关于利用JDK1.5中自带的Java.util生成GUID的方法

GUID简介
开发软件基金会(Open Software Foundation，OSF)研究出一种能产生唯一标识符的算法，产生全球唯一标识符(Universally Unique Identifier，UUID)。在COM的命名标准上，微软使用同样的算法！在COM中微软将它重命名为Globally Unique Identifier(GUID)。　　
生成GUID的算法根据以下几个方面：1.当前日期和时间。2.网络适配器卡地址。3.时针序。4.自动递增计数器。其中，网卡地址是相互不同的，对没有网卡的机器，地址对使用中的机器保持唯一性。

GUID其实就是一个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、一个随机数来生成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。

GUID实现

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

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

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

还有两种网上给出的生成方法:

import java.net.*;
import java.util.*;
import java.security.*;

public class Guid extends Object {

private String seedingString = "";

private String rawGUID = "";

private boolean bSecure = false;

private static Random myRand;

private static SecureRandom mySecureRand;

private static String s_id;

public static final int BeforeMD5 = 1;

public static final int AfterMD5 = 2;

public static final int FormatString = 3;

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 Guid() {
}

/*
* 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 Guid(boolean secure) {
bSecure = secure;
}

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

try {
md5 = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException e) {
System.out.println("Error: " + e);
}

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

if (secure) {
rand = mySecureRand.nextLong();
} else {
rand = myRand.nextLong();
}

// This StringBuffer can be a long as you need; the MD5
// hash will always return 128 bits. You can change
// the seed to include anything you want here.
// You could even stream a file through the MD5 making
// the odds of guessing it at least as great as that
// of guessing the contents of the file!
sbValueBeforeMD5.append(s_id);
sbValueBeforeMD5.append(":");
sbValueBeforeMD5.append(Long.toString(time));
sbValueBeforeMD5.append(":");
sbValueBeforeMD5.append(Long.toString(rand));

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

byte[] array = md5.digest();
StringBuffer sb = new StringBuffer();
for (int j = 0; j < array.length; ++j) {
int b = array[j] & 0xFF;
if (b < 0x10)
sb.append('0');
sb.append(Integer.toHexString(b));
}

rawGUID = sb.toString();

} catch (Exception e) {
System.out.println("Error:" + e);
}
return rawGUID;
}

public String createNewGuid(int nFormatType, boolean secure) {
getRandomGUID(secure);
String sGuid = "";
if (BeforeMD5 == nFormatType) {
sGuid = this.seedingString;
} else if (AfterMD5 == nFormatType) {
sGuid = this.rawGUID;
} else {
sGuid = this.toString();
}
return sGuid;
}

public String createNewGuid(int nFormatType) {
return this.createNewGuid(nFormatType, this.bSecure);
}

/*
* Convert to the standard format for GUID (Useful for SQL Server
* UniqueIdentifiers, etc.) Example: C2FEEEAC-CFCD-11D1-8B05-00600806D9B6
*/
public String toString() {
String raw = rawGUID.toUpperCase();
StringBuffer sb = new StringBuffer();
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();
}

public static void main(String args[])
{
for (int i=0;i<10;i++)
{
Guid guid=new Guid();
System.out.println("id===="+guid.getRandomGUID(true));
}

}
}

同样

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

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)。