While working on an unrelated project, I found an issue in Yasson when trying to serialize a POJO with a SQL Timestamp field with a custom date-time formatter enabled in Yasson configuration. The default formatters works just fine, but any explicitly specified formatter produces an NPE in SqlTimestampSerializer::formatWithFormatter method as it calls AbstractDateSerializer::toTemporalAccessor method which simply casts its argument to a TemporalAccessor. This does not work with legacy java.sql.Timestamp object, however, as Timestamp does not implement a TemporalAccessor read-only interface.

Here is a small patch to the SqlTimestampSerializer class to convert a Timestamp into a LocalDateTime (Java 8 Date API) as both hold the same moment-in-time value, UTC time zone, so no information loss occurs. Resulting temporal field cannot be still serialized to JSON with a custom date-time format that includes unsupported elements such as time zone, for example, as neither SQL Timestamp nor LocaDateTime contain any time zone information. This is expected and is by design (of a rather messy java.sql.Timestamp class).