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.
71 lines
2.8 KiB
Markdown
71 lines
2.8 KiB
Markdown
# Part 2 — Records, Optionals and sealed types
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Post: <https://ankurm.com/jackson-java-records-optionals/>
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The single biggest simplification in Jackson 3 lands here. All three features are in
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core: the project's [`pom.xml`](../pom.xml) has one dependency, no
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`jackson-datatype-jdk8`, no `jackson-module-parameter-names`, and no `-parameters`
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compiler argument.
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## C01 — records
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[`C01RecordRoundTrip.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C01RecordRoundTrip.java)
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```
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{"productId":101,"productName":"Wireless Keyboard","unitPrice":49.99}
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Wireless Keyboard
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round-trip equal: true
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```
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The post's own dependency block here is wrong twice over: it suggests
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`com.fasterxml.jackson.module:jackson-module-parameter-names:3.1.2`, which is both an
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artifact Jackson 3 does not need and a version that does not exist at that coordinate.
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## C02 — Optional
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[`C02OptionalFields.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C02OptionalFields.java)
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```
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present : {"customerName":"Alice","middleName":"Marie"}
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empty : {"customerName":"Bob","middleName":null}
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absent : {"customerName":"Bob"}
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isPresent: true
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missing -> Optional.empty (null? false)
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```
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Two behaviours worth committing to memory: an empty `Optional` serialises as `null`
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unless you ask for `NON_ABSENT`, and a *missing* property deserialises to
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`Optional.empty()` rather than to `null`, so the field is never null.
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## C03 — sealed types with an explicit registry
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[`C03SealedWithSubTypes.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C03SealedWithSubTypes.java)
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```
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Circle with radius: 5.0
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Rectangle 10.0 x 4.0
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lossy : [{"radius":5.0},{"width":10.0,"height":4.0}]
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correct : [{"shapeType":"circle","radius":5.0},{"shapeType":"rectangle","width":10.0,"height":4.0}]
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```
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Note the `lossy` line. Serialising a `List<Shape>` drops the `shapeType` discriminator,
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because a `List` gives Jackson no element type to dispatch on. This is the same defect
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covered at length in [Part 5](part5-polymorphic.md).
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## C04 — sealed auto-discovery
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[`C04SealedAutoDiscovery.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C04SealedAutoDiscovery.java)
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Beyond the post: Jackson 3 reads the `permits` clause, so `@JsonSubTypes` can be dropped
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entirely when each permitted type carries `@JsonTypeName`. There is no `@JsonSubTypes`
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anywhere in that file, and a third subtype added later just works.
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```
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Circle -> {"shapeType":"circle","radius":5.0} -> Circle[radius=5.0]
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Rectangle -> {"shapeType":"rectangle","width":10.0,"height":4.0} -> Rectangle[width=10.0, height=4.0]
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Triangle -> {"shapeType":"triangle","base":3.0,"height":6.0} -> Triangle[base=3.0, height=6.0]
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```
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In Jackson 2 the `@JsonSubTypes` registry had to be maintained in parallel with
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`permits`, and drifted whenever someone added a case.
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