Hi,
The way to layer a cake like this is as follows:
[ class ] <- describes the general structure of the
type (the types, names, and order of the parameter lists)
[ instance ] <- describes the specific structure of the
instance (the values)
[ structure ] <- converts to/from structure to instance (think
constructor / deconstructor pairs)
[ specific format ] <- converts to/from structure to specific format (think
MyCompany JSON Order format version 5)
[ general format] <- parses / generates the general format (think XML /
JSON / CSV / etc)
[ IO ] <- reads / writes the general format
(think file / socket / etc)
As for the question how to represent a Timestamp or similar, is answered in the
[specific format] layer of this cake. Why? Because the next version of the
specification of that layer might change how timestamps should be represented
at that layer.
Now, while it is possible to short-circuit some of the layers of the cake by
embedding specific format decisions at the class level, it means that there's
now a tight (and somewhat exclusive) link between a specific format and class,
with the implication that you cannot output the same instance as both XML and
JSON (as it embeds the decisions of a specific json schema. If that was the
default, and only, way to go about this, what specific format should a library
decide to adhere to?
The cake layering at the beginning of this email allows types to be used by
many different specific formats and even general formats, such that you could
read something from JSON-over-HTTPS and output it as XML-over-SFTP.
Cheers,
√
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: core-libs-dev <core-libs-dev-r...@openjdk.org> on behalf of Cay Horstmann
<cay.horstm...@gmail.com>
Sent: Wednesday, 21 May 2025 09:55
To: core-libs-dev@openjdk.org <core-libs-dev@openjdk.org>
Subject: Re: Towards a JSON API for the JDK
Data binding is indeed complex because real-life JSON is messy. I reviewed some
code in which I use Jackson data binding. Here is an example of a sticky issue.
I have Content that's either a list of strings or a list of string pairs (don't
ask). The JSON is
{ "strings": [..., ..., ... ] }
or
{ "pairs": [ { "first", ..., "second", ... }, ..., ... ]
The JavaScript client relies on the fact that exactly one of "strings" and
"pairs" exists. No, I can't change the JSON.
I deserialize into a record Content(List<String> strings, List<Pair<String>>
pairs) {}. And tweak Jackson to not to serialize entries with null values. It's
not pretty, but it is easier than doing tree navigation, since this sits deeply
in an otherwise fairly regular structure.
Could that work with Serialization 2.0? I suppose. Looking at
https://www.youtube.com/watch?v=mIbA2ymCWDs, a marshaller can't dynamically
produce either a List<String> or a List<Pair<String>>, so it would offer both
in the Marshalled<Content> object, with one of them being null. I would need to
tell the wire format generator to write the non-null one. Which is ok--that's
what I do now with Jackson.
Here is another common issue. You have a record TimedPoint(int x, int y,
Instant when) {}. How do you want to serialize the Instant? As an ISO 8601
string? Millis since the epoch? Who makes that choice? Slide 29 says "Let the
class author be in charge". WHICH class author? Can java.time.Instant make a
universal choice, for all possible wire formats? Surely not. Can TimedPoint?
Maybe. Or it is the job of the wire format generator to do that with
Marshalled<TimedPoint>?.
So here is my point. If the JDK were to include a JSON data mapper, that data
mapper is either rigid or flexible. Designing a flexible data mapper is hard.
Serialization 2.0 would not make it any easier.
My hunch is that flexible JSON data mapping is hard to do within the scope of
the JDK. Would it follow Jakarta JSON Binding and be tied to that spec? Or
strike out on its own? Neither seems attractive.
Ethan and Rémi have presented rigid JSON mappers. They handle nothing but
numbers, strings, booleans, maps, lists, records. Is that useful? Surely, for
simple programs. For that, there is no reason to wait for Serialization 2.0. If
and when that comes, it would be a natural extension.
Cheers,
Cay
Il 20/05/25 21:46, Paul Sandoz ha scritto:
> Data binding is a complex feature, even if some examples make it appear
> simple. Our intention is to explore alignment with the serialization 2.0
> effort, when we are ready to so. Hence, I would urge folks to be patience and
> watch out Brian and Viktor’s Devoxx 2024 talk on the topic.
>
> Paul.
>
>> On May 20, 2025, at 12:21 PM, Swaranga Sarma <sarma.swara...@gmail.com>
>> wrote:
>>
>>> A potential advantage that we (the OpenJDK community) can more easily do is
>>> devise an API that resonates with direction the Java platform is heading,
>>> specifically around the pattern matching and serialization 2.0
>>
>> Right, but most of the discussion here seems to be on the low level tree API
>> and how one might navigate it (perhaps using pattern matching). I am not
>> seeing any discussion on the serialization/deserialization or any reasoning
>> why that is not the goal of the new JEP. Basically a very focused and narrow
>> API:
>>
>> ```
>> var jsonStr = "..."
>> var myRecord = Json.deserialize(jsonString, UserRecord.class); //and a few
>> other variants
>> var serialized = Json.serialize(myRecord)
>> ```
>>
>> This leaves the tree API open to reimplementation later when pattern
>> matching and other features are firmly in place in the JDK. I am making a
>> case that this would be more immediately useful for the most common and
>> simpler cases.
>>
>>
>> Regards
>> Swaranga
>
--
Cay S. Horstmann | https://horstmann.com
