@Override
	public String create() {
		long id = System.currentTimeMillis();
		synchronized (this) {
			if (id <= lastAllocatedId) {
				id = lastAllocatedId + 1;
			}
			lastAllocatedId = id;
		}
		return Long.toString(id, 36).toUpperCase(Locale.ENGLISH);
	}
TimeBasedSessionIdFactory is a very simple {@link SessionIdFactory}, which
assigns numeric identifiers based on the current milliseconds time, amending
it as necessary to make it unique.
src/main/java/de/gessnerfl/fakesmtp/smtp/server/TimeBasedSessionIdFactory.java