Java实现雪花算法(snowflake)

public class SnowFlake {
 // 起始的时间戳
 private final static long START_STMP = 1577808000000L; //2020-01-01
 // 每一部分占用的位数，就三个
 private final static long SEQUENCE_BIT = 12; //序列号占用的位数
 private final static long MACHINE_BIT = 5; //机器标识占用的位数
 private final static long DATACENTER_BIT = 5; //数据中心占用的位数
 // 每一部分最大值
 private final static long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
 private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
 private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);
 // 每一部分向左的位移
 private final static long MACHINE_LEFT = SEQUENCE_BIT;
 private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
 private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;
 private long datacenterId; //数据中心
 private long machineId; //机器标识
 private long sequence = 0L; //序列号
 private long lastStmp = -1L; //上一次时间戳

 public SnowFlake(long datacenterId, long machineId) {
  if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
   throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
  }
  if (machineId > MAX_MACHINE_NUM || machineId < 0) {
   throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
  }
  this.datacenterId = datacenterId;
  this.machineId = machineId;
 }

 //产生下一个ID
 public synchronized long nextId() {
  long currStmp = timeGen();
  if (currStmp < lastStmp) {
   throw new RuntimeException("Clock moved backwards. Refusing to generate id");
  }

  if (currStmp == lastStmp) {
   //if条件里表示当前调用和上一次调用落在了相同毫秒内，只能通过第三部分，序列号自增来判断为唯一，所以+1.
   sequence = (sequence + 1) & MAX_SEQUENCE;
   //同一毫秒的序列数已经达到最大，只能等待下一个毫秒
   if (sequence == 0L) {
    currStmp = getNextMill();
   }
  } else {
   //不同毫秒内，序列号置为0
   //执行到这个分支的前提是currTimestamp > lastTimestamp，说明本次调用跟上次调用对比，已经不再同一个毫秒内了，这个时候序号可以重新回置0了。
   sequence = 0L;
  }

  lastStmp = currStmp;
  //就是用相对毫秒数、机器ID和自增序号拼接
  return (currStmp - START_STMP) << TIMESTMP_LEFT //时间戳部分
    | datacenterId << DATACENTER_LEFT  //数据中心部分
    | machineId << MACHINE_LEFT    //机器标识部分
    | sequence;        //序列号部分
 }

 private long getNextMill() {
  long mill = timeGen();
  while (mill <= lastStmp) {
   mill = timeGen();
  }
  return mill;
 }

 private long timeGen() {
  return System.currentTimeMillis();
 }
}

@Configuration
public class WebMvcConfig implements WebMvcConfigurer {

 @Autowired
 public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
  converters.add(toStringConverter());
 }

 /**
  * BigDecimal Long 转化为String
  *
  * @return
  */
 @Bean
 public MappingJackson2HttpMessageConverter toStringConverter() {
  MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
  ObjectMapper mapper = new ObjectMapper();
  SimpleModule simpleModule = new SimpleModule();
  simpleModule.addSerializer(BigDecimal.class, BigDecimalToStringSerializer.instance);
  simpleModule.addSerializer(Long.class, ToStringSerializer.instance);
  simpleModule.addSerializer(Long.TYPE, ToStringSerializer.instance);
  simpleModule.addSerializer(long.class, ToStringSerializer.instance);
  mapper.registerModule(simpleModule);
　　　　　// Include.Include.ALWAYS 默认
　　　　　// Include.NON_DEFAULT 属性为默认值不序列化
　　　　　// Include.NON_EMPTY 属性为 空（""） 或者为 NULL 都不序列化，则返回的json是没有这个字段的。这样对移动端会更省流量
　　　　　// Include.NON_NULL 属性为NULL 不序列化
　　　　　mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
　　　　　mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
　　　　　mapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_CONTROL_CHARS, true);// 允许出现特殊字符和转义符
　　　　　mapper.configure(JsonParser.Feature.ALLOW_SINGLE_QUOTES, true); // 允许出现单引号

　　　　　converter.setObjectMapper(mapper);

　　　　　return converter;
 }

 @JacksonStdImpl
 static class BigDecimalToStringSerializer extends ToStringSerializer {
  public final static BigDecimalToStringSerializer instance = new BigDecimalToStringSerializer();

  public BigDecimalToStringSerializer() {
   super(Object.class);
  }

  public BigDecimalToStringSerializer(Class<?> handledType) {
   super(handledType);
  }

  @Override
  public boolean isEmpty(SerializerProvider prov, Object value) {
   if (value == null) {
    return true;
   }
   String str = ((BigDecimal) value).stripTrailingZeros().toPlainString();
   return str.isEmpty();
  }

  @Override
  public void serialize(Object value, JsonGenerator gen, SerializerProvider provider)
    throws IOException {
   gen.writeString(((BigDecimal) value).stripTrailingZeros().toPlainString());
  }

  @Override
  public JsonNode getSchema(SerializerProvider provider, Type typeHint) throws JsonMappingException {
   return createSchemaNode("string", true);
  }

  @Override
  public void serializeWithType(Object value, JsonGenerator gen,
          SerializerProvider provider, TypeSerializer typeSer)
    throws IOException {
   // no type info, just regular serialization
   serialize(value, gen, provider);
  }
 }
}