Towards a JSON API for the JDK

[Cay Horstmann]

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.swaranga at 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