# Part 4 — Custom serialisers, deserialisers and mix-ins

Post: <https://ankurm.com/jackson-custom-serializer-mixin/>

This is the part with the most API churn. The post's code is Jackson 2 throughout.

| Jackson 2 | Jackson 3 |
|---|---|
| `JsonSerializer<T>` | `ValueSerializer<T>` |
| `JsonDeserializer<T>` | `ValueDeserializer<T>` |
| `SerializerProvider` | `SerializationContext` |
| `com.fasterxml.jackson.databind.ser.std.StdSerializer` | `tools.jackson.databind.ser.std.StdSerializer` |
| `gen.writeNumberField` / `writeStringField` | `gen.writeNumberProperty` / `writeStringProperty` |
| `p.getCodec().readTree(p)` | `ctxt.readTree(p)` |
| `throws IOException` | removed — exceptions are unchecked |
| `new SimpleModule(name, Version)` | `new SimpleModule(name)` |
| `mapper.registerModule(m)` | `builder.addModule(m)` |
| `mapper.addMixIn(a, b)` | `builder.addMixIn(a, b)` |

## E01 / E02 — the handlers

[`E01MoneyValueSerializer.java`](../src/main/java/com/ankurm/jackson3/part4custom/E01MoneyValueSerializer.java) ·
[`E02MoneyValueDeserializer.java`](../src/main/java/com/ankurm/jackson3/part4custom/E02MoneyValueDeserializer.java) ·
[`Money.java`](../src/main/java/com/ankurm/jackson3/part4custom/Money.java)

The deserialiser uses `path()` rather than `get()`. The post's version calls
`rootNode.get("amount").decimalValue()`, which throws `NullPointerException` on
`{"currency":"USD"}`. There is a second trap: in Jackson 3, a bare `decimalValue()` on
a `MissingNode` throws, where Jackson 2 returned `BigDecimal.ZERO`. The defaulting
overload `decimalValue(BigDecimal.ZERO)` is what you want.

## E03 — registration

[`E03SimpleModuleRegistration.java`](../src/main/java/com/ankurm/jackson3/part4custom/E03SimpleModuleRegistration.java)

```
serialised   : {"amount":20.00,"currency":"USD"}
amount       : 20.0  (scale lost)
amount exact : 20.00  (scale preserved)
missing field: Money[amount=0, currencyCode=EUR]
no module    : {"amount":19.999,"currencyCode":"usd"}
```

The post prints `20.00` for the deserialised amount. You get `20.0` — Jackson parses the
literal as a double before handing it over, so the scale is gone. If scale matters, and
for money it does, enable `USE_BIG_DECIMAL_FOR_FLOATS`, as the third line shows.

## E04 — mix-ins

[`E04MixinAnnotations.java`](../src/main/java/com/ankurm/jackson3/part4custom/E04MixinAnnotations.java)

```
with mixin   : {"city":"Springfield","street":"123 Main St","zip":"12345"}
without mixin: {"city":"Springfield","internalTrackingCode":"INTERNAL-X99","postalCode":"12345","street":"123 Main St"}
```

The second line is the proof that nothing was modified at the bytecode level: a mapper
built without the mix-in still sees `internalTrackingCode` and `postalCode`.

## E05 — ValueSerializer directly

[`E05ValueSerializerDirect.java`](../src/main/java/com/ankurm/jackson3/part4custom/E05ValueSerializerDirect.java)

Beyond the post. `StdSerializer` kept its name, which hides the rename; extending
`ValueSerializer` directly makes it obvious, and is the leaner form for a simple wrapper.

```
custom  : {"assignee":"u-42","title":"Fix build"}
default : {"assignee":{"value":"u-42"},"title":"Fix build"}
base class: tools.jackson.databind.ValueSerializer
```