java 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();

2. 第三方开源类库(推荐使用):
最著名的是 JUG .特点上是: 纯Java实现，开源，LGPL协议。采用了Native的方式产生真正的Uuid.而且提供了不同平台的实现,包括:

Linux / x86
Windows (98, ME, NT, 2K, XP?) / x86
Solaris / Sparc
Mac OS X
FreeBSD / x86
 import  org.doomdark.uuid.UUID;
 import  org.doomdark.uuid.UUIDGenerator;

UUIDGenerator generator  =  UUIDGenerator.getInstance();
UUID uuid  =  generator.generateRandomBasedUUID();

3. Java代码实现（jdk1.4可运行）:

 

 

Java代码
import java.io.IOException;  
import java.io.ObjectInputStream;  
import java.io.Serializable;  
import java.security.MessageDigest;  
import java.security.NoSuchAlgorithmException;  
import java.security.SecureRandom;  
 
import java.util.ArrayList;  
import java.util.HashSet;  
import java.util.Iterator;  
import java.util.List;  
import java.util.Set;  
 
public final class UUID  
    implements Serializable, Comparable  
{  
    private static final long serialVersionUID = 0xbc9903f7986d852fL;  
    private final long mostSigBits;  
    private final long leastSigBits;  
    private transient int version;  
    private transient int variant;  
    private volatile transient long timestamp;  
    private transient int sequence;  
    private transient long node;  
    private transient int hashCode;  
    private static volatile SecureRandom numberGenerator = null;  
   // static final boolean $assertionsDisabled = !UUID.desiredAssertionStatus();     
      
      
    public static void main(String[] args) {  
        System.out.println(UUID.randomUUID());  
        List list = new ArrayList();  
        String uid = "";  
                  
        for (int i = 0; i < 100000; i++) {  
            uid = UUID.randomUUID().toString();       
            list.add(uid);  
            }  
        System.out.println(list.size());  
          
        removeDuplicateObj(list);  
          
          
          
    }  
      
    private  static void   removeDuplicateObj(List   list)   {     
        
        Set   someSet   =   new   HashSet(list);     
   
        Iterator   iterator   =   someSet.iterator();     
        List   tempList   =   new   ArrayList();     
        int   i   =   0;     
        while   (iterator.hasNext())   {     
                tempList.add(iterator.next().toString());     
                i++;     
        }     
        System.out.println(tempList.size());  
}  
      
    private UUID(byte abyte0[])  
    {  
        version = -1;  
        variant = -1;  
        timestamp = -1L;  
        sequence = -1;  
        node = -1L;  
        hashCode = -1;  
        long l = 0L;  
        long l1 = 0L;  
       // if(!$assertionsDisabled && abyte0.length != 16)  
        //    throw new AssertionError();  
        for(int i = 0; i < 8; i++)  
            l = l << 8 | (long)(abyte0[i] & 0xff);  
        for(int j = 8; j < 16; j++)  
            l1 = l1 << 8 | (long)(abyte0[j] & 0xff);  
        mostSigBits = l;  
        leastSigBits = l1;  
    }  
    public UUID(long l, long l1)  
    {  
        version = -1;  
        variant = -1;  
        timestamp = -1L;  
        sequence = -1;  
        node = -1L;  
        hashCode = -1;  
        mostSigBits = l;  
        leastSigBits = l1;  
    }  
    public static UUID randomUUID()  
    {  
        SecureRandom securerandom = numberGenerator;  
        if(securerandom == null)  
            numberGenerator = securerandom = new SecureRandom();  
        byte abyte0[] = new byte[16];  
        securerandom.nextBytes(abyte0);  
        abyte0[6] &= 0xf;  
        abyte0[6] |= 0x40;  
        abyte0[8] &= 0x3f;  
        abyte0[8] |= 0x80;  
        UUID uuid = new UUID(abyte0);  
        return new UUID(abyte0);  
    }  
    public static UUID nameUUIDFromBytes(byte abyte0[])  
    {  
        MessageDigest messagedigest;  
        try 
        {  
            messagedigest = MessageDigest.getInstance("MD5");  
        }  
        catch(NoSuchAlgorithmException nosuchalgorithmexception)  
        {  
            throw new InternalError("MD5 not supported");  
        }  
        byte abyte1[] = messagedigest.digest(abyte0);  
        abyte1[6] &= 0xf;  
        abyte1[6] |= 0x30;  
        abyte1[8] &= 0x3f;  
        abyte1[8] |= 0x80;  
        return new UUID(abyte1);  
    }  
    public static UUID fromString(String s)  
    {  
        String as[] = s.split("-");  
        if(as.length != 5)  
            throw new IllegalArgumentException((new StringBuffer())  
.append("Invalid UUID string: ").append(s).toString());  
        for(int i = 0; i < 5; i++)  
            as[i] = (new StringBuffer()).append("0x").append(as[i]).toString();  
        long l = Long.decode(as[0]).longValue();  
        l <<= 16;  
        l |= Long.decode(as[1]).longValue();  
        l <<= 16;  
        l |= Long.decode(as[2]).longValue();  
        long l1 = Long.decode(as[3]).longValue();  
        l1 <<= 48;  
        l1 |= Long.decode(as[4]).longValue();  
        return new UUID(l, l1);  
    }  
    public long getLeastSignificantBits()  
    {  
        return leastSigBits;  
    }  
    public long getMostSignificantBits()  
    {  
        return mostSigBits;  
    }  
    public int version()  
    {  
        if(version < 0)  
            version = (int)(mostSigBits >> 12 & 15L);  
        return version;  
    }  
    public int variant()  
    {  
        if(variant < 0)  
            if(leastSigBits >>> 63 == 0L)  
                variant = 0;  
            else 
            if(leastSigBits >>> 62 == 2L)  
                variant = 2;  
            else 
                variant = (int)(leastSigBits >>> 61);  
        return variant;  
    }  
    public long timestamp()  
    {  
        if(version() != 1)  
            throw new UnsupportedOperationException("Not a time-based UUID");  
        long l = timestamp;  
        if(l < 0L)  
        {  
            l = (mostSigBits & 4095L) << 48;  
            l |= (mostSigBits >> 16 & 65535L) << 32;  
            l |= mostSigBits >>> 32;  
            timestamp = l;  
        }  
        return l;  
    }  
    public int clockSequence()  
    {  
        if(version() != 1)  
            throw new UnsupportedOperationException("Not a time-based UUID");  
        if(sequence < 0)  
            sequence = (int)((leastSigBits & 0x3fff000000000000L) >>> 48);  
        return sequence;  
    }  
    public long node()  
    {  
        if(version() != 1)  
            throw new UnsupportedOperationException("Not a time-based UUID");  
        if(node < 0L)  
            node = leastSigBits & 0xffffffffffffL;  
        return node;  
    }  
    public String toString()  
    {  
        return (new StringBuffer()).append(digits(mostSigBits >> 32, 8)).append("-").append(digits(mostSigBits >> 16, 4))  
.append("-").append(digits(mostSigBits, 4)).append("-")  
.append(digits(leastSigBits >> 48, 4)).append("-").append(digits(leastSigBits, 12)).toString();  
    }  
    private static String digits(long l, int i)  
    {  
        long l1 = 1L << i * 4;  
        return Long.toHexString(l1 | l & l1 - 1L).substring(1);  
    }  
    public int hashCode()  
    {  
        if(hashCode == -1)  
            hashCode = (int)(mostSigBits >> 32 ^ mostSigBits ^ leastSigBits >> 32 ^ leastSigBits);  
        return hashCode;  
    }  
    public boolean equals(Object obj)  
    {  
        if(!(obj instanceof UUID))  
            return false;  
        if(((UUID)obj).variant() != variant())  
        {  
            return false;  
        } else 
        {  
            UUID uuid = (UUID)obj;  
            return mostSigBits == uuid.mostSigBits && leastSigBits == uuid.leastSigBits;  
        }  
    }  
    public int compareTo(UUID uuid)  
    {  
        return mostSigBits >= uuid.mostSigBits ? mostSigBits <= uuid.mostSigBits ?   
leastSigBits >= uuid.leastSigBits ?   
((byte) (((byte)(leastSigBits <= uuid.leastSigBits ? 0 : 1)))) : -1 : 1 : -1;  
    }  
    private void readObject(ObjectInputStream objectinputstream)  
        throws IOException, ClassNotFoundException  
    {  
        objectinputstream.defaultReadObject();  
        version = -1;  
        variant = -1;  
        timestamp = -1L;  
        sequence = -1;  
        node = -1L;  
        hashCode = -1;  
    }  
    public int compareTo(Object obj)  
    {  
        return compareTo((UUID)obj);  
    }  
      
}