How to generate a unique order number

First, the order number has 3 properties: 1. Uniqueness  2. Not speculative 3. Efficiency

Uniqueness and unpredictability needless to say, efficiency is not a frequent database query to avoid duplication.
Moreover, the order number is short enough to meet these conditions.
I customized the order number in Java to generate the following way:
int r1= (int) (Math.random () * (10))//generating 2 0-9 random numbers
int r2= (int) (Math.random () * (10));
Long now = System.currenttimemillis ();//A 13-bit timestamp
String Paymentid =string.valueof (R1) +string.valueof (R2) +string.valueof (now);//Order ID

The current rule, two people in the same microsecond to submit orders repeat the probability of 1%
Order Effect: 251393292999921