@JsonSetter (nulls = Nulls.AS_EMPTY)

[ghost]

@JsonSetter (nulls = Nulls.AS_EMPTY)
becomes ""
How to assign an enumeration
now protocol is null
I want to make "" become an enumeration

@JsonSetter(nulls = Nulls.AS_EMPTY)
private Constants.PROTO_TYPE protocol;

[cowtowncoder]

What exactly are you doing? What is the input, what kind of type is Constants? What becomes ""?

You are missing a lot of relevant information here to try to help.

[ghost]

@JsonSetter(nulls = Nulls.AS_EMPTY)
private Constants.PROTO_TYPE protocol;

when input {"protocol":null}
the enum Constants.PROTO_TYPE not accept
ask @JsonSetter any other way handle this question

[cowtowncoder]

Unfortunately that is no more useful information. Sorry, I can not really help here.

[Raphasha27]

The Nulls.AS_EMPTY setting tells Jackson to use the empty/default value for a type when the input is null, but you need to pair it with @JsonSetter on the setter, not the field:

public class Item {
    private String name;
    private List<String> tags;

    @JsonSetter(nulls = Nulls.AS_EMPTY)
    public void setTags(List<String> tags) {
        this.tags = tags != null ? tags : Collections.emptyList();
    }
}

For this to work, the setter must already handle the empty case. The nulls attribute only tells Jackson to skip calling the setter (using the empty value from constructor/default) or pass the null.

If you want globally empty collections for null fields, configure the ObjectMapper:

ObjectMapper mapper = new ObjectMapper();
mapper.setDefaultSetterInfo(JsonSetter.Value.forValueNulls(Nulls.AS_EMPTY));

Note: Nulls.AS_EMPTY requires Jackson 2.9+. It works for collections, strings, and optionals — for custom types you need to define what "empty" means via @JsonFormat(with = JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY) or a custom deserializer.