Add support for handling `Map.Entry`

[cowtowncoder]

Since use of Map.Entry as type is relatively common for constructs like Iterable<Map.Entry<>> or List<Map.Entry<>>, it'd make sense to support it.
By default, serialization should probably occur as JSON Object with just one entry. Deserializer will need to do bit of verification after entry, to give more meaningful error for possible case of structural mismatch.

A potential added twist is whether to try to optimize more compact forms: specifically, Iterable (or Iterator) of Map.Entry instances could conceivably be simply serialized as JSON Object, instead of JSON Array of single-entry JSON Objects.

One possibility would be to initially support deserialization of such structure (as it should be detectable from JSON Array vs JSON Object difference); and later on maybe add support for serialization as well, using @JsonFormat to indicate difference.

[cowtowncoder]

Let's start with basics tho: simple read/write of Map.Entry as single entry JSON Object.

[cfieber]

may be worth calling this out a little more in the 2.5.x release notes - this changes the shape of a serialized collection of Map.Entry from [{'key':'key1', 'value':'value1'}] to {'key1':'value1'}

definitely think the new format is nicer, but we had a legacy api that happened to be emitting the former

[cowtowncoder]

Oh! Apologies for breaking your code. I did not realize serialization was actually working before 2.5, although in hindsight that makes complete change. Had I realized this, I would have added a SerializationFeature. That may not be very useful addition at this point.

I will definitely add a note about this, in (in)compatibility section.

Thank you for reporting this, and once again apologies for causing the breakage. This was definitely not intentional.

[cowtowncoder]

@cfieber Thank you again for pointing it out; I added a note on https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.5 to hopefully help.

[darinmanica]

Will there be a way to configure 2.5 to render maps in the pre-2.5 manner? What is the recommended upgrade path to 2.5 for a public API (that was using maps) and a substantial user base?

[cowtowncoder]

@darinmanica Rendering of Maps has not changed. What is your specific problem?

[codecracker2014]

@cowtowncoder i'm facing challenge due to this change as our js api is using key, value attributes. Can i still get earlier serialization of Map.Entry<> with jackson 2.7+ using some configuration?

[cowtowncoder]

@codecracker2014 for serialization, #865 will add alternative to "old" method, but unfortunately it'll only be in 2.9(.0); it would be by using @JsonFormat(shape=Shape.OBJECT) annotation.
Until then custom serializer is the way to go; it should be relatively easy to do that.

[ivascaflavius]

@cowtowncoder, out of curiosity, do you know by when the 2.9 stable release should be available? I see 2.9-pr3 is already available, and the original release date for 2.9 was originally intended to be April 2017.

[cowtowncoder]

@ivascaflavius Unfortunately I do not know, but fwtw I hope pr3 was the last pre-release. If all went well (... rarely does) I would estimate end of May, but more likely some time in June. The big missing piece that I still want to get done is non-blocking ("async") parsing for json; otherwise almost everything else remaining is optional.

[ivascaflavius]

@cowtowncoder
Ok, thanks! I have another question.

If we upgrade to version 2.8.x, in order to support backwards compatibility with data that was generated with pre 2.5 for collection of Map.Entry, you mentioned implementing a custom serializer for Map.Entry is the way to go. That is easy to do, and we were able to do that, however what we are struggling with is implementing a deserializer to do the same, specially given that we may have a convoluted object (the value in a Map.Entry may be another map of objects for example etc.).

Can you point us to an easy solution on implementing the deserializer for Map.Entry (by falling back to / reusing other serializers?) or any other way of maintaining the old (pre 2.5) behaviour, if we upgrade to Jackson 2.8.x?

Thanks!

[pedro93]

Hi,

Assuming the current newest version of jackson (2.90-pr4), how could I set an ObjectMapper object with this serialization/deserialization property to allow backwards-compatibility with 2.4.4?

[cowtowncoder]

@pedro93 You can use:

    @JsonFormat(shape=JsonFormat.Shape.OBJECT)

annotation on property, to serialize "as POJO" (i.e. with 2 fields); and:

@JsonFormat(shape=JsonFormat.Shape.NATURAL)

to serialize using new compact notation.

And to set this as the default, you should be able to use:

        mapper.configOverride(Map.Entry.class)
            .setFormat(JsonFormat.Value.forShape(JsonFormat.Shape.OBJECT)

as "per-type config override".