# Part 0 — Jackson 101: setup and the ObjectMapper lifecycle

Post: <https://ankurm.com/jackson-java-tutorial/>

## A01 — first round-trip

[`A01FirstRoundTrip.java`](../src/main/java/com/ankurm/jackson3/part0setup/A01FirstRoundTrip.java)

A plain POJO out to JSON and back, using `JsonMapper.builder().build()` in place of
`new ObjectMapper()`. Note there is no `throws` clause: Jackson 3 exceptions are unchecked.

```
{"listPrice":79.99,"productId":1,"productName":"Mechanical Keyboard"}
Mechanical Keyboard
```

The property order is **alphabetical**, not declaration order — the post shows
`productId` first. That is not a Jackson 3 change; it is how getter-based POJOs have
always been introspected. Records behave differently. See
[Part 1 / B04](part1-objectmapper.md#b04--property-ordering).

## A02 — the shared mapper

[`A02SharedMapperConfiguration.java`](../src/main/java/com/ankurm/jackson3/part0setup/A02SharedMapperConfiguration.java)

The post's central rule — build once, share everywhere — is enforced by the API in
Jackson 3 rather than left to discipline. Three of the four lines people copy from
Jackson 2 configuration are unnecessary or invalid here:

| Jackson 2 line | Status in Jackson 3 |
|---|---|
| `.registerModule(new JavaTimeModule())` | Delete. Built in; the class does not exist under `tools.jackson`. |
| `.registerModule(new Jdk8Module())` | Delete. Same. |
| `.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)` | **Does not compile.** Moved to `DateTimeFeature`, and already off. |
| `.configure(FAIL_ON_UNKNOWN_PROPERTIES, false)` | Redundant — already the default. |
| `.setSerializationInclusion(NON_NULL)` | Replaced by `.changeDefaultPropertyInclusion(...)`. |

```
configured : {"invoiceId":500,"customerName":"Alice","issuedOn":"2026-04-09"}
defaults   : {"invoiceId":500,"customerName":"Alice","issuedOn":"2026-04-09","note":null}
ObjectMapper set*() methods in Jackson 3: 0
```

The last line is the point: `tools.jackson.databind.ObjectMapper` has **zero** `set*`
methods. Reconfiguring a shared mapper is not discouraged in Jackson 3, it is impossible.

## A03 — the three processing models

[`A03ThreeProcessingModels.java`](../src/main/java/com/ankurm/jackson3/part0setup/A03ThreeProcessingModels.java)

The same payload read by data binding, the tree model and the streaming parser.

```
1. data binding : Order[orderId=1001, status=SHIPPED]
2. tree model   : orderId=1001 status=SHIPPED
3. streaming    : orderId=1001 status=SHIPPED
```

For what each approach costs on a real file, see
[Part 6 / G04](part6-streaming.md#g04--the-decision-table-measured).