Ankur
c438afc33b
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.
75 lines
3.1 KiB
Markdown
75 lines
3.1 KiB
Markdown
# Part 3 — Annotations
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Post: <https://ankurm.com/jackson-annotations-guide/>
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Every `@Json*` annotation is imported from `com.fasterxml.jackson.annotation`, even in
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Jackson 3. `jackson-annotations` deliberately keeps the old group ID and package so a
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single copy can serve Jackson 2 and Jackson 3 code on one classpath — the
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[migration repo's coexistence module](https://ankurm.com/git.app/asmhatre/jackson2-to-3-migration)
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proves that with both majors loaded at once.
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## D01 — @JsonProperty and @JsonIgnore
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[`D01RenameAndIgnore.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D01RenameAndIgnore.java)
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```
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rename : {"order_id":1001,"customer_name":"Alice"}
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read back : OrderSummary[orderId=1001, customerName=Alice]
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ignore : {"username":"alice"}
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read back : passwordHash=null
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```
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`@JsonIgnore` is bidirectional — the last line shows an injected `passwordHash` in the
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input being discarded, which is the security-relevant half people forget.
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## D02 — @JsonInclude and @JsonFormat
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[`D02InclusionAndFormat.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D02InclusionAndFormat.java)
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```
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NON_NULL : {"productName":"Keyboard"}
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NON_EMPTY : {"productName":"Keyboard"}
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formats : {"invoiceId":500,"defaultDate":"2026-04-09","ukStyleDate":"09/04/2026","totalAmount":"199.99"}
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```
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`defaultDate` carries no annotation and still comes out as `2026-04-09`. The post's
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claim that "without `@JsonFormat`, Jackson writes `LocalDate` as a numeric array" was
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true in Jackson 2; in Jackson 3 ISO-8601 is the default and the annotation is only
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needed for a non-standard pattern such as `ukStyleDate`.
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## D03 — @JsonAlias and unknown fields
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[`D03AliasAndUnknownFields.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D03AliasAndUnknownFields.java)
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```
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alias {"q":"jackson"} -> jackson
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alias {"query":"jackson"} -> jackson
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alias {"search_term":"jackson"} -> jackson
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default mapper : LenientResponse[status=OK, message=done]
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strict mapper : UnrecognizedPropertyException (as expected)
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strict + anno : OptedOutResponse[status=OK, message=done]
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```
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The post frames `@JsonIgnoreProperties(ignoreUnknown = true)` as the per-class
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alternative to disabling `FAIL_ON_UNKNOWN_PROPERTIES` globally. In Jackson 3 that
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feature is already off, so the default mapper tolerates the extra field with no
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annotation. The annotation earns its keep only on a mapper where you have deliberately
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re-enabled strictness — which is the `strict` case above.
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## D04 — creators, unwrapping and catch-alls
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[`D04CreatorsAndUnwrapping.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D04CreatorsAndUnwrapping.java)
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`@JsonUnwrapped` appears in the post's summary table but is never demonstrated;
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`@JsonAnyGetter`/`@JsonAnySetter` are not in the post at all, and are the cleanest way
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to keep fields you did not model instead of silently dropping them.
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```
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creator : ImmutablePoint(x=3.5, y=7.2)
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round-trip: {"x":3.5,"y":7.2}
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unwrapped : {"street":"123 Main St","city":"Springfield","customerName":"Alice"}
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any-setter: {surprise=1, another=[true, false]}
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any-getter: {"knownField":"a","surprise":1,"another":[true,false]}
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```
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