Introduction: Lightweight and high-performance jackson-module-kotlin
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jackson-module-kogera is an experimental project to develop jackson-module-kotlin.
This project has the following features compared to jackson-module-kotlin.

  • Lightweight
  • high-performance
    • Fast deserialization
    • Smaller memory consumption
  • More Kotlin friendly behavior

This project is experimental, but passes all the tests implemented in jackson-module-kotlin except for the intentional incompatibility.

Features of jackson-module-kogera

The main feature of jackson-module-kogera is that it replaces kotlin-reflect with kotlinx.metadata.jvm.
As of 1.7.21, kotlin-reflect is a huge library(3MB), and replacing it with kotlinx.metadata.jvm(1MB) makes it lightweight.

Several performance improvements have also been made. First, by implementing the equivalent of https://github.com/FasterXML/jackson-module-kotlin/pull/439, deserialization is now up to three times faster, depending on the use case.
The cache has also been reorganized based on benchmark results to achieve smaller memory consumption.
The performance degradation when the strictNullChecks option is enabled is also greatly reduced.

The next main feature is value class support.
The jackson-module-kogera supports many use cases for value class, including deserialization.
See here for basic policies and notes on handling value class.

Here is a list of issues that are not resolved in jackson-module-kotlin but are or will be resolved in kogera.

About intentional destructive changes

This project makes several disruptive changes to achieve more Kotlin-like behavior.
Details are summarized in KogeraSpecificImplementations.


  • jackson 2.14.2
  • Java 8+
  • Kotlin 1.7.21+

About compatibility checks

Compatibility checks for Java and Kotlin are done by CI grid tests.

The Java test covers all currently supported LTS versions and the latest versions.
Currently 8,11,17 and 19 are covered.

Kotlin is tested on the latest patch version and the latest Beta or RC version within each minor version since 1.7.21.
Currently 1.7.21, 1.8.10 and 1.8.20-Beta are covered.
I hope to lower this version in the future, but currently the minimum Kotlin version that can be supported is 1.7 due to kotlinx-metadata-jvm constraints.


The package is temporarily published in JitPack.
Please refer to jitpack.io for the released version.

ProjectMapK / jackson-module-kogera

repositories {
    // ...

    maven { setUrl("https://jitpack.io") }

dependencies {
  // ...


Migration in existing projects

The following descriptions are for the alpha version.
I plan to change the package and module names when I move to the beta version(see https://github.com/FasterXML/jackson-module-kotlin/issues/450#issuecomment-1384788717).

When replacing jackson-module-kotlin in an existing project, please replace the dependencies of jackson-module-kotlin with jackson-module-kogera.
Since the package/module name of jackson-module-kogera is the same as that of jackson-module-kotlin, it is basically possible to migrate by simply replacing dependencies (although there is a possibility of compile errors due to some destructive changes).

. /gradlew dependencies and if jackson-module-kotlin does not appear, you have successfully migrated.
If jackson-module-kotlin is still there, please exclude it from the dependencies.
At least for the Spring project at hand, I have confirmed that this method works fine up to the Jackson2ObjectMapperBuilder auto-configuration.

If you find any problems, it would be appreciated if you could share them in an issue.

About the future

Currently, this project is in alpha.
After the following features are implemented, this project will be moved to the beta version if there is enough demand for it.

  • Deserialization support for value class
    • Support for policy decisions and basic use cases was completed in #68, only edge cases and hard-to-fix issues remain
  • Support for less than Kotlin 1.6.x(including grid test building with CI)
    • Due to limitations of kotlinx-metadata-jvm, only 1.7 or later is supported.
    • However, it will be pulled down when possible.
  • Rename module and package(see https://github.com/FasterXML/jackson-module-kotlin/issues/450#issuecomment-1384788717).

About license

This project is based on jackson-module-kotlin, so the license follows jackson-module-kotlin.
The current license is Apache License 2.0.

jackson-module-kotlin/LICENSE at 2.14 · FasterXML/jackson-module-kotlin

About Kogera

Kogera is the Japanese name for Japanese pygmy woodpecker.
This bird is the smallest woodpecker in Japan.

About Me
GitHub: Trinea
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