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.
62 lines
2.5 KiB
Markdown
62 lines
2.5 KiB
Markdown
# S07 — Defaults that flipped silently
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Guide: <https://ankurm.com/jackson-3-vs-jackson-2/> ("Removed and Changed Default Features")
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[`before/S07DefaultsThatFlipped.java`](../jackson2-before/src/main/java/com/ankurm/migration/before/S07DefaultsThatFlipped.java) ·
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[`after/S07DefaultsThatFlipped.java`](../jackson3-after/src/main/java/com/ankurm/migration/after/S07DefaultsThatFlipped.java)
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These are the expensive ones. No compile error, no warning — behaviour just changes.
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The two programs run identical probes so the diff is the migration surface:
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```bash
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diff docs/output/before-S07DefaultsThatFlipped.txt \
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docs/output/after-S07DefaultsThatFlipped.txt
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```
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## Output
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**Jackson 2 — `jackson2-before`**
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```
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FAIL_ON_TRAILING_TOKENS : false
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FAIL_ON_UNKNOWN_PROPERTIES : true
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ALLOW_FINAL_FIELDS_AS_MUTATORS: true
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DEFAULT_VIEW_INCLUSION : true
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AUTO_DETECT_CREATORS exists : true
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concatenated JSON -> accepted: OrderDto[orderId=1]
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trailing garbage -> accepted: OrderDto[orderId=1]
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unknown property -> rejected: UnrecognizedPropertyException
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```
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**Jackson 3 — `jackson3-after`**
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```
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FAIL_ON_TRAILING_TOKENS : true
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FAIL_ON_UNKNOWN_PROPERTIES : false
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ALLOW_FINAL_FIELDS_AS_MUTATORS: false
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DEFAULT_VIEW_INCLUSION : false
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AUTO_DETECT_CREATORS exists : false
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concatenated JSON -> rejected: MismatchedInputException
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trailing garbage -> rejected: StreamReadException
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unknown property -> accepted: OrderDto[orderId=1]
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```
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| Setting | Jackson 2 | Jackson 3 | Effect |
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|---|---|---|---|
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| `FAIL_ON_TRAILING_TOKENS` | off | **on** | Concatenated or double-encoded JSON that used to parse now throws |
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| `FAIL_ON_UNKNOWN_PROPERTIES` | **on** | off | Unknown fields are tolerated by default; strictness must be opted into |
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| `ALLOW_FINAL_FIELDS_AS_MUTATORS` | on | **off** | Jackson no longer overwrites `final` fields by reflection |
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| `DEFAULT_VIEW_INCLUSION` | on | **off** | Review any `@JsonView` usage |
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| `AUTO_DETECT_CREATORS` | present | **gone** | Constant removed — but see below |
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The `FAIL_ON_UNKNOWN_PROPERTIES` flip is worth dwelling on because it moves in the
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*lenient* direction. Code that relied on a strict mapper to reject malformed payloads
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silently stops rejecting them after the upgrade. If you were using deserialisation
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failure as input validation, re-enable it explicitly.
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On `AUTO_DETECT_CREATORS`: the constant is gone, but the behaviour it governed is not.
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Single-argument constructors are still detected as delegating creators — demonstrated in
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the [features repo](https://ankurm.com/git.app/asmhatre/jackson3-by-example).
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