【Java】UUID的生成

  UUID(Universally Unique Identifier) : 全局唯一标识符,是指在一台机器上生成的数字，它保证对在同一时空中的所有机器都是唯一的。按照开放软件基金会(OSF)制定的标准计算，用到了以太网卡地址、纳秒级时间、芯片ID码和许多可能的数字。由以下几部分的组合：当前日期和时间(UUID的第一个部分与时间有关，如果你在生成一个UUID之后，过几秒又生成一个UUID，则第一个部分不同，其余相同)，时钟序列，全局唯一的IEEE机器识别号（如果有网卡，从网卡获得，没有网卡以其他方式获得），UUID的唯一缺陷在于生成的结果串会比较长。

1. 优点

1.1. 随机性
  从定义中可以看出，时间和随机数的加入使得生成的UUID是基本随机的。算法的核心思想是结合机器的网卡、当地时间、一个随即数来生成GUID。
1.2. 唯一性
  由于给予的IEEE机器识别号的唯一性，保证了不同设备的UUID也是不同的，而时间加随机数，从理论上讲，如果一台机器每秒产生10000000个GUID，则可以保证（概率意义上）3240年不重复。

2. 样式

  UUID是由一个十六位的数字组成,表现出来的形式例如

550E8400-E29B-11D4-A716-446655440000   

3. 适用范围

适用于需要唯一的ID，例如数据库主键，分布式系统的模块ID等

4. 使用方法（Java）

UUID uuid  =  UUID.randomUUID(); 
String s = UUID.randomUUID().toString();

5. 代码分析

/**
 * @author Administrator
 *
 * TODO To change the template for this generated type comment go to
 * Window - Preferences - Java - Code Style - Code Templates
 */
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Random;

public class RandomGUID extends Object {
   protected final org.apache.commons.logging.Log logger = org.apache.commons.logging.LogFactory
      .getLog(getClass());

   public String valueBeforeMD5 = "";
   public String valueAfterMD5 = "";
   private static Random myRand;
   private static SecureRandom mySecureRand;

   private static String s_id;
   private static final int PAD_BELOW = 0x10;
   private static final int TWO_BYTES = 0xFF;

   /*
    * Static block to take care of one time secureRandom seed.
    * It takes a few seconds to initialize SecureRandom.  You might
    * want to consider removing this static block or replacing
    * it with a "time since first loaded" seed to reduce this time.
    * This block will run only once per JVM instance.
      */

   static {
      mySecureRand = new SecureRandom();
      long secureInitializer = mySecureRand.nextLong();
      myRand = new Random(secureInitializer);
      try {
         s_id = InetAddress.getLocalHost().toString(); //s_id表示网卡地址，这个是固定的，所以只需要第一次调用定义就可以了
      } catch (UnknownHostException e) {
         e.printStackTrace();
      }

   }


   /*
    * Default constructor.  With no specification of security option,
    * this constructor defaults to lower security, high performance.
    */
   public RandomGUID() {
      getRandomGUID(false);
   }

   /*
    * Constructor with security option.  Setting secure true
    * enables each random number generated to be cryptographically
    * strong.  Secure false defaults to the standard Random function seeded
    * with a single cryptographically strong random number.
    */
   public RandomGUID(boolean secure) {
      getRandomGUID(secure);
   }

   /*
    * Method to generate the random GUID
    */
   private void getRandomGUID(boolean secure) {
      MessageDigest md5 = null;
      StringBuffer sbValueBeforeMD5 = new StringBuffer(128);

      try {
         md5 = MessageDigest.getInstance("MD5");
      } catch (NoSuchAlgorithmException e) {
         logger.error("Error: " + e);
      }

      try {
         long time = System.currentTimeMillis();
         long rand = 0;

         if (secure) {
            rand = mySecureRand.nextLong();
         } else {
            rand = myRand.nextLong();
         }
         sbValueBeforeMD5.append(s_id);
         sbValueBeforeMD5.append(":");
         sbValueBeforeMD5.append(Long.toString(time));
         sbValueBeforeMD5.append(":");
         sbValueBeforeMD5.append(Long.toString(rand));

         valueBeforeMD5 = sbValueBeforeMD5.toString();
         md5.update(valueBeforeMD5.getBytes());

         byte[] array = md5.digest();
         StringBuffer sb = new StringBuffer(32);
         for (int j = 0; j < array.length; ++j) {
            int b = array[j] & TWO_BYTES;
            if (b < PAD_BELOW)
               sb.append('0');
            sb.append(Integer.toHexString(b));
         }

         valueAfterMD5 = sb.toString();

      } catch (Exception e) {
         logger.error("Error:" + e);
      }
   }

   /*
    * Convert to the standard format for GUID
    * (Useful for SQL Server UniqueIdentifiers, etc.)
    * Example: C2FEEEAC-CFCD-11D1-8B05-00600806D9B6
    */
   public String toString() {
      String raw = valueAfterMD5.toUpperCase();
      StringBuffer sb = new StringBuffer(64);
      sb.append(raw.substring(0, 8));
      sb.append("-");
      sb.append(raw.substring(8, 12));
      sb.append("-");
      sb.append(raw.substring(12, 16));
      sb.append("-");
      sb.append(raw.substring(16, 20));
      sb.append("-");
      sb.append(raw.substring(20));

      return sb.toString();
   }