Why does JDBC make me commit a query?

Question

I was working with two DB2 database connections, A and B, and performed the following logic:
Turned off AutoCommit for A and B
Updated A
Updated B
Committted A
Rolled back B
Selected from A
Selected from B
Updated A
Committed A
Closed Connection A
Closed Connection B
Everything worked well until I tried to close Connection B. It threw a SQL Exception with the description "invalid transaction state". I read online that this means you are trying to close a connection with uncommitted data. It didn't make sense because the only thing I had done since rolling back was SELECT. When I tried it with a rollback or commit immediately before closing the connection, it worked. Any ideas why it would make me commit a SELECT? Is this just an unintended bug or could they have implemented it like this on purpose?

Answer 1

I also first was wondering why a select should be committed.
But also a select can cause locks that influence other connections. How, depends on the isolation level you set. For example SERIALIZABLE would block inserts by another connection that would change your retrieval in case you repeat it.
To make the DBMS know that after your retrieval your processing of these data is done - no changes following - you must end the transaction by commit (or rollback - would be the same here since you've done no changes).