Tracing commits on the server

I'm trying to debug some database transaction problems, 8.0.3

On the server I've enabled statement logging with the configuration
parameter:

log_statement = 'all'

Works peachy. But my jdbc transaction commits aren't logged,
and I can't figure out how to see when the server is processing logical
statement commits. (It is definitely not an auto-commit application).

I see one or two commits like this:

LOG: statement: begin; select getdatabaseencoding(); commit
LOG: statement: BEGIN; SELECT usesuper FROM pg_catalog.pg_user WHERE
usename = 'Foo'; COMMIT

I'm not sure where those are coming from, an artifact of some command
line tool or jdbc driver directive.

And I get some of these:

LOG: statement: begin; select getdatabaseencoding(); commit
LOG: statement: BEGIN;SELECT version();
LOG: statement: COMMIT;

I don't think those are comming from my app.

Other than that, I see some of these:

LOG: statement: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION
LEVEL SERIALIZABLE
LOG: statement: SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY
LOG: statement: SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE

Presumably they're being executed at the start of a new transaction, but
there's no sign of any COMMIT statements before them.

What can I do to see where the database is committing the transaction?