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  •    3年前 (2015-08-11)  JAVA WEB |   抢沙发  10 
    文章评分 0 次,平均分 0.0

    UUID是1.5中新增的一个类,在java.util下,用它可以产生一个号称全球唯一的ID,GUID是一个128位长的数字,一般用16进制表示。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。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。

    package com.mytest;
    import java.util.UUID;
    public class UTest {
        public static void main(String[] args) {
            UUID uuid = UUID.randomUUID();
            System.out.println(uuid);
        }
    }

    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。

    在Java中生成UUID主要有以下几种方式:
    1.JDK1.5
    如果使用的JDK1.5的话,那么生成UUID变成了一件简单的事,以为JDK实现了UUID:
    java.util.UUID,直接调用即可.

    UUID uuid  =  UUID.randomUUID(); 
    String s = UUID.randomUUID().toString();//用来生成数据库的主键id非常不错

    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

    //下面就是实现为数据库获取一个唯一的主键id的代码 
    public class UUIDGenerator { 
        public UUIDGenerator() { 
        } 
        /** 
         * 获得一个UUID 
         * @return String UUID 
         */ 
        public static String getUUID(){ 
            String s = UUID.randomUUID().toString(); 
            //去掉“-”符号 
            return s.substring(0,8)+s.substring(9,13)+s.substring(14,18)+s.substring(19,23)+s.substring(24); 
        } 
        /** 
         * 获得指定数目的UUID 
         * @param number int 需要获得的UUID数量 
         * @return String[] UUID数组 
         */ 
        public static String[] getUUID(int number){ 
            if(number < 1){ 
                return null; 
            } 
            String[] ss = new String[number]; 
            for(int i=0;i<number;i++){ 
                ss[i] = getUUID(); 
            } 
            return ss; 
        } 
        public static void main(String[] args){ 
            String[] ss = getUUID(10); 
            for(int i=0;i<ss.length;i++){ 
                System.out.println(ss[i]); 
            } 
        } 
    }   

    2.手动生成

    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());
           }
         }
    }
     

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