Cannot deserialize field with underscore in it's name

[mozzer39]

Hi!
I use OpenApi generator to create model classes based on provided API schema. The problem is that there is a field name starting with an underscore ("_"). With Jackson 2 there was no problem with deserializing the data. Unfortunately after upgrading to Jackson 3, this field is no longer deserialized and is null.

Here's an example model class generated by OpenApi:

@JsonInclude(com.fasterxml.jackson.annotation.JsonInclude.Include.NON_EMPTY)
@JsonIgnoreProperties(ignoreUnknown = true)

@Generated(value = "org.openapitools.codegen.languages.SpringCodegen", date = "2025-12-08T09:12:41.292705+01:00[Europe/Warsaw]",
           comments = "Generator version: 7.17.0")
public class ExampleClass {

    private @Nullable String _1;

    public ExampleClass _1(@Nullable String _1) {
        this._1 = _1;
        return this;
    }

    @Valid
    @JsonProperty("_1")
    public @Nullable String get_1() {
        return _1;
    }

    public void set_1(@Nullable String _1) {
        this._1 = _1;
    }
    ...
}

And here's an example json I try to deserialize:

{
    "_1" : "testString"
}

Any ideas what can I do in this situation? Obviously I can't make any changes in the API model. Is it a bug that Jackson 3 can no longer handle such situation? Thanks for helping!

[pjfanning]

Please provide a description of what goes wrong - for example, the exception with stacktrace.

[mozzer39]

There's no any exception. Simply after deserialization _1 field value is null, instead of testString (as it is with Jackson 2).

[cowtowncoder]

Yes, get_1() is not considered Bean-compatible getter. You will probably need to add @JsonSetter on setter to make things work. 3.0 is bit stricter on name validity wrt introspection.

There is a way to configure things to be more lenient, let me see if I can find the issue.

And no exception part is due to DeserializerFeature.FAIL_ON_UNKNOWN_PROPERTIES defaulting to false in Jackson 3, most likely (made due tons of users requesting default change -- but now hides real problems).

[cowtowncoder]

Found it: FasterXML/jackson-databind#2882 -- this is why getter/setter no longer recognized without annotations.

(and maybe FasterXML/jackson-databind#5355 too for work-around)