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();    }