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

Especially when it comes to deserialization throughput, it is several times higher than jackson-module-kotlin.

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 2.16.x
  • Java 8+
  • Kotlin 1.8.22+

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.

Kotlin is tested with the latest patch version and the latest Beta or RC version within each minor version after the minimum version.
The Kotlin version is the lowest version available with the latest kotlinx-metadata-jvm.

See Workflow for the currently tested versions.


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

When replacing jackson-module-kotlin in an existing project, please follow these steps

  1. Replace the dependencies of jackson-module-kotlin with jackson-module-kogera.
  2. Replace com.fasterxml.jackson.module.kotlin used for import with io.github.projectmapk.jackson.module.kogera.

In projects that use spring-boot, there is a possibility that jackson-module-kotlin will be used by auto-configuration.
Therefore, it is necessary to exclude jackson-module-kotlin from the dependencies and to configure it manually.

You can check if your project's dependencies include jackson-module-kotlin by looking at . /gradlew dependencies.

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

About the future

I will not be treating this project as "stable".

This project currently serves as a testing ground for all improvements to jackson-module-kotlin.
In prototyping, it is important to be willing to make disruptive changes.

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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