InstantAsStringAttributeConverter does not appear to have lexicographically correct sorting as claimed

Describe the bug

InstantAsStringAttributeConverter claims that its provided serialization is lexicographically sortable given that the year of the Instant is non-negative. It seems to also not be lexicographically sortable if any value falls directly on a second, since InstantAsStringAttributeConverter simply calls toString(), which uses an adapting ISO 8601 formatter that will truncate 3-groups of zero values:

From Instant's toString():

The format used is the same as DateTimeFormatter.ISO_INSTANT.

From DateTimeFormatter's ISO_INSTANT:

The nano-of-second outputs zero, three, six or nine digits as necessary.

The manifestation of this problem was a query on a sort key that, when re-sorted in application code, changed its order.

Expected Behavior

When using an Instant as a sort key on a DynamoDbBean, I expect sort orderings like this to be returned by a query:

2020-12-23T07:05:28.991Z
2020-12-23T07:05:29Z
2020-12-23T07:05:29.017Z

Current Behavior

Instead I'm getting orderings like this:

2020-12-23T07:05:28.991Z
2020-12-23T07:05:29.017Z
2020-12-23T07:05:29Z

since Z comes after . lexicographically. This disagrees with the natural sort order of the corresponding Instants.

Steps to Reproduce

Create a Dynamo schema with a string sort key, write two instants that share a second, but ensure one of them falls directly on the second while the other has some fractional component.

Possible Solution

Use new DateTimeFormatterBuilder().appendInstant(9).toFormatter() to control the number of zeros. For any values taken from System.currentTimeMillis() there will be a lot of unnecessary zeros, but I don't think there's any other correct default behavior.

Context

It's nice to combine the enhanced client's ability to use native Java types with querying by time. In this case the workaround is pretty straightforward for my case.

Your Environment

• AWS Java SDK version used: 2.15.50
• JDK version used: 11
• Operating System and version: Mac OS X 10.15.7

Parti da InstantAsStringAttributeConverter e verifica come serializza i valori Instant tramite toString(). Riproduci il caso con una chiave di ordinamento string e due istanti che condividono lo stesso secondo, di cui uno con un valore frazionario; il lavoro è completato quando le stringhe serializzate preservano l’ordinamento naturale di Instant durante la query e un nuovo ordinamento.