Ankur
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37 runnable examples covering the eight feature posts on ankurm.com, verified against Jackson 3.2.1 on Temurin 21.0.5. Every output committed under docs/ was produced by run-all.sh. Also documents 11 places where the published snippets do not compile or do not behave as printed against a real Jackson 3 build - most notably that writeValueAsString(List<Base>) silently drops the polymorphic type discriminator, so the post's serialised output cannot be read back.
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Part 3 — Annotations
Post: https://ankurm.com/jackson-annotations-guide/
Every @Json* annotation is imported from com.fasterxml.jackson.annotation, even in
Jackson 3. jackson-annotations deliberately keeps the old group ID and package so a
single copy can serve Jackson 2 and Jackson 3 code on one classpath — the
migration repo's coexistence module
proves that with both majors loaded at once.
D01 — @JsonProperty and @JsonIgnore
rename : {"order_id":1001,"customer_name":"Alice"}
read back : OrderSummary[orderId=1001, customerName=Alice]
ignore : {"username":"alice"}
read back : passwordHash=null
@JsonIgnore is bidirectional — the last line shows an injected passwordHash in the
input being discarded, which is the security-relevant half people forget.
D02 — @JsonInclude and @JsonFormat
NON_NULL : {"productName":"Keyboard"}
NON_EMPTY : {"productName":"Keyboard"}
formats : {"invoiceId":500,"defaultDate":"2026-04-09","ukStyleDate":"09/04/2026","totalAmount":"199.99"}
defaultDate carries no annotation and still comes out as 2026-04-09. The post's
claim that "without @JsonFormat, Jackson writes LocalDate as a numeric array" was
true in Jackson 2; in Jackson 3 ISO-8601 is the default and the annotation is only
needed for a non-standard pattern such as ukStyleDate.
D03 — @JsonAlias and unknown fields
alias {"q":"jackson"} -> jackson
alias {"query":"jackson"} -> jackson
alias {"search_term":"jackson"} -> jackson
default mapper : LenientResponse[status=OK, message=done]
strict mapper : UnrecognizedPropertyException (as expected)
strict + anno : OptedOutResponse[status=OK, message=done]
The post frames @JsonIgnoreProperties(ignoreUnknown = true) as the per-class
alternative to disabling FAIL_ON_UNKNOWN_PROPERTIES globally. In Jackson 3 that
feature is already off, so the default mapper tolerates the extra field with no
annotation. The annotation earns its keep only on a mapper where you have deliberately
re-enabled strictness — which is the strict case above.
D04 — creators, unwrapping and catch-alls
@JsonUnwrapped appears in the post's summary table but is never demonstrated;
@JsonAnyGetter/@JsonAnySetter are not in the post at all, and are the cleanest way
to keep fields you did not model instead of silently dropping them.
creator : ImmutablePoint(x=3.5, y=7.2)
round-trip: {"x":3.5,"y":7.2}
unwrapped : {"street":"123 Main St","city":"Springfield","customerName":"Alice"}
any-setter: {surprise=1, another=[true, false]}
any-getter: {"knownField":"a","surprise":1,"another":[true,false]}