Towards a JSON API for the JDK

Thu May 15 21:27:57 UTC 2025

Hi Paul,
yes, not having a simple JSON API in Java is an issue for beginners.

It's not clear to me why JsonArray (for example) has to be an interface instead of a record ?

I understand why Json.parse() only works on String and char[] but the API make it too easy to have many performance issues.
I think you need versions using a Reader and a Path.
Bonus point, if there is a method walk() that also returns a JsonValue but the List/Map inside JsonArray/JsonObject are populated lazily.

Minor point: Json.toDisplayString() should takes a second parameters indicating the number of spaces used for the indentation (like JSON.stringify in JS).

regards,
Rémi

----- Original Message -----
> From: "Paul Sandoz" <paul.sandoz at oracle.com>
> To: "core-libs-dev" <core-libs-dev at openjdk.org>
> Sent: Thursday, May 15, 2025 10:30:42 PM
> Subject: Towards a JSON API for the JDK

> Hi,
> 
> We would like to share with you our thoughts and plans towards a JSON API for
> the JDK.
> Please see the document below.
> 
> -
> 
> We have had the pleasure of using a clone of this API in some experiments we are
> conducting with
> ONNX and code reflection [1]. Using the API we were able to quickly write code
> to ingest and convert
> a JSON document representing ONNX operation schema into instances of records
> modeling the schema
> (see here [2]).
> 
> The overall out-of-box experience with such a minimal "batteries included” API
> has so far been positive.
> 
> Thanks,
> Paul.
> 
> [1] https://openjdk.org/projects/babylon/
> [2]
> https://github.com/openjdk/babylon/blob/code-reflection/cr-examples/onnx/opgen/src/main/java/oracle/code/onnx/opgen/OpSchemaParser.java#L87
> 
> # Towards a JSON API for the JDK
> 
> One of the most common requests for the JDK is an API for parsing and generating
> JSON. While JSON originated as a text-based serialization format for JSON
> objects ("JSON" stands for "JavaScript Object Notation"), because of its simple
> and flexible syntax, it eventually found use outside the JavaScript ecosystem as
> a general data interchange format, such as framework configuration files and web
> service requests/response formats.
> 
> While the JDK cannot, and should not, provide libraries for every conceivable
> file format or protocol, the JDK philosophy is one of "batteries included",
> which is to say we should be able to write basic programs that use common
> protocols such as HTTP, without having to appeal to third party libraries.
> The Java ecosystem already has plenty of JSON libraries, so inclusion in
> the JDK is largely meant to be a convenience, rather than needing to be the "one
> true" JSON library to meet the needs of all users. Users with specific needs
> are always free to select one of the existing third-party libraries.
> 
> ## Goals and requirements
> 
> Our primary goal is that the library be simple to use for parsing, traversing,
> and generating conformant JSON documents. Advanced features, such as data
> binding or path-based traversal should be possible to implement as layered
> features, but for simplicity are not included in the core API. We adopt a goal
> that the performance should be "good enough", but where performance
> considerations conflict with simplicity and usability, we will choose in favor
> of the latter.
> 
> ## API design approach
> 
> The description of JSON at `https:://json.org` describes a JSON document using
> the familiar "railroad diagram":
> ![image](https://www.json.org/img/value.png)
> 
> This diagram describes an algebraic data type (a sum of products), which we
> model directly with a set of Java interfaces:
> 
> ```
> interface JsonValue { }
> interface JsonArray extends JsonValue { List<JsonValue> values(); }
> interface JsonObject extends JsonValue { Map<String, JsonValue> members(); }
> interface JsonNumber extends JsonValue { Number toNumber(); }
> interface JsonString extends JsonValue { String value(); }
> interface JsonBoolean extends JsonValue  { boolean value(); }
> interface JsonNull extends JsonValue { }
> ```
> 
> These interfaces have (hidden) companion implementation classes that admit
> greater flexibility of implementation than modeling them directly with records
> would permit.
> Further, these interfaces are unsealed. We compromise on the sealed sum of
> products to enable
> alternative implementations, for example to support alternative formats that
> encode the same information in a JSON document but in a more efficient form than
> text.
> 
> The API has static methods for parsing strings into a `JsonValue`, conversion to
> and from purely untyped representations (lists and maps), and factory methods
> for building JSON documents. We apply composition consistently, e.g, a
> JsonString has a string, a JsonObject has a map of string to JsonValue, as
> opposed to extension for structural JSON values.
> 
> It turns out that this simple API is almost all we need for traversal. It gives
> us an immutable representation of a document, and we can use pattern matching to
> answer the myriad questions that will come up (Does this object have key X? Does
> it map to a number? Is that number representable as an integer?) when going
> from an untyped format like JSON to a more strongly typed domain model.
> Given a simple document like:
> 
> ```
>    {
>        "name": "John”,
>        "age": 30
>    }
> ```
> 
> we can parse and traverse the document as follows:
> 
> ```
> JsonValue doc = Json.parse(inputString);
> if (doc instanceof JsonObject o
>    && o.members().get("name") instanceof JsonString s
>    && s.value() instanceof String name
>    && o.members().get("age") instanceof JsonNumber n
>    && n.toNumber() instanceof Long l && l instanceof int age) {
>            // use "name" and "age"
>        }
> ```
> 
> Later, when the language acquires the ability to expose deconstruction patterns
> for arbitrary interfaces (similar to today's record patterns, see
> https://openjdk.org/projects/amber/design-notes/patterns/towards-member-patterns),
> this will be simplifiable to:
> 
> ```
> JsonValue doc = Json.parse(inputString);
> if (doc instanceof JsonObject(var members)
>    && members.get("name") instanceof JsonString(String name)
>    && members.get("age") instanceof JsonNumber(int age)) {
>            // use "name" and "age"
>        }
> ```
> 
> So, overtime, as more pattern matching features are introduced we anticipate
> improved use of the API. This is a primary reason why the API is so minimal.
> Convenience methods we add today, such as a method that accesses a JSON
> object component as say a JSON string or throws an exception, will become
> redundant in the future.
> 
> ## JSON numbers
> 
> The specification of JSON number makes no explicit distinction between integral
> and decimal numbers, nor specifies limits on the size of those numbers.
> This is a common source of interoperability issues when consuming JSON
> documents. Generally users cannot always but often do assume JSON numbers are
> parsable, without loss of precision, to IEEE double-precision floating point
> numbers or 32-bit signed integers.
> 
> In this respect the API provides three means to operate on the JSON number,
> giving the user full control:
> 
> 1. Underlying string representation can be obtained, if preserving syntactic
>   details such as leading or trailing zeros is important.
> 2. The string representation can be parsed to an instance of `BigDecimal`, using
>   `toBigDecimal` if preserving decimal numbers is important.
> 3. The string representation can be parsed into an instance of `Long`, `Double`,
>   `BigInteger`, or `BigDecimal`, using `toNumber`. The result of this method
>   depends on how the representation can be parsed, possibly losing precision,
>   choosing a suitably convenient numeric type that can then be pattern
>   matched on.
> 
> Primitive pattern matching will help as will further pattern matching features
> enabling the user to partially match.
> 
> ## Prototype implementation
> 
> The prototype implementation is currently located into the JDK sandbox
> repository
> under the `json` branch, see
> here
> https://github.com/openjdk/jdk-sandbox/tree/json/src/java.base/share/classes/java/util/json
> The prototype API javadoc generated from the repository is also available at
> https://cr.openjdk.org/~naoto/json/javadoc/api/java.base/java/util/json/package-summary.html
> 
> ### Testing and conformance
> 
> The prototype implementation passes all conformance test cases but two,
> available
> on https://github.com/nst/JSONTestSuite. The two exceptions are the ones which
> the
> prototype specifically prohibits, i.e, duplicated names in JSON objects
> (https://cr.openjdk.org/~naoto/json/conformance/results/parsing.html#35).
> 
> ### Performance
> 
> Our main focus so far has been on the API design and a functional
> implementation.
> Hence, there has been less focus on performance even though we know there are a
> number of performance enhancements we can make eventually.
> We are reasonably happy with the current performance. The
> implementation performs well when compared to other JSON implementations
> parsing from string instances and traversing documents.
> 
> An example of where we may choose simplicity and usability over performance
> is the rejection of JSON documents containing objects that in turn contain
> members
> with duplicate names. That may increase the cost of parsing, but simplifies the
> user
> experience for the majority of cases since if we reasonably assume JsonObjects
> are
> map-like, what should the user do with such members, pick one the last one?
> merge
> the values? or reject?
> 
> ## A JSON JEP?
> 
> We plan to draft JEP when we are ready. Attentive readers will observe that
> a JEP already exists, JEP 198: Light-Weight JSON API
> (https://openjdk.org/jeps/198). We will
> either update this JEP, or withdraw it and draft a new one.