Why does @Transactional not work across threads?

Spring keeps the active transaction and its connection in TransactionSynchronizationManager, which is ThreadLocal-backed. Work handed to another thread, by @Async, an executor or a parallel stream, starts outside that transaction. It will either run with no transaction at all or open its own on a second connection, which cannot see the first one's uncommitted writes and can deadlock against it at the database level.

That last part is the war story to tell: a parent transaction holding row locks, a child thread on another connection waiting for those same rows, and the parent waiting for the child. Neither is a Java deadlock, so a thread dump shows nothing and the database's lock monitor shows everything.

The rule: one transaction, one thread. Do the concurrent work outside the transaction and commit once, or give each task its own short transaction and make the whole thing idempotent.