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.
73 lines
3.2 KiB
Markdown
73 lines
3.2 KiB
Markdown
# Part 4 — Custom serialisers, deserialisers and mix-ins
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Post: <https://ankurm.com/jackson-custom-serializer-mixin/>
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This is the part with the most API churn. The post's code is Jackson 2 throughout.
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| Jackson 2 | Jackson 3 |
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|---|---|
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| `JsonSerializer<T>` | `ValueSerializer<T>` |
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| `JsonDeserializer<T>` | `ValueDeserializer<T>` |
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| `SerializerProvider` | `SerializationContext` |
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| `com.fasterxml.jackson.databind.ser.std.StdSerializer` | `tools.jackson.databind.ser.std.StdSerializer` |
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| `gen.writeNumberField` / `writeStringField` | `gen.writeNumberProperty` / `writeStringProperty` |
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| `p.getCodec().readTree(p)` | `ctxt.readTree(p)` |
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| `throws IOException` | removed — exceptions are unchecked |
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| `new SimpleModule(name, Version)` | `new SimpleModule(name)` |
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| `mapper.registerModule(m)` | `builder.addModule(m)` |
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| `mapper.addMixIn(a, b)` | `builder.addMixIn(a, b)` |
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## E01 / E02 — the handlers
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[`E01MoneyValueSerializer.java`](../src/main/java/com/ankurm/jackson3/part4custom/E01MoneyValueSerializer.java) ·
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[`E02MoneyValueDeserializer.java`](../src/main/java/com/ankurm/jackson3/part4custom/E02MoneyValueDeserializer.java) ·
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[`Money.java`](../src/main/java/com/ankurm/jackson3/part4custom/Money.java)
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The deserialiser uses `path()` rather than `get()`. The post's version calls
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`rootNode.get("amount").decimalValue()`, which throws `NullPointerException` on
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`{"currency":"USD"}`. There is a second trap: in Jackson 3, a bare `decimalValue()` on
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a `MissingNode` throws, where Jackson 2 returned `BigDecimal.ZERO`. The defaulting
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overload `decimalValue(BigDecimal.ZERO)` is what you want.
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## E03 — registration
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[`E03SimpleModuleRegistration.java`](../src/main/java/com/ankurm/jackson3/part4custom/E03SimpleModuleRegistration.java)
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```
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serialised : {"amount":20.00,"currency":"USD"}
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amount : 20.0 (scale lost)
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amount exact : 20.00 (scale preserved)
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missing field: Money[amount=0, currencyCode=EUR]
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no module : {"amount":19.999,"currencyCode":"usd"}
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```
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The post prints `20.00` for the deserialised amount. You get `20.0` — Jackson parses the
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literal as a double before handing it over, so the scale is gone. If scale matters, and
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for money it does, enable `USE_BIG_DECIMAL_FOR_FLOATS`, as the third line shows.
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## E04 — mix-ins
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[`E04MixinAnnotations.java`](../src/main/java/com/ankurm/jackson3/part4custom/E04MixinAnnotations.java)
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```
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with mixin : {"city":"Springfield","street":"123 Main St","zip":"12345"}
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without mixin: {"city":"Springfield","internalTrackingCode":"INTERNAL-X99","postalCode":"12345","street":"123 Main St"}
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```
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The second line is the proof that nothing was modified at the bytecode level: a mapper
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built without the mix-in still sees `internalTrackingCode` and `postalCode`.
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## E05 — ValueSerializer directly
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[`E05ValueSerializerDirect.java`](../src/main/java/com/ankurm/jackson3/part4custom/E05ValueSerializerDirect.java)
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Beyond the post. `StdSerializer` kept its name, which hides the rename; extending
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`ValueSerializer` directly makes it obvious, and is the leaner form for a simple wrapper.
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```
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custom : {"assignee":"u-42","title":"Fix build"}
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default : {"assignee":{"value":"u-42"},"title":"Fix build"}
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base class: tools.jackson.databind.ValueSerializer
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```
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