public class GenerateUUID {
 public static final void main(String... args) {
 // generate random UUIDs
 UUID idOne = UUID.randomUUID();
 UUID idTwo = UUID.randomUUID();
 log("UUID One: " + idOne);
 log("UUID Two: " + idTwo);
 }

 private static void log(Object object) {
 System.out.println(String.valueOf(object));
 }
}

public class GenerateId {
 public static void main(String... arguments) {
 try {
  SecureRandom prng = SecureRandom.getInstance("SHA1PRNG");

  String randomNum = Integer.valueOf(prng.nextInt()).toString();

  MessageDigest sha = MessageDigest.getInstance("SHA-1");
  byte[] result = sha.digest(randomNum.getBytes());

  System.out.println("Random number: " + randomNum);
  System.out.println("Message digest: " + hexEncode(result));
 } catch (NoSuchAlgorithmException ex) {
  System.err.println(ex);
 }
 }

 static private String hexEncode(byte[] input) {
 StringBuilder result = new StringBuilder();
 char[] digits = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a',
  'b', 'c', 'd', 'e', 'f'};
 for (int idx = 0; idx < input.length; ++idx) {
  byte b = input[idx];
  result.append(digits[(b & 0xf0) >> 4]);
  result.append(digits[b & 0x0f]);
 }
 return result.toString();
 }
}

/**
 * @ClassName: SnowflakeIdWorker3rd
 * @Description:snowflake算法改進
 * @author: wanghao
 * @date: 2019年12月13日 下午12:50:47
 * @version V1.0
 * 
 *     將產(chǎn)生的Id類型更改為Integer 32bit <br>
 *     把時間戳的單位改為分鐘,使用25個比特的時間戳（分鐘） <br>
 *     去掉機器ID和數(shù)據(jù)中心ID <br> 
 *     7個比特作為自增值，即2的7次方等于128。
 */
public class SnowflakeIdWorker3rd {
 /** 開始時間戳 (2019-01-01) */
 private final int twepoch = 25771200;// 1546272000000L/1000/60;

 /** 序列在id中占的位數(shù) */
 private final long sequenceBits = 7L;

 /** 時間截向左移7位 */
 private final long timestampLeftShift = sequenceBits;

 /** 生成序列的掩碼，這里為127 */
 private final int sequenceMask = -1 ^ (-1 << sequenceBits);

 /** 分鐘內(nèi)序列(0~127) */
 private int sequence = 0;
 private int laterSequence = 0;

 /** 上次生成ID的時間戳 */
 private int lastTimestamp = -1;

 private final MinuteCounter counter = new MinuteCounter();
 
 /** 預支時間標志位 */
 boolean isAdvance = false;

 // ==============================Constructors=====================================
 public SnowflakeIdWorker3rd() {

 }

 // ==============================Methods==========================================
 /**
 * 獲得下一個ID (該方法是線程安全的)
 * 
 * @return SnowflakeId
 */
 public synchronized int nextId() {
 
 
 int timestamp = timeGen();
 // 如果當前時間小于上一次ID生成的時間戳，說明系統(tǒng)時鐘回退過這個時候應當拋出異常
 if (timestamp < lastTimestamp) {
  throw new RuntimeException(String.format(
   "Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
 }
 
 if(timestamp > counter.get()) {
  counter.set(timestamp);
  isAdvance = false;
 }

 // 如果是同一時間生成的，則進行分鐘內(nèi)序列
 if (lastTimestamp == timestamp || isAdvance) {
  if(!isAdvance) {
  sequence = (sequence + 1) & sequenceMask;
  }

  // 分鐘內(nèi)自增列溢出
  if (sequence == 0) {
  // 預支下一個分鐘,獲得新的時間戳
  isAdvance = true;
  int laterTimestamp = counter.get();
  if (laterSequence == 0) {
   laterTimestamp = counter.incrementAndGet();
  }

  int nextId = ((laterTimestamp - twepoch) << timestampLeftShift) //
   | laterSequence;
  laterSequence = (laterSequence + 1) & sequenceMask;
  return nextId;
  }
 }
 // 時間戳改變，分鐘內(nèi)序列重置
 else {
  sequence = 0;
  laterSequence = 0;
 }

 // 上次生成ID的時間截
 lastTimestamp = timestamp;

 // 移位并通過或運算拼到一起組成32位的ID
 return ((timestamp - twepoch) << timestampLeftShift) //
  | sequence;
 }

 /**
 * 返回以分鐘為單位的當前時間
 * 
 * @return 當前時間(分鐘)
 */
 protected int timeGen() {
 String timestamp = String.valueOf(System.currentTimeMillis() / 1000 / 60);
 return Integer.valueOf(timestamp);
 }

 // ==============================Test=============================================
 /** 測試 */
 public static void main(String[] args) {
 SnowflakeIdWorker3rd idWorker = new SnowflakeIdWorker3rd();
 for (int i = 0; i < 1000; i++) {
  long id = idWorker.nextId();
  System.out.println(i + ": " + id);
 }
 }
}

public class MinuteCounter {
 private static final int MASK = 0x7FFFFFFF;
  private final AtomicInteger atom;
  
  public MinuteCounter() {
    atom = new AtomicInteger(0);
  }
  
  public final int incrementAndGet() {
    return atom.incrementAndGet() & MASK;
  }
  
  public int get() {
    return atom.get() & MASK;
  }
  
  public void set(int newValue) {
   atom.set(newValue & MASK);
  }
}