Why are both SQL_ID and PREV_SQL_ID null?

session A is blocking session B.
it was row-level lock.
But, why are both SQL_ID and PREV_SQL_ID null in session A?

Well, what do we know?
Session A must have an uncommitted transaction.
That uncommitted transaction is blocking session B.
But session A could well be doing nothing now, could be churning away doing some sort of processing outside of the database (java client you said), might well be off doing something elsewhere in a distributed transaction.
Who knows?
Not necessarily the database.
If you have ASH licensed, you might, might get a better picture of what session A was doing by looking at DBA_HIST_ACTIVE_SESS_HISTORY but as this is sampled data and your statements might fly in and out, it might not be helpful.
Note that even if prev_sql_id or sql_id were populated, these might not be the statements responsible for the lock.

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