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How to genarate Unique ID in Java

Style 1 - UUID

import java.util.UUID;

public class GenerateUUID {

public static final void main(String... aArgs){
//generate random UUIDs
UUID idOne = UUID.randomUUID();
UUID idTwo = UUID.randomUUID();
log("UUID One: " + idOne);
log("UUID Two: " + idTwo);
}

private static void log(Object aObject){
System.out.println( String.valueOf(aObject) );
}
}



Example run :

>java -cp . GenerateUUID
UUID One: 067e6162-3b6f-4ae2-a171-2470b63dff00
UUID Two: 54947df8-0e9e-4471-a2f9-9af509fb5889

If Java 5 is not available, then there are other more laborious ways to generate unique ids


Style 2 - SecureRandom and MessageDigest

import java.security.SecureRandom;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class GenerateId {

public static void main (String... arguments) {
try {
//Initialize SecureRandom
//This is a lengthy operation, to be done only upon
//initialization of the application
SecureRandom prng = SecureRandom.getInstance("SHA1PRNG");

//generate a random number
String randomNum = new Integer( prng.nextInt() ).toString();

//get its digest
MessageDigest sha = MessageDigest.getInstance("SHA-1");
byte[] result = sha.digest( randomNum.getBytes() );

System.out.println("Random number: " + randomNum);
System.out.println("Message digest: " + hexEncode(result) );
}
catch ( NoSuchAlgorithmException ex ) {
System.err.println(ex);
}
}

/**
* The byte[] returned by MessageDigest does not have a nice
* textual representation, so some form of encoding is usually performed.
*
* This implementation follows the example of David Flanagan's book
* "Java In A Nutshell", and converts a byte array into a String
* of hex characters.
*
* Another popular alternative is to use a "Base64" encoding.
*/
static private String hexEncode( byte[] aInput){
StringBuilder result = new StringBuilder();
char[] digits = {'0', '1', '2', '3', '4','5','6','7','8','9','a','b','c','d','e','f'};
for ( int idx = 0; idx < aInput.length; ++idx) {
byte b = aInput[idx];
result.append( digits[ (b&0xf0) >> 4 ] );
result.append( digits[ b&0x0f] );
}
return result.toString();
}
}



Example run :

>java -cp . GenerateId
Random number: -1103747470
Message digest: c8fff94ba996411079d7114e698b53bac8f7b037

Style 3 - UID

Finally, here is another method, using a java.rmi.server.UID. The Serializable identifiers generated by this class are unique on the host on which they are generated, provided that

* the host takes more than one millisecond to reboot
* the host's clock is never set to run backwards

In order to construct a UID that is globally unique, simply pair a UID with an InetAddress.


import java.rmi.server.UID;

public class UniqueId {

/**
* Build and display some UID objects.
*/
public static void main (String... arguments) {
for (int idx=0; idx<10; ++idx){
UID userId = new UID();
System.out.println("User Id: " + userId);
}
}
}


Example run :

User Id: 3179c3:ec6e28a7ef:-8000
User Id: 3179c3:ec6e28a7ef:-7fff
User Id: 3179c3:ec6e28a7ef:-7ffe
User Id: 3179c3:ec6e28a7ef:-7ffd
User Id: 3179c3:ec6e28a7ef:-7ffc
User Id: 3179c3:ec6e28a7ef:-7ffb
User Id: 3179c3:ec6e28a7ef:-7ffa
User Id: 3179c3:ec6e28a7ef:-7ff9
User Id: 3179c3:ec6e28a7ef:-7ff8
User Id: 3179c3:ec6e28a7ef:-7ff7

Clearly, these are not secure identifiers - knowing one, it is easy to guess another.

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