neitnaco

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。

import java.net.*;
import java.util.*;
import java.security.*;public class GuidCreator 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 GuidCreator() { }

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

/*
* Method to generate the random GUID
*/
private void 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 < 0×10) sb.append(’0′);
sb.append(Integer.toHexString(b));
}

rawGUID = sb.toString();

} catch (Exception e) {
System.out.println(”Error:” + e);
}
}

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();
}
}

/**
* Use Exceple
* public static void main(String args[]) {
* for (int i=0; i< 100; i++) {
* GuidCreator myGUID = new GuidCreator();
* System.out.println(”Seeding String=” + myGUID.createNewGuid(GuidCreator.BeforeMD5));
* System.out.println(”rawGUID=” + myGUID.createNewGuid(GuidCreator.AfterMD5));
* System.out.println(”RandomGUID=” + myGUID.createNewGuid(GuidCreator.FormatString));
* }
* }
*/