The suggested solution is based on the UUID for EJB pattern, that comes out from this question:
How can universally unique primary keys can be generated in menory without requiring a database or a singleton?
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.