# S07 — Defaults that flipped silently

Guide: <https://ankurm.com/jackson-3-vs-jackson-2/> ("Removed and Changed Default Features")

[`before/S07DefaultsThatFlipped.java`](../jackson2-before/src/main/java/com/ankurm/migration/before/S07DefaultsThatFlipped.java) ·
[`after/S07DefaultsThatFlipped.java`](../jackson3-after/src/main/java/com/ankurm/migration/after/S07DefaultsThatFlipped.java)

These are the expensive ones. No compile error, no warning — behaviour just changes.
The two programs run identical probes so the diff is the migration surface:

```bash
diff docs/output/before-S07DefaultsThatFlipped.txt \
     docs/output/after-S07DefaultsThatFlipped.txt
```

## Output

**Jackson 2 — `jackson2-before`**

```
FAIL_ON_TRAILING_TOKENS      : false
FAIL_ON_UNKNOWN_PROPERTIES   : true
ALLOW_FINAL_FIELDS_AS_MUTATORS: true
DEFAULT_VIEW_INCLUSION       : true
AUTO_DETECT_CREATORS exists  : true

concatenated JSON  -> accepted: OrderDto[orderId=1]
trailing garbage   -> accepted: OrderDto[orderId=1]
unknown property   -> rejected: UnrecognizedPropertyException
```

**Jackson 3 — `jackson3-after`**

```
FAIL_ON_TRAILING_TOKENS      : true
FAIL_ON_UNKNOWN_PROPERTIES   : false
ALLOW_FINAL_FIELDS_AS_MUTATORS: false
DEFAULT_VIEW_INCLUSION       : false
AUTO_DETECT_CREATORS exists  : false

concatenated JSON  -> rejected: MismatchedInputException
trailing garbage   -> rejected: StreamReadException
unknown property   -> accepted: OrderDto[orderId=1]
```

| Setting | Jackson 2 | Jackson 3 | Effect |
|---|---|---|---|
| `FAIL_ON_TRAILING_TOKENS` | off | **on** | Concatenated or double-encoded JSON that used to parse now throws |
| `FAIL_ON_UNKNOWN_PROPERTIES` | **on** | off | Unknown fields are tolerated by default; strictness must be opted into |
| `ALLOW_FINAL_FIELDS_AS_MUTATORS` | on | **off** | Jackson no longer overwrites `final` fields by reflection |
| `DEFAULT_VIEW_INCLUSION` | on | **off** | Review any `@JsonView` usage |
| `AUTO_DETECT_CREATORS` | present | **gone** | Constant removed — but see below |

The `FAIL_ON_UNKNOWN_PROPERTIES` flip is worth dwelling on because it moves in the
*lenient* direction. Code that relied on a strict mapper to reject malformed payloads
silently stops rejecting them after the upgrade. If you were using deserialisation
failure as input validation, re-enable it explicitly.

On `AUTO_DETECT_CREATORS`: the constant is gone, but the behaviour it governed is not.
Single-argument constructors are still detected as delegating creators — demonstrated in
the [features repo](https://ankurm.com/git.app/asmhatre/jackson3-by-example).