Ankur
f0a7053fc9
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.
3.0 KiB
S06 / S08 — Coexistence and wire-format equivalence
Guide: https://ankurm.com/jackson-3-vs-jackson-2/ (FAQs)
The coexistence/ module puts both major versions on one
classpath. Its 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
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
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.