Overview

Jackson is a fast JSON processor for Java that supports three models: streaming, node, and object mapping (akin to the three independent models SAX/[Stax], DOM and JAXB for XML processing).

The object mapping model is a high-level processing model that allows the user to project JSON data onto a domain-specific data model appropriate for their application, without having to deal with the low-level mechanics of JSON parsing. It is the standard object mapping parser implementaton in Jersey, the reference implementation for JSR-311 (Java API for Restful Web Services).

Scala is a functional programming language for the JVM that supports Java interoperability. Its standard library is quite distinct from Java, and does not fulfill the expectations of Jacksons default mappings. Notably, Scala collections do not derive from java.util.Collection or its subclasses, and Scala properties do not (by default) look like Java Bean properties.

The Scala Module supports serialization and limited deserialization of Scala Case Classes, Sequences, Maps, Tuples, Options, and Enumerations.

Version Support

Jackson-module-scala follows the same release strategy of jackson-databind. 3.x branch is used for Jackson 3 development.

Scala 2.12, 2.13, 3.3+ are supported. Scala 2.11 support was dropped in v3.0.0. Java 17 is the minimum supported version now (Jackson 3 generally has a minimum requirement of Java 17).

Scala 3

There are a few differences from Scala 2 support.

• There are still a few tests that work with Scala 2 that fail with Scala 3
• It is expected that most use cases should work ok with Scala 3
  • Known issues with using jackson-module-scala with Scala 3 are tracked at scala3
  • There has been limited testing of using Scala 3 classes with Scala 2 jackson-module-scala or Scala 2 classes with Scala 3 jackson-module-scala
• Scala 3 emits no generic signature for a local or anonymous class (scala/scala3#6349), so a Map[String, Base] member of new { ... } or of a class declared inside a method looks like a raw Map to Jackson. Values still serialize, but anything that depends on the declared value type - such as @JsonTypeInfo on Base - is not applied. Name the type with @JsonSerialize(contentAs = classOf[Base]), or declare the class at the top level or inside an object.
• Scala 3 enums are supported out of the box, and derives ScalaTypeInfo / derives SealedSubtypes replace the registrations Scala 2 needs for type-erased fields and sealed hierarchies - see Type-erased fields and Scala 3 enums and sealed hierarchies below.

Usage

To use the Scala Module in Jackson, simply register it with the ObjectMapper instance:

val mapper = JsonMapper.builder()
  .addModule(DefaultScalaModule)
  .build()

DefaultScalaModule is a Scala object that includes support for all currently supported Scala data types. If only partial support is desired, the component traits can be included individually (approach differs from Jackson 2):

val scalaModule = ScalaModule.builder()
  .addModule(OptionModule)
  .addModule(TupleModule)
  .build()

val mapper = JsonMapper.builder()
  .addModule(scalaModule)
  .build()

If you want to configure the behavior of the ScalaModule but have all the underlying Scala modules, you can do this :

val scalaModule = ScalaModule.builder()
  .addAllBuiltinModules()
  .addModule(TupleModule)
  .build()

val mapper = JsonMapper.builder()
  .addModule(scalaModule)
  .supportScala3Classes(false) //default of true
  .build()

ClassTagExtensions

You can also mixin ClassTagExtensions to get rich wrappers that automatically convert scala ClassTags directly into TypeReferences for Jackson to use:

val mapper = JsonMapper.builder().addModule(DefaultScalaModule).build() :: ClassTagExtensions
val myMap = mapper.readValue[Map[String, Tuple2[Int,Int]]](src)

ClassTagExtensions is a replacement for ScalaObjectMapper, which was recently deprecated because it relies on Manifests and they are not supported in Scala 3.

This is the equivalent of

val mapper = JsonMapper.builder().addModule(DefaultScalaModule).build()
val myMap = mapper.readValue(src, new TypeReference[Map[String,Tuple2[Int,Int]]]{})

Consult the Scaladoc for further details.

Type-erased fields (Option[Long], Seq[Int], Map[String, Long])

The JVM keeps a reference type argument in the generic signature - Option[String] is still Option<String> at runtime - but a Scala primitive is erased to Object, so Option[Long] and Option[Int] look the same to Jackson. On deserialization a small JSON number is read as an Integer, and unboxing it as a Long later fails. Serialization is unaffected. The FAQ covers the problem in more depth. There are three ways to name the erased type:

• Scala 3: derive ScalaTypeInfo on the class. The full type of each affected field is captured at compile time and nothing needs to be registered with the mapper:

  case class Erased(aLong: Option[Long], byId: Map[Long, String], pairs: Seq[(String, Long)]) derives ScalaTypeInfo

  A primitive is put back wherever it sits - the content of an Option or a collection, the key or the value of a Map, a slot of a tuple or an Either, the argument of a generic case class, or any nesting of those, and for a public var or a @JsonCreator companion method's parameter as much as for a constructor parameter. Nothing is captured for Option[String], which needs no help, or for a member that mentions a type parameter of the class, which Jackson resolves from the type it is asked to read.

  Derive it on an enum or on the base of a sealed hierarchy and one clause covers every case or implementation - a derives cannot be written on an enum case:

  enum Shape derives ScalaTypeInfo:
    case Circle(radius: Option[Long])
    case Dot

  A class you cannot change is described by a Jackson mix-in that derives it. A trait extending the class describes the class's own members, so none is repeated; for a final class, which no trait can extend, a class with members of the same names (not necessarily all of them) does the same:

  trait ErasedMixin extends Erased derives ScalaTypeInfo
  JsonMapper.builder().addModule(DefaultScalaModule).addMixIn(classOf[Erased], classOf[ErasedMixin])

• Annotate the field (any Scala version):

  case class OptionLong(@JsonDeserialize(contentAs = classOf[Long]) valueLong: Option[Long])

  A @JsonDeserialize on a field takes precedence over anything ScalaTypeInfo derived for it. For a class you cannot change, put the annotation on a mix-in instead:

  trait OptionLongMixin {
    @JsonDeserialize(contentAs = classOf[Long]) def valueLong: Option[Long]
  }
  JsonMapper.builder().addModule(DefaultScalaModule).addMixIn(classOf[OptionLong], classOf[OptionLongMixin])

• Register the type programmatically (any Scala version), for a class you cannot annotate:

  ScalaAnnotationIntrospectorModule.registerReferencedValueType(classOf[OptionLong], "valueLong", classOf[Long])

  A registration made this way takes precedence over one derived through ScalaTypeInfo. clearRegisteredReferencedTypes() removes them again. A module built with ScalaModule.builder() keeps its own registrations, separate from the ScalaAnnotationIntrospectorModule object.

Scala 3 enums and sealed hierarchies

A Scala 3 enum is supported by DefaultScalaModule without further configuration. A simple case is written as its name ("Red"); an enum with parameterized cases tags each value with @type:

enum Color(val rgb: Int):
  case Red extends Color(0xFF0000)
  case Mix(mix: Int) extends Color(mix)

// Color.Red      -> "Red"
// Color.Mix(4491519) -> {"@type":"Mix","mix":4491519,"rgb":4491519}

An enum annotated with @JsonTypeInfo is left to the standard Jackson handling.

A sealed hierarchy can be marked for automatic polymorphic handling - a @type property naming the implementation, without @JsonTypeInfo and @JsonSubTypes - in either of two equivalent ways:

sealed trait Animal extends SealedPolymorphismSupport   // any Scala version; needs scala-reflect on Scala 2
sealed trait Animal derives SealedSubtypes              // Scala 3; implementations captured at compile time
case class Dog(name: String) extends Animal
case object Unknown extends Animal

// Dog("rex") -> {"@type":"Dog","name":"rex"}
// Unknown    -> {"@type":"Unknown"}

The @type value is the implementation's simple name; an implementation declared inside some other object keeps that object in its name ({"@type":"Kept.Tame",...}). scala-reflect is an optional dependency of this module, needed only by Scala 2 applications that use SealedPolymorphismSupport. See the Scaladoc of SealedPolymorphismSupport and SealedSubtypes for the naming and lookup rules.

Sbt

To import in sbt:

libraryDependencies += "tools.jackson.module" %% "jackson-module-scala" % "3.1.2"

Java/Kotlin users

DefaultScalaModule is a Scala Object and to access it when you are not compiling with Scala compiler, you will need to use tools.jackson.module.scala.javadsl.DefaultScalaModule.getInstance() instead. You can access the Scala object using tools.jackson.module.scala.DefaultScalaModule$.MODULE$.

import tools.jackson.module.scala.javadsl.*;

ObjectMapper mapper = JsonMapper.builder().addModule(DefaultScalaModule.getInstance()).build();
// or ScalaModule.builder().addAllBuiltinModules().build() instead of DefaultScalaModule.getInstance()

Building

The branches often depends on SNAPSHOT versions of the core Jackson projects, which are published to the Sonatype OSS Repository. To make these dependencies available, create a file called sonatype.sbt in the same directory as build.sbt with the following content. The project .gitignore file intentionally prevents this file from being checked in.

resolvers ++= Resolver.sonatypeOssRepos("snapshots")
resolver += "Sonatype Central Snapshots" at "https://central.sonatype.com/repository/maven-snapshots"

Download, docs

Check out Wiki. API Scaladocs can be found on the project site but they are not really well suited to end users, as most classes are implementation details of the module.

Related Projects

• jackson-scala-reflect-extensions
• jackson-scala3-reflect-extensions
• jackson-module-enumeratum
• jackson-caffeine-cache

Contributing

The main mechanisms for contribution are:

• Reporting issues, suggesting improved functionality on Github issue tracker
• Participating in discussions on mailing lists, Gitter (see Jackson portal for details)
• Submitting Pull Requests (PRs) to fix issues, improve functionality.

Support

Community support

Jackson components are supported by the Jackson community through mailing lists, Gitter forum, Github issues. See Participation, Contributing for full details.

Enterprise support

Available as part of the Tidelift Subscription.

The maintainers of jackson-module-scala and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. Learn more.

Core Development Team

Currently active core developers (ones who can review, accept and merge Pull Requests) are:

• PJ Fanning (@pjfanning)

If you have questions on issues, implementation strategies, you may refer to core developers (and this is recommended if you are in doubt!), but keep in mind that these are voluntary positions: everyone is doing this because they want to, not because they are paid or contractually obligated to. This also means that time availability changes over time so getting answers may take time.

In addition, other Jackson developers with similar access (but less active) include:

• Christopher Currie (@christophercurrie) -- original author of Scala module
• Morten Kjetland (@mbknor)
• Nate Bauernfeind (@nbauernfeind)
• Tatu Saloranta (@cowtowncoder) -- main author of core Jackson components

Acknowledgements