The data type smallserial returns tableMetaData.getColumnTypeName()
int2 rather than smallserial, not even serial2. In addition the field
is also not identified as auto incrementing tableMetaData.isAutoIncrement().

PostgreSQL 12, 13 PgJDBC 42.2.16

Given:

CREATE TABLE smallserial_types (
data_type_id SERIAL NOT NULL PRIMARY KEY,
smallserial_type smallserial
);

Meta Data Returns:

ResultSetMetaData tableMetaData;
Name = tableMetaData.getColumnLabel(i);
Class = tableMetaData.getColumnClassName(i);
SQLType = tableMetaData.getColumnType(i);
TypeName = tableMetaData.getColumnTypeName(i);

Name Class SQLType TypeName
data_type_id java.lang.Integer 4 serial
smallserial_type java.lang.Integer 5 int2