# 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](https://ankurm.com/git.app/asmhatre/jackson2-to-3-migration)
proves that with both majors loaded at once.

## D01 — @JsonProperty and @JsonIgnore

[`D01RenameAndIgnore.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D01RenameAndIgnore.java)

```
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

[`D02InclusionAndFormat.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D02InclusionAndFormat.java)

```
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

[`D03AliasAndUnknownFields.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D03AliasAndUnknownFields.java)

```
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

[`D04CreatorsAndUnwrapping.java`](../src/main/java/com/ankurm/jackson3/part3annotations/D04CreatorsAndUnwrapping.java)

`@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]}
```