    πŸ›  Bugfixes

    • Correctly handle buffer boundaries while decoding escape sequences from json stream (#1706)
    • Properly skip unknown keys for objects and structures with zero properties (#1720)
    • Fix merging for maplikeSerializer when the map is not empty (by using the actual size * 2). (#1712) (thanks to pdvrieze)
    • Fix lookup of primitive array serializers by Java type token (#1708)
  • v1.3.0 Changes

    September 23, 2021

    πŸš€ This release contains all of the cool new features from 1.3.0-RC (see below) as well as minor improvements. 0️⃣ It uses Kotlin 1.5.31 by default.

    πŸ›  Bugfixes and improvements

    • Promote JsonConfiguration and its usages to stable (#1690)
    • Remove opt-in annotations from SerialFormat, StringFormat, BinaryFormat (#1688)
    • Correctly throw SerializationException instead of IOOBE for some cases with EOF in streams (#1677)
    • CBOR: ignore tags when reading (#1614) (thanks to David Robertson)
  • v1.3.0-RC Changes

    September 06, 2021

    πŸš€ This is a release candidate for the next version. It contains a lot of interesting features and improvements, so we ask you to evaluate it and share your feedback. 0️⃣ Kotlin 1.5.30 is used by default.

    Java IO stream-based JSON serialization

    Finally, in kotlinx.serialization 1.3.0 we’re presenting the first experimental version of the serialization API for IO streams: Json.encodeToStream and Json.decodeFromStream extension functions. With this API, you can decode objects directly from files, network connections, and other data sources without reading the data to strings beforehand. The opposite operation is also available: you can send encoded objects directly to files and other streams in a single API call. IO stream serialization is available only on the JVM platform and for the JSON format for now.

    Check out more in the PR.

    0️⃣ Property-level control over defaults values encoding

    0️⃣ Previous versions of the library allowed to specify whether to encode or drop default properties values with πŸ”§ format configuration flags such as Json { encodeDefaults = false }. 0️⃣ In 1.3.0 we’re extending this feature by adding a new way to fine-tune the serialization of default values: 0️⃣ you can now control it on the property level using the new @EncodeDefaults annotation.

    0️⃣ @EncodeDefaults annotation has a higher priority over the encodeDefaults property and takes one of two possible values:

    • 0️⃣ ALWAYS (default value) encodes a property value even if it equals to default.
    • πŸ”§ NEVER doesn’t encode the default value regardless of the format configuration.

    0️⃣ Encoding of the annotated properties is not affected by encodeDefault format flag and works as described for all serialization formats, not only JSON.

    To learn more, check corresponding PR.

    Excluding null values from JSON serialization

    In 1.3.0, we’re introducing one more way to reduce the size of the generated JSON strings: omitting null values. πŸ”§ A new JSON configuration property explicitNulls defines whether null property values should be included in the serialized JSON string. 0️⃣ The difference from encodeDefaults is that explicitNulls = false flag drops null values even if the property does not have a default value. 0️⃣ Upon deserializing such a missing property, a null or default value (if it exists) will be used.

    0️⃣ To maintain backwards compatibility, this flag is set to true by default. πŸ“š You can learn more in the [documentation](docs/ or the PR.

    Per-hierarchy polymorphic class discriminators

    In previous versions, you could change the discriminator name using the πŸ“„ classDiscriminator property of the Json instance. In 1.3.0, we’re adding a way to set a custom discriminator name for each class hierarchy to enable more flexible serialization. You can do it by annotating a class with @JsonClassDiscriminator with the discriminator name as its argument. A custom discriminator is applied to the annotated class and its subclasses. Only one custom discriminator can be used in each class hierarchy, thanks to the new @InheritableSerialInfo annotation.

    Check out corresponding PR for details.

    πŸ‘Œ Support for Java module system

    πŸš€ Now all kotlinx.serialization runtime libraries are shipped as a multi-release JAR with module-info.class file for Java versions 9 and higher. This enables possibilities to use kotlinx.serialization with modern tools such as jlink and various technologies such as TorandoFX.

    Many thanks to our contributor Gerard de Leeuw and his PR for making this possible.

    Native targets for Apple Silicon

    πŸš€ This release includes klibs for new targets, introduced in Kotlin/Native 1.5.30 β€” 🍎 macosArm64, iosSimulatorArm64, watchosSimulatorArm64, and tvosSimulatorArm64.

    πŸ›  Bugfixes and improvements

    • Properly handle quoted 'null' literals in lenient mode (#1637)
    • Switch on deep recursive function when nested level of JSON is too deep (#1596)
    • Support for local serializable classes in IR compiler
    • Support default values for @SerialInfo annotations in IR compiler
    • Improve error message for JsonTreeReader (#1597)
    • Add guide for delegating serializers and wrapping serial descriptor (#1591)
    • Set target JVM version to 8 for Hocon module in Gradle metadata (#1661)
  • v1.2.2 Changes

    July 08, 2021

    πŸ›  This release contains various bugfixes, some useful features and important performance improvements. 0️⃣ It also uses Kotlin 1.5.20 as default.

    πŸ”‹ Features

    • Support for @JsonNames and coerceInputValues in Json.decodeFromDynamic (#1479)
    • Add factory function to wrap a serial descriptor with a custom name for custom delegating serializers (#1547) (thanks to Fadenfire)
    • Allow contextually serialized types to be used as map keys in Json (#1552) (thanks to pdvrieze)

    πŸ›  Bugfixes and performance improvements

    • Update size in JsonStringBuilder slow-path to avoid excessive array-copies for large strings with escape symbols (#1491)
    • Optimize integer encoding length in CBOR (#1570) (thanks to davertay)
    • Throw JsonDecodingException instead of ClassCastException during unexpected null in TreeJsonDecoder (#1550)
    • Prohibit 'null' strings in lenient mode in order to get rid of 'null' and "null" ambiguity (#1549)
    • Avoid usage of reflective-like serialDescriptor<KType> in production sources (#1540)
    • Added correct error message when deserializing missing enum member for Properties format (#1539)
    • Make DescriptorSchemaCache in Json thread-local on Native (#1484)
  • v1.2.1 Changes

    May 14, 2021

    πŸ›  This release mainly contains bugfixes for various issues, including important broken thread-safety and improper encoding.

    πŸ”‹ Features

    • Added support for nullable values, nested and empty collections in protobuf (#1430)

    πŸ›  Bugfixes

    • Support @JsonNames for enum values (#1473)
    • Handle EOF in skipElement correctly (#1475)
    • Allow using value classes with primitive carriers as map keys (#1470)
    • Read JsonNull only for non-string literals in JsonTreeReader (#1466)
    • Properly reuse JsonStringBuilders in CharArrayPool (#1455)
    • Properly ensure capacity of the string builder on the append slow-path (#1441)