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Jackson 2 to 3 migration companion code

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Three Maven modules - jackson2-before (2.22.1), jackson3-after (3.2.1) and a
coexistence module with BOTH majors on one classpath - so every claim in the two
migration guides is executed rather than asserted. Paired class names make the
before/after outputs directly diffable via run-all.sh.

Confirms the guides on wire-format equivalence (10-case suite, zero mismatches),
classpath coexistence and the collapse of four artifacts into one. Corrects nine
points, including that enableDefaultTyping() is still present in Jackson 2.22.1
rather than removed in 2.16, and that the published "after" mapper snippet does
not compile.
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# S06 / S08 — Coexistence and wire-format equivalence
Guide: <https://ankurm.com/jackson-3-vs-jackson-2/> (FAQs)
The [`coexistence/`](../coexistence) module puts **both** major versions on one
classpath. Its [`pom.xml`](../coexistence/pom.xml) declares
`tools.jackson.core:jackson-databind:3.2.1` and
`com.fasterxml.jackson.core:jackson-databind:2.22.1` together, with no exclusions and
no shading.
## S06 — both majors in one class
[`S06BothOnOneClasspath.java`](../coexistence/src/main/java/com/ankurm/migration/coexist/S06BothOnOneClasspath.java)
```
jackson 2 class : com.fasterxml.jackson.databind.ObjectMapper
jackson 3 class : tools.jackson.databind.json.JsonMapper
jackson 2 output: {"booking_id":1,"travel_date":"2026-09-15"}
jackson 3 output: {"booking_id":1,"travel_date":"2026-09-15"}
wire-compatible : true
3 reads 2's JSON: Booking[id=1, travelDate=2026-09-15]
2 reads 3's JSON: Booking[id=1, travelDate=2026-09-15]
annotation from : jackson-annotations-2.22.jar
<- one jackson-annotations jar, used by both majors
```
One DTO, annotated once with `@JsonProperty` imported from
`com.fasterxml.jackson.annotation`, and both mappers honour it. That is precisely why
`jackson-annotations` deliberately kept the old group ID and package while everything
else moved to `tools.jackson`: it is the shared piece.
The last line resolves which jar the annotation actually came from at runtime —
a single `jackson-annotations-2.22.jar`, serving both.
One thing the guides do not mention: Jackson 3's built-in `java.time` support does not
extend to the Jackson 2 mapper sitting beside it. The coexistence module still needs
`jackson-datatype-jsr310` and `jackson-datatype-jdk8` for the Jackson 2 side. Nothing is
shared between them except the annotations.
## S08 — wire-format equivalence
[`S08WireFormatEquivalence.java`](../coexistence/src/main/java/com/ankurm/migration/coexist/S08WireFormatEquivalence.java)
The comparison post's final AI prompt asks for a cross-version regression suite. This is
that suite, written and run. Ten cases go through both mappers and the JSON strings are
compared exactly.
```
plain record identical
nested list identical
map identical
java.time identical
Optional present identical
Optional empty identical
NON_NULL identical
@JsonProperty identical
@JsonFormat identical
nulls and empties identical
cases : 10
mismatches: 0
Wire format is identical across both majors for these cases.
```
Zero mismatches. The guides' claim that the serialised form is unchanged holds for
records, nested lists, maps, `java.time`, present and empty `Optional`, `NON_NULL`
inclusion, `@JsonProperty` renaming, `@JsonFormat` patterns, and empty collections.
This matters most if your JSON is cached, signed, hashed, or consumed by a system you
do not control — those are the cases where a field-order or date-format change would be
a production incident rather than a cosmetic difference. Run this suite against your own
DTOs before switching such a service.