How to create user friendly unique IDs, UUIDs or other unique identifiers in Java

I开发者_如何学C usually use the UUID class to generate unique IDs. This works fine if these IDs are used by technical systems only, they don't care how long they are:

System.out.println(UUID.randomUUID().toString());

> 67849f28-c0af-46c7-8421-94f0642e5d4d

Is there a nice way to create user friendly unique IDs (like those from tinyurl) which are a bit shorter than the UUIDs? Usecase: you want to send out IDs via Mail to your customers which in turn visit your site and enter that number into a form, like a voucher ID.

I assume that UUIDs get generated equally through the whole range of the 128 Bit range of the UUID. So would it be sage to use just the lower 64 Bits for instance?

System.out.println(UUID.randomUUID().getLeastSignificantBits());

Any feedback is welcome.

I assume that UUIDs get generated equally through the whole range of the 128 Bit range of the UUID.

First off, your assumption may be incorrect, depending on the UUID type (1, 2, 3, or 4). From the Java UUID docs:

There exist different variants of these global identifiers. The methods of this class are for manipulating the Leach-Salz variant, although the constructors allow the creation of any variant of UUID (described below).

The layout of a variant 2 (Leach-Salz) UUID is as follows: The most significant long consists of the following unsigned fields:

0xFFFFFFFF00000000 time_low 
0x00000000FFFF0000 time_mid 
0x000000000000F000 version 
0x0000000000000FFF time_hi  

The least significant long consists of the following unsigned fields:

0xC000000000000000 variant 
0x3FFF000000000000 clock_seq 
0x0000FFFFFFFFFFFF node  

The variant field contains a value which identifies the layout of the UUID. The bit layout described above is valid only for a UUID with a variant value of 2, which indicates the Leach-Salz variant.

The version field holds a value that describes the type of this UUID. There are four different basic types of UUIDs: time-based, DCE security, name-based, and randomly generated UUIDs. These types have a version value of 1, 2, 3 and 4, respectively.

The best way to do what you're doing is to generate a random string with code that looks something like this (source):

public class RandomString {

          public static String randomstring(int lo, int hi){
                  int n = rand(lo, hi);
                  byte b[] = new byte[n];
                  for (int i = 0; i < n; i++)
                          b[i] = (byte)rand('a', 'z');
                  return new String(b, 0);
          }

          private static int rand(int lo, int hi){
                      java.util.Random rn = new java.util.Random();
                  int n = hi - lo + 1;
                  int i = rn.nextInt(n);
                  if (i < 0)
                          i = -i;
                  return lo + i;
          }

          public static String randomstring(){
                  return randomstring(5, 25);
          }

        /**
         * @param args
         */
        public static void main(String[] args) {
                System.out.println(randomstring());

        }

}

If you're incredibly worried about collisions or something, I suggest you base64 encode your UUID which should cut down on its size.

Moral of the story: don't rely on individual parts of UUIDs as they are holistically designed. If you do need to rely on individual parts of a UUID, make sure you familiarize yourself with the particular UUID type and implementation.

Here is another approach for generating user friendly IDs:
http://thedailywtf.com/Articles/The-Automated-Curse-Generator.aspx

(But you should go for the bad-word-filter)

Any UUID/Guid is just 16 Bytes of data. These 16 bytes can be easily encoded using BASE64 (or BASE64url), then stripped off all of the "=" characters at the end of the string.

This gives a nice, short string which still holds the same data as the UUID/Guid. In other words, it is possible to recreate the UUID/Guid from that data if such becomes necessary.

Here's a way to generate a URL-friendly 22-character UUID

public static String generateShortUuid() {
        UUID uuid = UUID.randomUUID();

        long lsb = uuid.getLeastSignificantBits();
        long msb = uuid.getMostSignificantBits();

        byte[] uuidBytes = ByteBuffer.allocate(16).putLong(msb).putLong(lsb).array();

        // Strip down the '==' at the end and make it url friendly   
        return Base64.encode(uuidBytes)
                    .substring(0, 22)
                    .replace("/", "_")
                    .replace("+", "-");
    }

For your use-case, it would be better to track a running count of registered user, and for each value, generate a string-token like this:

public static String longToReverseBase62(long value /* must be positive! */) {

        final char[] LETTERS = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();

        StringBuilder result = new StringBuilder(9);
        do {
            result.append(LETTERS[(int)(value % 62)]);
            value /= 62l;
        }
        while (value != 0);

        return result.toString();
    }

For security reasons, it would be better if you make the values non-sequential, so each time a user registers, you can increment the value let's say by 1024 (This would be good to generate uuids for 2^64 / 2^10 = 2^54 users which is quite certainly more than you'd ever need :)

At the time of this writing, this question's title is:

How to create user friendly unique IDs, UUIDs or other unique identifiers in Java

The question of generating a user-friendly ID is a subjective one. If you have a unique value, there are many ways to format it into a "user-friendly" one, and they all come down to mapping unique values one-to-one with "user-friendly" IDs — if the input value was unique, the "user-friendly" ID will likewise be unique.

In addition, it's not possible in general to create a random value that's also unique, at least if each random value is generated independently of any other. In addition, there are many things you should ask yourself if you want to generate unique identifiers (which come from my section on unique random identifiers):

1. Can the application easily check identifiers for uniqueness within the desired scope and range (e.g., check whether a file or database record with that identifier already exists)?
2. Can the application tolerate the risk of generating the same identifier for different resources?
3. Do identifiers have to be hard to guess, be simply "random-looking", or be neither?
4. Do identifiers have to be typed in or otherwise relayed by end users?
5. Is the resource an identifier identifies available to anyone who knows that identifier (even without being logged in or authorized in some way)?
6. Do identifiers have to be memorable?

In your case, you have several conflicting goals: You want identifiers that are unique, random, and easy to type by end users. But other things you should think about are:

• Are other users allowed to access the resource identified by the ID, whenever they know the ID? If not, then additional access control or a longer key length will be necessary.
• Can your application tolerate the risk of duplicate keys? If so, then the keys can be completely randomly generated (such as by a cryptographic RNG such as java.security.SecureRandom in Java). If not, then your goal will be harder to achieve, especially for keys intended for security purposes.

Also, if you want IDs that have to be typed in by end users, you should consider choosing a character set carefully or allowing typing mistakes to be detected.

Only for you :) :

private final static char[] idchars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();
private static String createId(int len) {
    char[] id = new char[len];
    Random r = new Random(System.currentTimeMillis());
    for (int i = 0;  i < len;  i++) {
        id[i] = idchars[r.nextInt(idchars.length)];

    }
    return new String(id);
}

How about this one? Actually, this code returns 13 characters(numbers and lowercase alphabets) max.

import java.nio.ByteBuffer;
import java.util.UUID;

/**
 * Generate short UUID (13 characters)
 * 
 * @return short UUID
 */
public static String shortUUID() {
  UUID uuid = UUID.randomUUID();
  long l = ByteBuffer.wrap(uuid.toString().getBytes()).getLong();
  return Long.toString(l, Character.MAX_RADIX);
}