# S01 — Mapper construction and immutability

Guide: <https://ankurm.com/jackson-3-migration-guide/> (Steps 2 and 3)

[`before/S01MapperConstruction.java`](../jackson2-before/src/main/java/com/ankurm/migration/before/S01MapperConstruction.java) ·
[`after/S01MapperConstruction.java`](../jackson3-after/src/main/java/com/ankurm/migration/after/S01MapperConstruction.java)

## The guide's "after" snippet does not compile

```java
JsonMapper mapper = JsonMapper.builder()
    .disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)   // no such constant
    .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
    .serializationInclusion(JsonInclude.Include.NON_NULL)      // no such method
    .build();
```

Two of those four lines are invalid against Jackson 3.2.1:

- `SerializationFeature.WRITE_DATES_AS_TIMESTAMPS` does not exist. The flag moved to
  `tools.jackson.databind.cfg.DateTimeFeature` and now defaults to off, so ISO-8601
  output needs no configuration and there is nothing to disable.
- `JsonMapper.Builder.serializationInclusion(...)` does not exist. The real API is
  `changeDefaultPropertyInclusion(UnaryOperator<JsonInclude.Value>)`.

The third line is valid but redundant — `FAIL_ON_UNKNOWN_PROPERTIES` already defaults
to `false` in Jackson 3.

## Output

**Jackson 2 — `jackson2-before`**

```
output          : {"id":1,"travelDate":"2026-09-15","seatPreference":"aisle"}
after mutation  : { "id" : 1, "travelDate" : "2026-09-15", "seatPreference" : "aisle" }
mutation stuck  : true
set*() mutators : 40
```

**Jackson 3 — `jackson3-after`**

```
output          : {"id":1,"travelDate":"2026-09-15","seatPreference":"aisle"}
forked indented : { "id" : 1, "travelDate" : "2026-09-15", "seatPreference" : "aisle" }
original intact : false
set*() mutators : 0   <- mutation is impossible, not merely discouraged
```

Both produce the same JSON, which is the reassuring part. The interesting lines are the
last two of each.

In Jackson 2 the mapper stays mutable forever. `mapper.enable(INDENT_OUTPUT)` after the
mapper has been injected into a dozen components takes effect for all of them —
`mutation stuck : true`. Forty `set*` methods are available to do it with.

In Jackson 3 there are zero. To vary configuration you fork: `rebuild()` returns a
builder seeded from the existing mapper, and the original is untouched
(`original intact : false`).