# S03 — Custom serialiser and deserialiser renames

Guide: <https://ankurm.com/jackson-3-vs-jackson-2/> ("Custom Serializers and Deserializers: Class Renames")

[`before/S03CustomHandlers.java`](../jackson2-before/src/main/java/com/ankurm/migration/before/S03CustomHandlers.java) ·
[`after/S03CustomHandlers.java`](../jackson3-after/src/main/java/com/ankurm/migration/after/S03CustomHandlers.java)

If your project has custom handlers, this is where the upgrade time goes. Every item
below is a compile error until fixed.

| Jackson 2 | Jackson 3 |
|---|---|
| `JsonSerializer<T>` | `ValueSerializer<T>` |
| `JsonDeserializer<T>` | `ValueDeserializer<T>` |
| `SerializerProvider` | `SerializationContext` |
| `throws IOException` | remove — unchecked now |
| `gen.writeNumberField(...)` | `gen.writeNumberProperty(...)` |
| `gen.writeStringField(...)` | `gen.writeStringProperty(...)` |
| `p.getCodec().readTree(p)` | `ctxt.readTree(p)` |
| `new SimpleModule(name, Version)` | `new SimpleModule(name)` |
| `mapper.registerModule(m)` | `builder.addModule(m)` |

## Output

**Jackson 2 — `jackson2-before`**

```
serialised   : {"amount":19.99,"currency":"USD"}
deserialised : Money[amount=20.0, currencyCode=USD]
base classes : com.fasterxml.jackson.databind.JsonSerializer / com.fasterxml.jackson.databind.JsonDeserializer
```

**Jackson 3 — `jackson3-after`**

```
serialised   : {"amount":19.99,"currency":"USD"}
deserialised : Money[amount=20.0, currencyCode=USD]
base classes : tools.jackson.databind.ValueSerializer / tools.jackson.databind.ValueDeserializer
```

Identical JSON on both sides; only the base class names differ. That is the useful
result — this is a mechanical rename, not a behavioural change, so it is safe to do in
bulk with OpenRewrite and review as a diff.