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.
61 lines
2.6 KiB
Markdown
61 lines
2.6 KiB
Markdown
# Part 0 — Jackson 101: setup and the ObjectMapper lifecycle
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Post: <https://ankurm.com/jackson-java-tutorial/>
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## A01 — first round-trip
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[`A01FirstRoundTrip.java`](../src/main/java/com/ankurm/jackson3/part0setup/A01FirstRoundTrip.java)
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A plain POJO out to JSON and back, using `JsonMapper.builder().build()` in place of
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`new ObjectMapper()`. Note there is no `throws` clause: Jackson 3 exceptions are unchecked.
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```
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{"listPrice":79.99,"productId":1,"productName":"Mechanical Keyboard"}
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Mechanical Keyboard
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```
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The property order is **alphabetical**, not declaration order — the post shows
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`productId` first. That is not a Jackson 3 change; it is how getter-based POJOs have
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always been introspected. Records behave differently. See
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[Part 1 / B04](part1-objectmapper.md#b04--property-ordering).
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## A02 — the shared mapper
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[`A02SharedMapperConfiguration.java`](../src/main/java/com/ankurm/jackson3/part0setup/A02SharedMapperConfiguration.java)
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The post's central rule — build once, share everywhere — is enforced by the API in
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Jackson 3 rather than left to discipline. Three of the four lines people copy from
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Jackson 2 configuration are unnecessary or invalid here:
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| Jackson 2 line | Status in Jackson 3 |
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|---|---|
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| `.registerModule(new JavaTimeModule())` | Delete. Built in; the class does not exist under `tools.jackson`. |
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| `.registerModule(new Jdk8Module())` | Delete. Same. |
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| `.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)` | **Does not compile.** Moved to `DateTimeFeature`, and already off. |
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| `.configure(FAIL_ON_UNKNOWN_PROPERTIES, false)` | Redundant — already the default. |
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| `.setSerializationInclusion(NON_NULL)` | Replaced by `.changeDefaultPropertyInclusion(...)`. |
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```
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configured : {"invoiceId":500,"customerName":"Alice","issuedOn":"2026-04-09"}
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defaults : {"invoiceId":500,"customerName":"Alice","issuedOn":"2026-04-09","note":null}
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ObjectMapper set*() methods in Jackson 3: 0
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```
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The last line is the point: `tools.jackson.databind.ObjectMapper` has **zero** `set*`
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methods. Reconfiguring a shared mapper is not discouraged in Jackson 3, it is impossible.
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## A03 — the three processing models
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[`A03ThreeProcessingModels.java`](../src/main/java/com/ankurm/jackson3/part0setup/A03ThreeProcessingModels.java)
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The same payload read by data binding, the tree model and the streaming parser.
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```
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1. data binding : Order[orderId=1001, status=SHIPPED]
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2. tree model : orderId=1001 status=SHIPPED
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3. streaming : orderId=1001 status=SHIPPED
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```
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For what each approach costs on a real file, see
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[Part 6 / G04](part6-streaming.md#g04--the-decision-table-measured).
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