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Re: Object Creation in ozone
Don, allow me to comment,
[Business rules require a valid object network]
> So if the valid state of 'ObjectOne' upon instantiation requires the
> existence of another new object (the 'ObjectTwo' in my example) then it
must
> created from the ObjectOne constructor or created and passed to the
> constructor. The latter approach may require knowledge of the internals
of
> ObjectOne which is unacceptable.
If you would neglect the necessity of storing objects in an OODB, you would
model your business rule by only allowing a parameterized constructor.
To reininstantiate objects an OODB needs a public constructor (or a VM which
does not call the constructor when instantiating objects from persistent
storage).
> One way around this in ozone that I tried as an interim work around was a
> 'secondary' init() method similar in purpose to Tim Brown's suggestion
that
> is called immediately after object creation. However this requires that an
> object be created in an invalid state thus objects can no longer be
> responsible for their own state.
This is specific to the ozone way of letting objects "live" on the server.
(no doubt, this system has advantages)
On other OODBs you would model your rules by using constructors with
parameters.
Specifically for the OODB you would add a public constructor with zero
parameters.
To check your logic you could do this:
- add a comment, that this public constructor is for the db only
- use grep or a similar tool to make all constructors of this kind private
- compilation should be successful
- use grep again to restore the needed state
> If one is
> using explicit transactions it could be made to work by
> aborting the transaction and the object's creation.
I guess, this is the correct ozone solution for your problem, but Falko will
surely comment on that.
['ozoneInit']
Callback functions are a common concept.
The JDO specification proposes the following optional callbacks:
- jdoPostLoad
- jdoPreStore
- jdoPreClear
PreStore is what you want, but you can easily implement it yourself:
Override the ozone commit function and call your 'ozoneInit' on all new
objects and check the integrity there.
A few if(... == null) should even be less work than parameterized
constructors, because you don't have to change the entire hierarchy of your
objects every time you add a new member.
(My function's name is onCreate().)
Regards,
Carl Rosenberger
db4o - database for objects - http://www.db4o.com