Java中同步方法的问题

Question

I have following implementation in Java where I am trying to use a synchronized method:

class dbAccess{  
     public synchronized void getGUID(){  
           counter=/*Access last count from txn_counter table */
           /*Insert a unique value to txn_counter table based on the acquired value of counter */ 
           /*Insert new counter value to GUID_log table */
     }  
}

The portion between /* */ represent some sql queries. The implementation has 10 threads. I was hoping that counter value returned everytime would be unique. But it so happens that multiple runs return same value of counter.

Can you please point out if I am doing anything wrong. And, is it the right way to do this?

Answer 1

Just because it is synchronised in java, does not mean it is synchronised on the database. This method needs to run in a database transaction with read locking enabled.

Answer 2

Perhaps you have several instances of dbAccess ? (The synchronized keyword works on object level not on class level.) In that case you need to make the method static (may not be feasible in your situation), or try to have a static lock protecting the method body, like this:

class dbAccess{  
    private final static Object o = new Object();

    public void getGUID(){  
        synchronized (o) {
            counter=/*Access last count from txn_counter table */
            // Insert a unique value to txn_counter table based on
            // the acquired value of counter
            // Insert new counter value to GUID_log table
        }
    }  
}