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
2.9.8+ Releases are compiled with Kotlin 1.3.x, other older releases are Kotlin 1.2.x. All should be compatible with
current Kotlin if you also ensure the kotlin-reflect dependency is included with the same version number as stdlib.
- release
2.10.10(for Jackson2.10.x) - release
2.9.10(for Jackson2.9.x) - release
2.8.11.1(for Jackson2.8.x) - release
2.7.9.1(for Jackson2.7.x) lacking in some new features from 2.8 branch
Releases require that you have included Kotlin stdlib and reflect libraries already.
Gradle:
compile "com.fasterxml.jackson.module:jackson-module-kotlin:2.10.+"
Maven:
<dependency>
<groupId>com.fasterxml.jackson.module</groupId>
<artifactId>jackson-module-kotlin</artifactId>
<version>2.10.10</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:
val mapper = ObjectMapper().registerModule(KotlinModule())
or with the extension functions imported from import com.fasterxml.jackson.module.kotlin.*, one of:
val mapper = jacksonObjectMapper()
val mapper = ObjectMapper().registerKotlinModule()
A simple data class example:
import com.fasterxml.jackson.module.kotlin.*
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.
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
@JsonCreatorannotation is optional unless you have more than one constructor that is valid, or you want to use a static factory method (which also must haveplatformStaticannotation). In these cases, annotate only one method asJsonCreator. - Serializing a member or top-level Kotlin class that implements Iterator requires a workaround, see Issue #4 for easy workarounds.
- If using proguard:
kotlin.Metadataannotations may be stripped, preventing deserialization. Add a proguard rule to keep thekotlin.Metadataclass:-keep class kotlin.Metadata { *; }- If you're getting
java.lang.ExceptionInInitializerError, you may also need:-keep class kotlin.reflect.** { *; }
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)
(others are likely to work, but may not be tuned for Jackson)