The DvdDatabase Class - SCJD Exam with J2SE 5 - page 155

We have decided to allow any UPC to be reserved, even if no such record exists. This
ensures that a DVD can also be reserved when we are first adding it to the system. The alterna-
tive would be to have the reserveDVD method start by verifying that the record does exist,
throwing some exception if the record does not exist. If we had any delete methods, we would
also have to verify that the record existed after we acquired the lock. However, if we did this,
we would also have to change the logic of the addDVD method, and possibly the updateDVD
method (remember, they both defer to the same private persistDVD method, which might also
have to be updated).
Our reserveDVD method acquires a mutual exclusion lock, and then goes into a while
loop, waiting until we either time out or acquire the lock.
Our DBClient interface specifies that we must return false if we were unable to lock the
record within 5 seconds. Under JDK 1.4 there was no guaranteed way of determining whether
a call to wait(timeout) had timed out or whether notification had been received. JDK 5 has a
new Condition.await method, which will return a Boolean true if we were notified by the
unlock method that we can continue processing, and false if we timed out.
If we have acquired the lock, we add a record to the map of lock owners, indicating that
we own the lock.
Finally, we release the mutual exclusion lock.
The Logical Release Method
The releaseDVD method is the counterpoint to the reserveDVD method shown earlier, and the
code is very similar.
void releaseDvd(String upc, DvdDatabase renter) {
log.entering("ReservationsManager", "releaseDvd",
new Object[]{upc, renter});
lock.lock();
if (reservations.get(upc) == renter) {
reservations.remove(upc);
log.fine(renter + " released lock for " + upc);
lockReleased.signal();
} else {
log.warning(renter + " cant release lock for " + upc + ": not owner");
}
lock.unlock();
log.exiting("ReservationsManager", "releaseDvd");
}
First, we ensure we have a lock on the mutual exclusion Lock ; then if it is the owner of the
reservation who is releasing it, we remove our lock indication from the map. Then we signal
any waiting threads that they can try to acquire locks. If the wrong renter instance is passed to
this method, a warning message is logged. Finally, we release our mutual exclusion Lock .