lock table to avoid race condition?

Does Oracle have any equivalent to MySQL's feature that allows a transaction to acquire a lock on one or more tables until the lock is either explicitly released or the connection associated with it dies?

I know it's a performance-killer if applied blindly to actively-used tables, which is why I planned to use it in conjunction with a semaphore table that would only be used by threads/transactions actually performing a specific maintenance operation to ensure that only one thread/transaction can be doing maintenance on the main table at any instant in time -- allowing other threads that don't bother to look at the semaphore table to get dirty reads in the meantime without delay or interruption (relying on oracle to release the lock and roll back the transaction if something caused the thread with the lock to die or lose its connection).

My problem is that I have a table with a nested table and date field that needs to enforce uniqueness between the nested table + date pair. Oracle can't attach unique constraints to nested tables, so I need to kludge it myself... with an application that's multithreaded AND likely to be running simultaneously on multiple machines (ie, a race condition eagerly waiting to happen). I kind of came up with a gruesome Rube Goldberg-esque way to achieve it by creating a semaphore table and doing an elaborate dance of inserting, committing, sleeping, and reading until it manages to insert the one and only record in the semaphore table before doing the maintenance on the main table and removing the record from the semaphore table... but even that's vulnerable to a thread dying outright before it deletes its record from the semaphore table (adding more second-guessing into the insert-sleep-select dance).

What I'd LIKE to do is have my Connection ask Oracle, "Please let me have exclusive access to the Semaphore table (allowing me to silently wait up to xx seconds for it to become available, then notifying me that I'm out of luck if it's still not available), but roll back everything I'm about to do in this transaction and release my hold if something causes an automatic transaction rollback".