How to serialize a date to a specific format?

The mandatory output format of any date in a JSON string is:

{
    "element": [
        "~#date",
        [
            "2013-05-31"
        ]
    ],
    ...
}

I have to adhere to this specific format.

Building an object with an array of objects will not work. I get something like this:

{
    "element": [
        {
            "~#date": "2024-09-17"
        }
    ],
    ...
}

I could write a Serializer for LocalDate printing the '[...]'. That feels like cheating. Notice that the number of dates is always 1.

Is there a standard way of doing this?

This is what I did so far: The object contains:

@JsonProperty("aDate")
private MyMDate[] myDate = null;

With 'MyDate' being:

@Setter
@Getter
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class MyDate {
    @JsonProperty("~#date")
    @JsonSerialize(using = LocalDateArraySerializer.class)
    private LocalDate[] date;
}

And the serializer:

public class LocalDateArraySerializer extends JsonSerializer<LocalDate[]> {
    private final DateTimeFormatter format = DateTimeFormatter.ofPattern( "yyyy-MM-dd");

    @Override
    public void serialize(LocalDate[] values, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        String seperator = "";
        for ( LocalDate ld: values) {
            gen.writeString( seperator + ld.format(format));
            seperator = ",";
        }
    }
}

This gives the actual output: an array with an object. That is not the requested output.

You just need these lines :

public class CustomLocalDateSerializer extends StdSerializer<LocalDate> {

public CustomLocalDateSerializer() {
    super(LocalDate.class);
}

@Override
public void serialize(LocalDate value, JsonGenerator gen, SerializerProvider provider) throws IOException {
    gen.writeStartArray();   // start outer array
    gen.writeString("~#date");
    gen.writeStartArray();   // start inner array
    gen.writeString(value.format(DateTimeFormatter.ofPattern("yyyy-MM-dd"))); //date format
    gen.writeEndArray();     // end inner array
    gen.writeEndArray();     // end outer array
    }
}

@JsonSerialize(using = CustomLocalDateSerializer.class)
public class DateHolder {
private LocalDate date;

public DateHolder(LocalDate date) {
    this.date = date;
}

// getters and setters
}

and then call the custom serializer

ObjectMapper mapper = new ObjectMapper();

DateHolder dateHolder = new DateHolder(LocalDate.of(2013, 5, 31));
    String json = mapper.writeValueAsString(dateHolder); //serialize
System.out.println(json);