How to format date correctly using Spring Data Elasticsearch

As I am a new joiner,I can't comment under @P.J.Meisch's anwser by the stack rules. I also faced the problem, and solved it with @P.J.Meisch's anwser. But just a little change with the @ReadingConverter. Infact, the raw type read from ES, is Long, and the result type in java we need is LocalDateTime. Thus, the read converter shoud be Long to LocalDateTime. Code follows below:

@Configuration
public class ElasticsearchClientConfig extends AbstractElasticsearchConfiguration {

    public final static int TIME_OUT_MILLIS = 50000;

    @Autowired
    private ElasticsearchProperties elasticsearchProperties;

    @Override
    @Bean
    public RestHighLevelClient elasticsearchClient() {

        final ClientConfiguration clientConfiguration = ClientConfiguration.builder()
                .connectedTo(elasticsearchProperties.getHost() + ":" + elasticsearchProperties.getPort())
                .withBasicAuth(elasticsearchProperties.getName(), elasticsearchProperties.getPassword())
                .withSocketTimeout(TIME_OUT_MILLIS)
                .withConnectTimeout(TIME_OUT_MILLIS)
                .build();

        return RestClients.create(clientConfiguration).rest();
    }

    /**
     * Java LocalDateTime to ElasticSearch Date mapping
     *
     * @return EntityMapper
     */
    @Override
    @Bean
    public EntityMapper entityMapper() {
        ElasticsearchEntityMapper entityMapper = new ElasticsearchEntityMapper(elasticsearchMappingContext(), new DefaultConversionService());
        entityMapper.setConversions(elasticsearchCustomConversions());
        return entityMapper;
    }

    @Override
    public ElasticsearchCustomConversions elasticsearchCustomConversions() {
        return new ElasticsearchCustomConversions(Arrays.asList(DateToStringConverter.INSTANCE, LongToLocalDateTimeConverter.INSTANCE));
    }

    @WritingConverter
    enum DateToStringConverter implements Converter<Date, String> {
        /**
         * instance
         */
        INSTANCE;

        @Override
        public String convert(@NonNull Date date) {
            return DateUtil.format(date, DateConstant.TIME_PATTERN);
        }
    }

    **@ReadingConverter
    enum LongToLocalDateTimeConverter implements Converter<Long, LocalDateTime> {
        /**
         * instance
         */
        INSTANCE;
        @Override
        public LocalDateTime convert(@NonNull Long s) {
            return LocalDateTime.ofInstant(Instant.ofEpochMilli(s), ZoneId.systemDefault());
        }
    }**

}

and the DateUtil file:

public class DateUtil {
    /**
     * lock obj
     */
    private static final Object LOCK_OBJ = new Object();

    /**
     * sdf Map for different pattern
     */
    private static final Map<String, ThreadLocal<SimpleDateFormat>> LOCAL_MAP = new HashMap<>();


    /**
     * thread safe
     *
     * @param pattern pattern
     * @return SimpleDateFormat
     */
    private static SimpleDateFormat getSdf(final String pattern) {
        ThreadLocal<SimpleDateFormat> tl = LOCAL_MAP.get(pattern);

        if (tl == null) {
            synchronized (LOCK_OBJ) {
                tl = LOCAL_MAP.get(pattern);
                if (tl == null) {
                    System.out.println("put new sdf of pattern " + pattern + " to map");
                    tl = ThreadLocal.withInitial(() -> {
                        System.out.println("thread: " + Thread.currentThread() + " init pattern: " + pattern);
                        return new SimpleDateFormat(pattern);
                    });
                    LOCAL_MAP.put(pattern, tl);
                }
            }
        }

        return tl.get();
    }

    /**
     * format
     *
     * @param date    date
     * @param pattern pattern
     * @return String
     */
    public static String format(Date date, String pattern) {
        return getSdf(pattern).format(date);
    }

}

at last,

pls vote for @P.J.Meisch, not me.