RE: reference to the database

> -----Original Message-----
> From: Leo Mekenkamp [mailto:[email protected]]
> Sent: Wednesday, March 17, 2004 4:49 PM
> To: Wong Liong Hung
> Subject: RE: [Ozone-users] reference to the database
>
>
> On Wed, 2004-03-17 at 09:38, Wong Liong Hung wrote:
> > I face problem when using the "setting the db object as a
> static member of a
> > class" approach with transaction. When calling
> > engine.setModel("BMW") within transaction (See the code below).
> I got the
> > following exception
> >
> > java.lang.IllegalStateException: Attempt to receive() without
> prior send().
> >         at EngineImpl_Proxy.setModel(Unknown Source)
> >         at Client.main(Unknown Source)
> >
> > The setModel() method calls add() method of Store. StoreImpl is
> a singleton
> > class with the static getInstance() method for
> > creating StoreImpl object if it doesn't already exist and
> getting the one
> > and only instance of StoreImpl object from ozone.
> > Both setModel() and add() are ozone update methods.
> >
> > If I change the following statement
> > singleton = (Store)Client.db.objectForName("Store" );
> > to
> > singleton = (Store)Env.currentEnv().database.objectForName("Store" );
> >
> > The program works fine. However, if I call
> StoreImpl.getInstance() outside
> > of transaction block, I got the exception mentioned
> > in my previous message.
> > Exception in thread "main" java.lang.NullPointerException
> >          at org.ozoneDB.Database.objectForName(Database.java:132)
> >          at Client.main(Unknown Source)
>
> In your case using external transactions are complicating matters
> unnecessary. You are better of not using them. Ozone uses implicit
> transactions for you most of the time.

Below is what I want to do:-
void doSomething() {
try {
transaction.begin
a = db.createObject
a.initialize() ---> a.value = 1         //called from a.initialize()
                    b = db.createObject //called from a.initialize()
                    b.initialize()      //called from a.initialize()
a.update()
c = db.createObject
c.initialize()

d.getInstance().add(a); //singleton object
e.getInstance().addName(a); //singleton object
transaction.commit()
} catch (Exception e) {
   transaction.rollback()
}
}

1. doSomething() is a method of non-ozone object. Hence, I cannot depend on
implicit
transactions to control the commit and rollback of all of the method which I
call in doSomething, right?

2. Without using external transaction, how can I rollback the change made to
the a object if c.initialize() failed?

3. If I depend on implicit transactions and by making a.initialize() as an
ozone update method, exception thrown in b.initialize() will cause the
a.value NOT being updated, right?


>
> (...)
> >     public static Store getInstance() throws Exception {
> >         if(singleton == null) {
> >             synchronized(Store.class) {
> >                 singleton = (Store)Client.db.objectForName("Store" );
>
> What you are doing here in called double checked locking. Double checked
> locking is an anti-pattern (i.e. should NOT be used); it has serious
> repercussions if your program runs on a multiprocessor machine and even
> on a single cpu machine.

What I'm trying to do is create a singleton class to make sure that only one
instance exist in any point of time and hide the use of ozone from the
caller. Any idea this can be achieved without using double checked locking.


Your help is very much appreciated.
Thanks.

>
> Cheers,
> Leo
>



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