Without enetring in the specifics (you can fully check out the pattern by reading the appropriate chapter), the solution is to generate a 32 digit key, encoded in hexadecimal composed as follows:1: Unique down to the millisecond. Digits 1-8 are are the hex encoded lower 32 bits of the System.currentTimeMillis() call. 2: Unique across a cluster. Digits 9-16 are the encoded representation of the 32 bit integer of the underlying IP address. 3: Unique down to the object in a JVM. Digits 17-24 are the hex representation of the call to System.identityHashCode(), which is guaranteed to return distinct integers for distinct objects within a JVM. 4: Unique within an object within a millisecond. Finally digits 25-32 represent a random 32 bit integer generated on every method call using the cryptographically strong java.security.SecureRandom class.
1. (0-7) IPAddress as HEX - 8 bytes 2. (8-19) CurrentTimeMillis() as HEX - Display all 12 bytes 3. (20-23) SecureRandom() as HEX - Keep only 4 significant bytes. Since this is "random" it doesn't really matter how many bytes you keep or eliminate 4. (24-31) System.identityHashCode as Hex - 8 bytes
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.logging.Logger;
public class UID {
private static Logger m_log = Logger.getAnonymousLogger();
private static char separator;
private static StringBuilder IPAddressSegment;
private static SecureRandom prng;
static {
try {
// Get IPAddress Segment
IPAddressSegment = new StringBuilder();
InetAddress addr = InetAddress.getLocalHost();
StringBuilder strTemp = new StringBuilder();
byte[] ipaddr = addr.getAddress();
for (int i = 0; i < ipaddr.length; i++) {
Byte b = new Byte(ipaddr[i]);
strTemp = new StringBuilder(Integer.toHexString(b.intValue() & 0x000000ff));
while (strTemp.length() < 2) {
strTemp.insert(0, '0');
}
IPAddressSegment.append(strTemp);
}
if (separator != '\u0000') {
IPAddressSegment.append(separator);
}
//Get Random Segment Algoritm
prng = SecureRandom.getInstance("SHA1PRNG");
} catch (UnknownHostException ex) {
m_log.severe("Unknown Host Exception Caught: " + ex.getMessage());
} catch (NoSuchAlgorithmException nsae) {
m_log.severe("No Such Algorithm Exception Caught: " + nsae.getMessage());
}
}
public static final String getUID() {
StringBuilder strRetVal = new StringBuilder(IPAddressSegment);
StringBuilder strTemp = new StringBuilder();
//Get CurrentTimeMillis() segment
strTemp = new StringBuilder(Long.toHexString(System.currentTimeMillis()));
while (strTemp.length() < 12) {
strTemp.insert(0, '0');
}
strRetVal.append(strTemp);
if (separator != '\u0000') {
IPAddressSegment.append(separator);
}
// Get Random Segment
strTemp = new StringBuilder(Integer.toHexString(prng.nextInt()));
while (strTemp.length() < 8) {
strTemp.insert(0, '0');
}
strRetVal.append(strTemp.substring(4));
if (separator != '\u0000') {
IPAddressSegment.append(separator);
}
//Get IdentityHash() segment
strTemp = new StringBuilder(Long.toHexString(System.identityHashCode((Object) new UID())));
while (strTemp.length() < 8) {
strTemp.insert(0, '0');
}
strRetVal.append(strTemp);
return strRetVal.toString().toUpperCase();
}
public static void main(String[] args) throws Exception {
for (int i = 0; i < 10; i++) {
long lngStart = System.currentTimeMillis();
m_log.info(UID.getUID());
m_log.info("Elapsed time: " + (System.currentTimeMillis() - lngStart));
}
}
}
source
