Java生成唯一GUID

GUID（Global unique identifier）全局唯一标识符，它是由网卡上的标识数字(每个网卡都有唯一的标识号)以及 CPU 时钟的唯一数字生成的的一个 16 字节的二进制值。

GUID 的格式为“xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx”，其中每个 x 是 0-9 或 a-f 范围内的一个十六进制的数字。例如：76895313-839E-4E89-BAFC-B253BFF3173F

世界上的任何两台计算机都不会生成重复的 GUID 值。GUID 主要用于在拥有多个节点、多台计算机的网络或系统中，分配必须具有唯一性的标识符。在 Windows 平台上，GUID 应用非常广泛：注册表、类及接口标识、数据库、甚至自动生成的机器名、目录名等。

1.SQL Server数据库
以前开发SQL Server数据库将表定义中将列类型指定为uniqueidentifier，则列的值就为 GUID 类型。

2.使用T-SQL生产一个GUID
insert into table1(id,name,...) values(NewID(),'张三',...)

3.在C#中创建一个GUID
Guid guid = Guid.NewGuid();
Console.Writeln(guid.ToString());

4.在Java中创建UUID
在网上查资料才知道在Java中，变成了UUID。创建方式也出奇简单System.out.println( java.util.UUID.randomUUID());

如下例子：

import java.util.UUID;   
  
public class GetGuid   
{   
    /**  
     * 获取guid码  
     * @param args  
     */  
    public static void main(String[] args) {   
          UUID uuid = UUID.randomUUID();   
          String a = uuid.toString();            
          System.out.println(a);     
          System.out.println(a.length());   
         }   
}  

还有一种方法是自己写一个GUID生成用：需要comm log 库

如下实例：

/**
 * @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();
      } 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();
   }


     // Demonstraton and self test of class
     public static void main(String args[]) {
       for (int i=0; i< 100; i++) {
         RandomGUID myGUID = new RandomGUID();
         System.out.println("Seeding String=" + myGUID.valueBeforeMD5);
         System.out.println("rawGUID=" + myGUID.valueAfterMD5);
         System.out.println("RandomGUID=" + myGUID.toString());
       }
     }


}

同样

UUID uuid = UUID.randomUUID();
System.out.println("{"+uuid.toString()+"}");

UUID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。通常平台会提供生成UUID的API。UUID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。关于UUID这个标准使用最普遍的是微软的GUID(Globals Unique Identifiers)。