jackson-module-kotlin

Overview

Module that adds support for serialization/deserialization of Kotlin classes and data classes. Previously a default constructor must have existed on the Kotlin object for Jackson to deserialize into the object. With this module, single constructor classes can be used automatically, and those with secondary constructors or static factories are also supported.

Status

• release 3.2.0 (for Jackson 3.2.x)
• release 3.1.0 (for Jackson 3.1.x)
• release 2.21.1 (for Jackson 2.21.x)

Releases require that you have included Kotlin stdlib and reflect libraries already.

Gradle:

implementation "tools.jackson.module:jackson-module-kotlin:3.2.+"

Maven:

<dependency>
    <groupId>tools.jackson.module</groupId>
    <artifactId>jackson-module-kotlin</artifactId>
    <version>3.2.0</version>
</dependency>

Usage

For any Kotlin class or data class constructor, the JSON property names will be inferred from the parameters using Kotlin runtime type information.

To use, just register the Kotlin module with your ObjectMapper instance:

// With Jackson 3.0 and later
import tools.jackson.module.kotlin.jacksonObjectMapper
...
val mapper = jacksonObjectMapper()
// or
import tools.jackson.module.kotlin.registerKotlinModule
...
val mapper = ObjectMapper().registerKotlinModule()
// or
import tools.jackson.module.kotlin.jsonMapper
import tools.jackson.module.kotlin.kotlinModule
...
val mapper = jsonMapper {
  addModule(kotlinModule())
}

In 2.17 and later, the jacksonObjectMapper {} and registerKotlinModule {} lambdas allow configuration for KotlinModule.
See #Configuration for details on the available configuration items.

A simple data class example:

import tools.jackson.module.kotlin.jacksonObjectMapper
import tools.jackson.module.kotlin.readValue

data class MyStateObject(val name: String, val age: Int)

...
val mapper = jacksonObjectMapper()

val state = mapper.readValue<MyStateObject>(json)
// or
val state: MyStateObject = mapper.readValue(json)
// or
myMemberWithType = mapper.readValue(json)

All inferred types for the extension functions carry in full generic information (reified generics). Therefore, using readValue() extension without the Class parameter will reify the type and automatically create a TypeReference for Jackson.

Also, there are some convenient operator overloading extension functions for JsonNode inheritors.

import tools.jackson.databind.node.ArrayNode
import tools.jackson.databind.node.ObjectNode
import tools.jackson.databind.node.JsonNodeFactory
import tools.jackson.module.kotlin.*

// ...
val objectNode: ObjectNode = JsonNodeFactory.instance.objectNode()
objectNode.put("foo1", "bar").put("foo2", "baz").put("foo3", "bax")
objectNode -= "foo1"
objectNode -= listOf("foo2")
println(objectNode.toString()) // {"foo3":"bax"}

// ...
val arrayNode: ArrayNode = JsonNodeFactory.instance.arrayNode()
arrayNode += "foo"
arrayNode += true
arrayNode += 1
arrayNode += 1.0
arrayNode += "bar".toByteArray()
println(arrayNode.toString()) // ["foo",true,1,1.0,"YmFy"]

Shorthands for deserialization

Shorthands for the deserialization methods of ObjectMapper and ObjectReader are provided as extension functions.
Since their type parameters are reified, Class and TypeReference do not need to be passed explicitly.

Receiver	Shorthands
ObjectMapper	readValue, readValues, treeToValue, convertValue
ObjectReader	readValueTyped, readValuesTyped, treeToValue

ObjectMapper.readValue accepts the same sources as the original, namely JsonParser, File, String, Reader, InputStream and ByteArray.

Note that these are not merely shorthands: since 2.19.0, most of them check the deserialized value to preserve Kotlin null safety.
If a null is deserialized while the reified type is non-null, DatabindException is thrown instead of returning it.

// Throws DatabindException, because String is non-null but null was deserialized
mapper.readValue<String>("null")

// Returns null, because the reified type is nullable
mapper.readValue<String?>("null")

The same check also detects values whose type is unrelated to the reified type, which indicates that ObjectMapper is incorrectly customized.

For readValues / readValuesTyped, the check is applied to each value by the returned iterator, so the exception is thrown from next() / nextValue() rather than from the function itself.
ObjectReader.treeToValue is the only function that does not perform the check and declares a nullable return type instead.

Compatibility

Kotlin

(NOTE: incomplete! Please submit corrections/additions via PRs!)

Different kotlin-core versions are supported by different Jackson Kotlin module minor versions. Here is an incomplete list of supported versions:

• Jackson 3.3.x: Kotlin-core 2.2 - 2.4
• Jackson 3.2.x: Kotlin-core 2.1 - 2.3
• Jackson 3.1.x: Kotlin-core 2.1 - 2.3
• Jackson 2.21.x: Kotlin-core 2.1 - 2.3
  • Starting with version 2.21.2, compatibility with Kotlin 1.9 is provided(see #1129).

Please note that the versions supported by 2.17 are tentative and may change depending on the release date.

Android

Supported Android SDK versions are determined by jackson-databind. Please see this link for details.

Annotations

You can intermix non-field values in the constructor and JsonProperty annotation in the constructor. Any fields not present in the constructor will be set after the constructor call. An example of these concepts:

@JsonInclude(JsonInclude.Include.NON_EMPTY)
class StateObjectWithPartialFieldsInConstructor(val name: String, @JsonProperty("age") val years: Int) {
    @JsonProperty("address") lateinit var primaryAddress: String // set after construction
    var createdDt: DateTime by Delegates.notNull()               // set after construction
    var neverSetProperty: String? = null                         // not in JSON so must be nullable with default
}

Note that using lateinit or Delegates.notNull() will ensure that the value is never null when read, while letting it be instantiated after the construction of the class.

Caveats

• The @JsonCreator annotation is optional unless you have more than one constructor that is valid, or you want to use a static factory method (which also must have platformStatic annotation, e.g. @JvmStatic). In these cases, annotate only one method as JsonCreator.
• During deserialization, if the definition on Kotlin is a non-null primitive and null is entered explicitly on JSON, processing will continue with an unintended default value. This problem is fixed by enabling DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES on ObjectMapper.
• Serializing a member or top-level Kotlin class that implements Iterator requires a workaround, see Issue #4 for easy workarounds.
• If using proguard:
  • kotlin.Metadata annotations may be stripped, preventing deserialization. Add a proguard rule to keep the kotlin.Metadata class: -keep class kotlin.Metadata { *; }
  • If you’re getting java.lang.ExceptionInInitializerError, you may also need: -keep class kotlin.reflect.** { *; }
  • If you’re still running into problems, you might also need to add a proguard keep rule for the specific classes you want to (de-)serialize. For example, if all your models are inside the package com.example.models, you could add the rule -keep class com.example.models.** { *; }
  • Also, please refer to this page for settings related to jackson-databind.

Support for Kotlin Built-in classes

These Kotlin classes are supported with the following fields for serialization/deserialization (and other fields are hidden that are not relevant):

• Pair (first, second)
• Triple (first, second, third)
• IntRange (start, end)
• CharRange (start, end)
• LongRange (start, end)

Deserialization for value class is also supported since 2.17.
Please refer to this page for more information on using value class, including serialization.

(others are likely to work, but may not be tuned for Jackson)

Sealed classes without @JsonSubTypes

Subclasses can be detected automatically for sealed classes, since all possible subclasses are known at compile-time to Kotlin. This makes com.fasterxml.jackson.annotation.JsonSubTypes redundant. A com.fasterxml.jackson.annotation.@JsonTypeInfo annotation at the base-class is still necessary.

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
sealed class SuperClass{
    class A: SuperClass()
    class B: SuperClass()
}

...
val mapper = jacksonObjectMapper()
val root: SuperClass = mapper.readValue(json)
when(root){
    is A -> "It's A"
    is B -> "It's B"
}

Configuration

The Kotlin module may be given a few configuration parameters at construction time; see the inline documentation for details on what options are available and what they do.

val kotlinModule = KotlinModule.Builder()
    .enable(KotlinFeature.StrictNullChecks)
    .build()
val mapper = JsonMapper.builder()
    .addModule(kotlinModule)
    .build()

If your ObjectMapper is constructed in Java, there is a builder method provided for configuring these options:

KotlinModule kotlinModule = new KotlinModule.Builder()
        .enable(KotlinFeature.StrictNullChecks)
        .build();
ObjectMapper objectMapper = JsonMapper.builder()
        .addModule(kotlinModule)
        .build();

Development

Maintainers

Following developers have committer access to this project.

• Author: Jayson Minard (@apatrida) wrote this module originally (no longer active)
• Active Maintainers:
  • Dmitry Spikhalskiy (@Spikhalskiy) – since 2.14
  • Drew Stephens (@dinomite)
  • Vyacheslav Artemyev (@viartemev)
  • WrongWrong (@k163377) – since 2.15
• Co-maintainers:
  • Tatu Saloranta (@cowtowncoder)

You may at-reference maintainers as necessary but please keep in mind that all maintenance work is strictly voluntary (no one gets paid to work on this or any other Jackson components) so there is no guarantee for timeliness of responses.

All Pull Requests should be reviewed by at least one of active maintainers; bigger architectural/design questions should be agreed upon by majority of active maintainers.

Releases & Branches

This module follows the release schedule of the rest of Jackson—the current version is consistent across all Jackson components & modules. See the jackson-databind README for details.

Contributing

We welcome any contributions—reports of issues, ideas for enhancements, and pull requests related to either of those.

See the main Jackson contribution guidelines for more details.

Branches

If you are going to write code, choose the appropriate base branch:

• 3.1 for bugfixes against the LTS version
• 3.2 for bugfixes against the current stable version
• 3.x for additive functionality & features or minor, backwards compatible changes to existing behavior to be included in the next minor version release
• 2.x/2.21 for bugfixes against the LTS version

Failing tests

There are a number of tests for functionality that is broken, mostly in the failing package but a few as part of other test suites. Instead of ignoring these tests (with JUnit’s @Ignore annotation) or excluding them from being run as part of automated testing, the tests are written to demonstrate the failure (either making a call that throws an exception or with an assertion that fails) but not fail the build, except if the underlying issue is fixed. This allows us to know when the tested functionality has been incidentally fixed by unrelated code changes.

See the tests readme for more information.