Register a CustomConverter in a MongoTemplate with Spring Boot

You need to create a configuration class for converter config.

@Configuration
@EnableAutoConfiguration(exclude = { EmbeddedMongoAutoConfiguration.class })
@Profile("!testing")
public class MongoConfig extends AbstractMongoConfiguration {
    @Value("${spring.data.mongodb.host}")  //if it is stored in application.yml, else hard code it here
    private String host;

    @Value("${spring.data.mongodb.port}")
    private Integer port;

    @Override
    protected String getDatabaseName() {
        return "test";
    }

    @Bean
    public Mongo mongo() throws Exception {
        return new MongoClient(host, port);
    }

    @Override
    public String getMappingBasePackage() {
        return "com.base.package";
    }

    @Override
    public CustomConversions customConversions() {
        List<Converter<?, ?>> converters = new ArrayList<>();
        converters.add(new LongToDateTimeConverter());
        return new CustomConversions(converters);
    }
}
@ReadingConverter
static class LongToDateTimeConverter implements Converter<Long, Date> {
    @Override
    public Date convert(Long source) {
        if (source == null) {
            return null;
        }
        return new Date(source);
    }
}