Only letting one instance of my application run

Question

hi guys,
My application will allow the user to interact with the database. The database will be stored on a local machine, and not on a network, so that only one person can access it at a time.
The problem is, I believe a user can launch two instances of my application simultaneously, and therefore there are potential problems.
So I want the user to only ever be able to launch one instance of the JFrame through which all interactions to the DB are made.
So MyApp.java contains
class MyApp
public void main (String[] args)
MyFrame mF = new MyFrame();
}Now, is it the right idea of mine to make mF singleton? Or is it the wrong approach?
But what is confusing me is...if at the java prompt I type
java MyApp
and then I type, once again
java MyApp
will these two be treated as two completely different running of programs? Say I had mF as a singleton. Does this mean there can exist one instance of mF in each of my two different runnings of MyApp, or are the two somehow connected, which means that if I ran MyApp a second time theer'll be some sort of error cos mF already exists and cos it is a singleton, it can't exist twice?
Sorry if I havent made myself clear. I'm very confused myself.
Thanks.
AU

Answer 1

My application will allow the user to interact with
the database. The database will be stored on a local
machine, and not on a network, so that only one
person can access it at a time.If using jdbc then local or remote only really effect your database driver choice not your code.
The problem is, I believe a user can launch two
instances of my application simultaneously,Yes.
and therefore there are potential problems. Potential but not that difficult to avoid.
So I want the user to only ever be able to launch one
instance of the JFrame through which all interactions
to the DB are made.If you think about responsibilities, then data integrity is not the responsibility of the UI, its the responsibility of the DBMS.
Now, is it the right idea of mine to make mF
singleton? Or is it the wrong approach?A Singleton will only ensure that a jre has a single instance. If you start multiple instances of the jre then there can be one instance of the application in each.
The best solution is to use your databases normal locking process through the JDBC api
The simplest solution is for your program to lock either the database, the table or the record you are interested in. You can do this by creating a lock record on a seperate table. If the insert fails to create the record, somebody else is already using the dataset. If it succeeds then is available. You will need a way to forcible clear the lock if your application stops unexpectedly.