Is Oracle 12.2 backward compatible with Oracle 12.1
Here is the list of changes:

Behavior Changes, Deprecated and Desupported Features for Oracle Database 12c Release 2 (12.2)

Such basic functionality that you mentioned (table joins, stored procedures, JDBC connections) should of course work.

Sure, there are new features, and some changes, but I can not recall anything significant related to those areas that worked in 12.1 but did not in 12.2.