//第一种
SerializeConfig config = new SerializeConfig();  
config.put(Date.class, new SimpleDateFormatSerializer("yyyy-MM-dd"));  
String str = JSON.toJSONString(u,config);
//第二种
JSONObject.DEFFAULT_DATE_FORMAT="yyyy-MM-dd";
String str = JSON.toJSONString(u,SerializerFeature.WriteDateUseDateFormat);
//第三种
String str = JSON.toJSONStringWithDateFormat(u, "yyyy-MM-dd", SerializerFeature.WriteDateUseDateFormat);

try{
    insert();
}catch(DuplicateKeyExceprion e){
    update();
}

@JsonComponent
public class DateConfigure {
    @Value("${spring.jackson.date-format:yyyy-MM-dd HH:mm:ss}")
    private String pattern;
    @Bean
    public Jackson2ObjectMapperBuilderCustomizer jackson2ObjectMapperBuilder() {
        return builder -> {
            TimeZone tz = TimeZone.getTimeZone("GMT+8");
            DateFormat df = new SimpleDateFormat(pattern);
            df.setTimeZone(tz);
            builder.failOnEmptyBeans(false)
                    .failOnUnknownProperties(false)
                    .featuresToDisable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
                    .dateFormat(df);
        };
    }
    @Bean
    public Jackson2ObjectMapperBuilderCustomizer localDateTimeCustomizer() {
        return builder -> builder.serializerByType(LocalDateTime.class,
                new LocalDateTimeSerializer(DateTimeFormatter.ofPattern(pattern)));
    }
}
//如果有特殊格式，可以用@JsonFormat定制：
@JsonFormat(locale = "zh", timezone = "GMT+8", pattern = "yyyy-MM-dd")
//还可以这样
@JsonSerialize(using = DateToStringSerializer.class)
public class DateToStringSerializer extends JsonSerializer<Date> {

    @Override
    public void serialize(Date date, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException {
        if (date != null) {
            jsonGenerator.writeString(DateUtil.format(date,DateUtil.FORMAT_YMDHMS));
        } else {
            jsonGenerator.writeNull();
        }
    }
}

@Configuration
public class WebConfigure implements WebMvcConfigurer {
    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        MappingJackson2HttpMessageConverter jackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
        ObjectMapper objectMapper = jackson2HttpMessageConverter.getObjectMapper();
        //不显示为null的字段
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        SimpleModule simpleModule = new SimpleModule();
        simpleModule.addSerializer(Long.class, ToStringSerializer.instance);
        simpleModule.addSerializer(Long.TYPE, ToStringSerializer.instance);
        objectMapper.registerModule(simpleModule);
        jackson2HttpMessageConverter.setObjectMapper(objectMapper);
        //放到第一个
        converters.add(0, jackson2HttpMessageConverter);
    }
}
// 或者可以这样
@JsonSerialize(using = LongToStringSerializer.class)
public class LongToStringSerializer extends JsonSerializer<Long> {
    @Override
    public void serialize(Long value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        if (value != null) {
            gen.writeString(value.toString());
        } else {
            gen.writeNull();
        }
    }
}

@Configuration
public class CorsConfig {
 
    @Bean
    public CorsFilter corsFilter() {
        final UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
        final CorsConfiguration config = new CorsConfiguration();
        config.setAllowCredentials(true); //支持cookie 跨域
        config.setAllowedOrigins(Arrays.asList("*"));
        config.setAllowedHeaders(Arrays.asList("*"));
        config.setAllowedMethods(Arrays.asList("*"));
        config.setMaxAge(300L);//设置时间有效
 
        source.registerCorsConfiguration("/**", config);
        return new CorsFilter(source);
    }
}