Should I synchronize on data in bookHotel method?

[Sean Gildea]

Do I need to synchronize on data in my client GUIController method BookHotel?

I already synchronize on my low level class , FileIO which Data implements

     public boolean bookHotel(long recNo, String custID,
    String hostname, String port) {
    
//will store existing record recNo
String[] existingRecord = null; 

long cookie = 0;

if (connectMode == GUIController.ALONE) {

DBAccess data = new Data();

synchronized(data) {
 
try { 
existingRecord = data.readRecord(recNo);
} catch (RecordNotFoundException rnfx) {
GUIExceptionHandler.warning("Record " + 
recNo +" was not found");
return false;
}
 
if (BookingRules.isBooked(existingRecord)) {
return false;
}

if (!BookingRules.isWithin48Hrs(existingRecord)) {
return false;
}

// remove the recNo from first subscript and create a record
// that the server can accept and use to update the db
String [] record = new String[7];
for ( int i = 1; i < 7; i++ ) {
record[i-1] = existingRecord[i];
}
// place the customer id in the last subscript
record[6] = custID;
 
try { // locking the record
cookie = data.lockRecord(recNo);
} catch (RecordNotFoundException rnfEx) {
return false;
}

try { // booking the hotel room
data.updateRecord(recNo, record, cookie);
} catch (SecurityException secEx) {
return false;
} catch (RecordNotFoundException rnfEx) {
return false;
}

try { // unlocking the record
data.unlock(recNo, cookie);
} catch (SecurityException secEx) {
return false;
}
}
} 

[peter wooster]

I don't synchronize at that level, the question to ask yourself is "what resource am I protecting from multiple accesses?". In my implementation only a single thread accesses a Data instance at a time. That thread is the thread that is performing actions for the client over the network.

All the syncronization is in the lower level objects that represents the database file and the lock manager.

I syncronize on the RandomAccessFile instance to prevent problems where one thread moves the file pointer and another moves it elsewhere before it can read or write the data.

I also synchronize on the Map of locked resources and the Lock object that is waiting or being notified.

All this is carefully done to ensure that the order is always Map->Lock or Map->RandomAccessFile, never the other way around, so there is no danger of deadlock.

[Sean Gildea]

Thanks peter!

That answers my question!