LocalDateTime not displaying as timestamps (ie as a long) java 8 spring boot 2.2.0

After visiting all pages about it where I found and tried many many ideas, none worked for me so I write this post. My API in java 8 spring boot 2.2.0 have beans that are generated from xsd files. I recently changes the Date type to LocalDateTime (because deprecation so it's time to change). The problem is, before, Date were display in a timestamp format (as a long) and now LocalDateTime are display in an array and I don't want it.

What I tried to solve:

• I had a WebMvc annotation on a confiuration class, removed it, not solved the issue.
• I added WRITE_DATES_AS_TIMESTAMPS: true in my application.yml (tried many formats on this properties)
• I have a configuration class for message converters where I tried to manually force timestamps.
• I wanted to add a Json annotation in my beans but as it generate from xsd, I don't know if it is possible. it is ?

Here is a snippet of the configuration class for message converters :

@Configuration
   public class MVCConfigure implements WebMvcConfigurer {
    @Autowired
    private ObjectMapper jacksonObjectMapper;

    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {

        converters.removeIf(c -> c instanceof MappingJackson2HttpMessageConverter);

        jacksonObjectMapper.enable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        converters.add(httpMessageConverterV1V2);

Nothing of theses solutions worked and I still get an array for my LocalDateTime (and I want a long).

Here is how I get the LocalDateTime :

private LocalDateTime getFirstLocalDateTime(final SolrDocument doc, final String key) {
        LocalDateTime ldt = null;
        Date solrDate = (Date) doc.getFirstValue(key);
        if(solrDate != null){
            ldt = solrDate.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime();
        }
        return ldt;
    }

ps : The reason I use Date to get the Date from SolrDoc is because I can't cast to LocalDateTime directly

actual value : "lastIndexedDate": [ 2022, 1, 4, 2, 2, 2, 655000000 ]

desire value : "lastIndexedDate": 1641261722655

So, it is possible to get what I want ?

Thanks to @deHaar who gave me some hints I've found how to proceed.

I created a custom serializer :

public class LocalDateTimeSerialize extends StdSerializer<LocalDateTime> {

    public LocalDateTimeSerialize(Class<LocalDateTime> t) {
        super(t);
    }

    @Override
    public void serialize(LocalDateTime localDateTime, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        Long l = localDateTime.toInstant(OffsetDateTime.now().getOffset())
                .toEpochMilli();
        jsonGenerator.writeString(l.toString());
    }
}

I change my extendMessageConverters to use this custom serializer in my objectMapper :

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new JavaTimeModule()
                .addSerializer(LocalDateTime.class, new LocalDateTimeSerialize(LocalDateTime.class)));