Skip to content


Subversion checkout URL

You can clone with
Download ZIP
Scala JSON library designed for easy, type-safe, and non-reflective (dis)assembly of JSON payloads
Scala Java


latest commit 9e797740ae
Robert J. Macomber authored



Getting it

Starting with version 2.0.0, rojoma-json is published on Maven central, so setting up SBT is as simple as

libraryDependencies += "com.rojoma" %% "rojoma-json-v3" % "3.3.0"

While for Maven, the pom snippet is:


rojoma-json-v3 is published for Scala version 2.10 and 2.11.


package com.rojoma.json.v3.ast

  • JValue: An AST for JSON
    • JAtom
      • JNull
      • JBoolean(boolean: Boolean)
      • JString(string: String)
      • JNumber
    • JCompound
      • JArray(toSeq: scala.collection.Seq[JValue])
      • JObject(fields: scala.collection.Map[String, JValue])

The JCompound classes extend Iterable and have convenience methods that make them act like Seq and Map respectively, but are not themselves actually Seqs or Maps. Use the toSeq, or fields or toMap, method to get a real one.

JNumber is not a case class; it is an abstract class whose implementation varies depending on how it was constructed. Use the various toX methods to access its value.

All JValues have a cast[T] method that can be used to safely downcast to a more specific type.

There is also support for "dynamically typed" access to JValues:

someJValue = j"{outer : {inner : [0,1,2,3]}}"
someJValue.dyn.outer.inner(2).?             // returns Right(JNumber(2))
someJValue.dyn.outer("inner")(2).?          // returns Right(JNumber(2))
someJValue.dyn.outer.inner(2).!             // returns JNumber(2)
someJValue.dyn.outer.nonexistant.inner(2).? // returns Left(DecodeError.MissingField("nonexistant", .outer))
someJValue.dyn.outer.nonexistant.inner(2).! // throws a NoSuchElementException

This is implemented via scala's Dynamic trait; as a result, applyDynamic, selectDynamic, and apply, plus the methods on Object, will resolve to real methods instead of path elements.

package com.rojoma.json.v3.codec

  • JsonEncode[T]: a typeclass for converting objects to JValues
  • JsonDecode[T]: a typeclass for converting objects from JValues
  • FieldEncode[T]: a typeclass for converting objects to object keys
  • FieldDecode[T]: a typeclass for converting objects from object keys

Encoding is assumed to always succeed; decoding can fail, returning a DecodeError containing the cause of the failure and the path at which the failure happened.

The following types have implicit codecs in JsonEncode's and JsonDecode's companions:

  • String
  • Boolean
  • Numeric types, including BigInt, BigDecimal, and their java.math counterparts
  • JValue and all its subclasses
  • Any S[T] <: Seq[T] if T has a JsonCodec and S has an implicit CanBuild
  • Any S[T] <: Set[T] if T has a JsonCodec and S has an implicit CanBuild
  • Any M[T, U] <: Map[T,U] if T has a FieldCodec, U has a JsonCodec and M has an implicit CanBuild
  • Either (biased on decoding to Right)
  • Unit
  • Tuples up to Tuple22
  • java.util.List[T] if T has a JsonCodec
  • java.util.Set[T] if T has a JsonCodec
  • java.util.Map[T, U] if T has a FieldCodec and U has a JsonCodec
  • Java enumerations
  • java.util.UUID

The "atomic" ones (String, Boolean, numbers, enums, UUID, and URL) also have field codecs in FieldEncode and FieldDecode.

Numeric codecs are "lenient" -- that is, if a number is out of range of the requested type, it undergoes the normal truncation BigDecimal.toXXX does. If this is not desired, request a BigDecimal and use the .toXXXExact alternatives.

The codecs' companion objects themselves can be used as values that represent the result of an implicit search. That is:

JsonEncode[T] === implicitly[JsonEncode[T]]
JsonDecode[T] === implicitly[JsonDecode[T]]
FieldEncode[T] === implicitly[FieldEncode[T]]
FieldDecode[T] === implicitly[FieldDecode[T]]

Default JsonEncodes will be as lazy as possible. Modifying the object that was encoded before serializing the resulting JValue or calling forced on it has undefined behavior.


  • JsonReader: Convert character data to JValues
  • JsonWriter: Convert JValues to character data
    • CompactJsonWriter
    • PrettyJsonWriter

The two concrete JsonWriter classes have toWriter and toString convenience methods on their companion objects. JsonReader, similarly, has fromString and fromWriter. Neither JsonReader nor the JsonWriters make any effort to minimize the number of calls to read() or write() on the IO handle they're given, so it is probably a good idea to ensure that it is buffered.

As extensions to the JSON specification, JsonReader accepts the following:

  • single-quote delimited strings
  • unquoted object keys
  • Javascript-style comments

The only limits on the sizes of strings and depth of nesting are those of the JVM. Parsing is done recursively, and so stack space is the limiting factor in nesting. Numbers are restricted to those which can fit in a Java BigDecimal. The reader does validate surrogate pairs and will replace stray halves with the Unicode REPLACEMENT_CHARACTER character. JsonReader also guarantees to read exactly as much as necessary as to read a single JValue -- i.e., only to the closing delimiter for objects, arrays, and strings, and one character past for all other types of JSON datum.

Below the JsonReader level lives the JsonEvent level, which can be used to implement streamed processing. A JsonReader consumes an Iterator[JsonEvent]; that iterator can itself be either a JsonEventIterator (which consumes an Iterator[JsonToken]) or a FusedBlockJsonEventIterator (which reads character data directly). In the former case, Iterator[JsonToken] is provided by JsonTokenIterator and BlockJsonTokenIterator. Building on top of the non-Block versions will ensure, at a performance penalty, that no more is read from the underlying source of character data than is necessary to read a complete JSON datum; the Block variants are faster at the cost of reading more than that. If you do not specify, rojoma-json will always pick the non-Block versions when reading from Readers.

package com.rojoma.json.v3.matcher

  • OptPattern: The base class of all Patterns, plus POption
    • Pattern: A specification for extracting data from JValues
      • Literal(x: JValue): match a literal value
      • FLiteral(f: JValue => Boolean): conditionally match a value
      • PArray(subpatterns: Pattern*): match an array containing values that match a series of patterns
      • PObject(subpatterns: (String, OptPattern)*): match an object containing fields that match patterns
      • FirstOf(subpatterns: Pattern*): try to match a series of patterns in turn
      • AllOf(subpatterns: OptPattern*): match a series of patterns in turn
      • Variable[T : JsonCodec]: match a value of type T
    • POption(subpattern: Pattern): Optionally match a pattern. Only valid in a PObject and AllOf.

These are probably best understood with a simple example:

val channel = Variable[String]() // Could be anything with a JsonCodec instance
val text = Variable[String]()
val ChatPattern =
  PObject("command" -> "chat",
          "to" -> channel,
          "message" -> text)
val JoinPattern =
  PObject("command" -> "join",
          "channel" -> channel)
val LeavePattern =
  PObject("command" -> "leave",
          "channel" -> channel,
          "message" -> POption(text))

def process(message: JValue) = message match {
  case ChatPattern(results) =>
    sendText(channel(results), text(results))
  case JoinPattern(results) =>
  case LeavePattern(results) =>
    departChannel(channel(results), text.get(results))
  case _ =>
    error("unknown command", message)

def leaveChannel(channelName: String, message: Option[String]): JValue =
  LeavePattern.generate(channel := channelName, text :=? message)

OptPattern's companion contains implicit conversions from various literal forms into Patterns.

Information is extracted by means of Variable patterns. Variables can be created with the apply method on the companion object. They are typed and will only succeed in matching if the value at their position is of the correct type. If a single Variable appears more than once in a Pattern, it is not an error, but all appearances must match the same value.

Any object with an implicit JsonDecode instance in scope can be automatically coerced to a literal Pattern. If there is also a JsonEncode instance, that pattern can be used as a generator.

The result of a match is an opaque object which can be given to a Variable to extract the data, either by applying the Variable like a function or calling its get method. Variables and Patterns themselves are immutable.

PObject will accepts multiple patterns for a single field; all must accept for the containing PObject to accept. If the target of a field is marked optional by wrapping it in a POption, it either must match the subpattern or not appear at all. To tolerate random unparsable data in a field, use FirstOf with a final branch that accepts anything.

In AllOf, if a field is marked with POption, the value under consideration is allowed to not match that particular subpattern. In this context, POption(p) is a shorthand for FirstOf(p, Variable[JValue]()).

Custom matchers can be defined by subclassing Pattern and implementing the method evaluate(x: JValue, environment: Pattern.Results): Either[DecodeError, Pattern.Results].

Most Patterns can also be used to generate JSON using the generate method, passing in a list of variable bindings in the form variable := value or, if the variable occurs in an "optional" position (i.e., inside a POption or FirstOf) variable :=? optValue, where optValue is an Option.

package com.rojoma.json.v3.zipper

A zipper for navigating JSON. There are five interfaces:

  • JsonZipper
    • JAtomZipper
    • JArrayZipper
    • JObjectZipper
  • NothingZipper

An array or object zipper may be acquired by calling asArray or asObject on a generic zipper.

Each of the first five allows you to move up, to the top of the object, or find the zipper's current value, replace, or remove it. In addition, the array and object zippers allow replaceing or removeing child elements. All of the motion operators (except top) return Options; suffix the operator with _! to make it return the value directly or throw a NoSuchElementException if the motion is impossible.

The NothingZipper is special -- it is what is returned from removing the current object. With a NothingZipper you can either put a new object in the hole it represents (via the replace method) or move up or to the top. Unlike the JsonZipper classes, when you have nothing top might not return anything, since it is possible that the root object is what was removed.

package com.rojoma.json.v3.jpath

The JPath class is a simple wrapper over JsonZippers for doing "xpath-style" queries on a JValue.

// Find the first name of all users that live in Tucson
new JPath(myObject).down("users").*.having(_.down("city").where( == JString("Tucson"))).down("firstName").finish

The result is a Stream[JValue]. Currently JPath is a read-only interface.

package com.rojoma.json.v3.util

Utility operations that combine parts from other packages. The main member of this package is the object JsonUtil which contains convenience methods for moving data all the way between character data and usable objects.

The package includes helpers for building JsonEncode and JsonDecode instances. For single classes, there is the macro-based AutomaticJsonCodecBuilder (also AutomaticJsonEncodeBuilder and AutomaticJsonDecodeBuilder for unidirectional conversion) which can produce instances for case-like classes automatically. There is also the older SimpleJsonCodecBuilder (with encode and decode variants) that requires the programmer to say how to (de)construct objects. They are very straightforward to use:

case class Foo(a: Int, b: Option[String])
object Foo {
  implicit val jCodec = AutomaticJsonCodecBuilder[Foo]
  // alternately: SimpleJsonCodecBuilder[Foo].build("a", _.a, "b", _.b)

These can be used to build codecs for any classes which have accessors that match up to their constructor parameters. For the Simple case, the names and accessors must be provided in the same order the constructor takes them, or you will either get an exception when build is called (because it can't find the right constructor) or you'll get a JsonCodec that doesn't roundtrip properly (if the types of the accessors just happen to line up with the types of the constructor parameters). The types of the values to be serialized must either have JsonCodecs themselves, or be Options wrapping around such types. build comes in variants that will handle up to 22 fields.

The generation for the Automatic builders can be affected by placing annotations (also defined in the util package) on the subject class's constructor's parameters. The annotations are:

  • @JsonKey("string literal") which overrides automatic selection of the field's name in the generated JSON.
  • @NullForNone which causes the field to generate a null if it is an empty Option. Ordinarily empty Options are simply omitted from generation altogether.
  • @LazyCodec which causes the codec for the field to be resolved lazily. It is ordinarily not necessary, but can be used to stop stack overflows or NullPointerExceptions building codecs for recursive data structures.
  • @JsonKeyStrategy(strategy) to override the subject class's default key generation strategy.

The two defined strategies are Strategy.Identity (the default) and Strategy.Underscore (which converts camel-case names to lower-case, underscore-separated names). The @JsonKeyStrategy annotation can also be used on the class level to set the default strategy for all fields. If two names map to the same JSON identifier (whether automatically or through use of @JsonKey) a compiler error occurs.

For hierarchies of classes, there are analagous Simple and Automatic hierarchy codec builders. The simple builders expect a chain of branches; the automatic ones derives the chain automatically. The means by which the different cases are differentiated is configurable. The options are TagToValue which produces a wrapper object shaped as {"typeTag" : "value"}, TagAndValue which produces a wrapper shaped as {"tagName" : "typeTag", "valueName" : "value"}, InternalTag which only works with values that encode as objects and which adds an additional field to them to serve as the type tag, and NoTag which on decoding simply tries all branches in the order specified until one succeeds.

Note that the AutomaticHierarchy builders can be affected by SI-7046 and SI-7588.

package com.rojoma.json.v3.interpolation

A string interpolator for building JValues in a syntactically lightweight way. The package contains interpolators json and j; they are exact synonyms.

import com.rojoma.json.v3.interpolation._

val x = "world"

// generates { "hello" : "world", "world" : "wide web" }
j"""{ "hello" : $x, $x : "wide web" }"""

// Any encodable can be used in a non-field position
case class A(x: Int)
implicit val aCodec = AutomaticJsonCodecBuilder[A]
j"""{ "a" : ${A(5)} }""" // { "a" : { "x" : 5 } }

The interpolators are implemented as macros. The well-formedness of the template is checked at compile time.

package com.rojoma.json.v3.conversions.v2

While version 3 of rojoma-json is not binary or source compatible with v2, it can exist alongside it. This package contains helpers to convert between the two versions, to assist in transitions.

The package is meant to have all its contents imported. It contains implicit conversions which add toV3 methods onto rojoma-json-2's JsonTokens, JsonEvents, and JValues and toV2 methods onto rojoma-json-3's. In addition, it can create rojoma-json-2 JsonCodecs from instances of 3's JsonEncode and JsonDecode.

Incompatible changes from rojoma-json 2

  • JNumber is no longer a case class.
  • JsonDecode returns Either[DecodeError, T] instead of Option[T].
  • Pattern#matches and Pattern#evaluate return Either[DecodeError, Pattern.Results] instead of Option[Pattern.Results].
  • JsonDiff is gone. It was cute but useless.
  • PArray now requires an exact length match.
  • The position on JsonTokens and JsonEvents is now provided in a secondary constructor parameter, rather than being a mutable field.
  • The various high-level readers prefer to use block IO instead of character-by-character IO.
Something went wrong with that request. Please try again.