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From "Adam Zimowski" <zimowsk...@gmail.com>
Subject Re: POJO dependency injection (with interface) into TAP4 application
Date Thu, 16 Mar 2006 16:28:57 GMT
Well, the restriction I'm trying to impose is that entire codebase
refernces DAO interfaces only. The goal I'm striving to achieve, is
not to have a single place in the entire codebase where specific DAO
instance is used.

The application will be open source, and eventually it will be harder
and harder to check the incomming code for usage of specific instances
of DAOs. A far better way would be to flat out prohibit getting a
specific instance, and allow it only thru Hivemind.

I could be totally wrong with what I'm trying to achieve. But because
the application eventually will be large, I think it would be very
beneficial to use DAOs only thru interfaces within the codebase.

Adam

On 3/16/06, Mike Snare <mikesnare@gmail.com> wrote:
> Hmm... If you make it a singleton, it's a singleton.  It shouldn't
> really matter whether or not they access it via hivemind or directly,
> should it?
>
> Not sure I see much point in the exception if you make it a singleton.
>  It won't matter how they access it -- it will always be the same
> instance.
>
> I think the question is, does it really *need* to be a singleton or
> are you trying to impose an arbitrary restriction.  If it really
> *needs* to be a singleton then make it so and move on.  Accessing it
> via hivemind becomes of secondary importance at that point.
>
> On 3/16/06, Adam Zimowski <zimowski74@gmail.com> wrote:
> > You're awesome.  Thanks much, you clearly know this stuff well.
> >
> > I think I got 99% of what I was looking for from your answer. I'll
> > reply to this post with my working solution later today.
> >
> > Then, ideally what I will try is to figure out some way that if
> > developer tries this:
> >
> > SessionDAO dao = SessionDAO.getInstance();
> >
> > they would get ForbiddenInvocationException or something like that.
> > Have you seen such approach?
> >
> > Adam
> >
> > On 3/16/06, Mike Snare <mikesnare@gmail.com> wrote:
> > > Of course, you could make the dao implementation an *actual* singleton.
> > >
> > > The purpose of the 'singleton' service model in hivemind is, as I
> > > understand it, to avoid needless re-creation of objects when such
> > > re-creation is not necessary, and NOT to provide the protection
> > > features of a standard java singleton.  If that's correct than it's a
> > > bit of a misnomer, IMHO.
> > >
> > > If what you REALLY want is an object that is guaranteed to be a
> > > singleton no matter what, than fall back on standard java and make it
> > > a singleton.  You can still access the singleton using some other
> > > hivemind service and return it.
> > >
> > > It depends on what you want to say about your library:  if the
> > > 'proper' use is to bootstrap the registry to get the service (and the
> > > DAO) then it may be fine to have the DAO be a simple POJO.  Failure to
> > > access the DAO via hivemind is a misuse of the library and the user
> > > gets whatever failures they deserve.  If, however, it is acceptable
> > > (and/or expected) that the user will access the object outside of
> > > hivemind you may want to make it a proper singleton.
> > >
> > > As to the session issue -- yeah, I misunderstood.  The simple answer
> > > is to just return SessionDAOImpl.Instance() -- assuming you made it an
> > > instance.  In that scenario the request is unnecessary.  The exists()
> > > method may be unnecessary too, I was just trying to sort of mimic the
> > > functionality available in standard ASO usage.
> > >
> > > On 3/16/06, Adam Zimowski <zimowski74@gmail.com> wrote:
> > > > Thank you very much Mike.  I like the concept, and your point is right
> > > > on the money. I want to force other developers to use Hivemind to
> > > > obtain ISessionDAO, not the specific implementation.
> > > >
> > > > In your example my dao would be stored inside the session. Is there a
> > > > reason for this? I would prefer it to be a singleton, and not create a
> > > > session just to get the DAO. My session DAO is to provide session
> > > > persistence in the clustered environment, I didn't mean it to live
> > > > inside the session. I'm sorry if that may have been confusing, I could
> > > > have used another dao name, like MemberDAO and IMemberDAO.
> > > >
> > > > Anyway, I will play with your example and try to change it so that it
> > > > doesn't use the session.
> > > >
> > > > One last question. Let's assume I have it wired such that Hivemind
> > > > retuns IMemberDAO or whatever other DAO interface I specify. If my DAO
> > > > implementations live in some package as part of the application, what
> > > > there to prevent other developers from doing inevitable:
> > > >
> > > > SessionDAO dao = new SessionDAO();
> > > > MemberDAO dao = new MemberDAO();
> > > > etc...
> > > >
> > > > I really would like to prohibit this kind of thing totally. There may
> > > > be developers not familiar with IOC that would do this kind of thing,
> > > > yet if it weren't possible they would eventually find the right way
> > > > (the IOC way) to do this type of thing.
> > > >
> > > > Sincere Regards,
> > > > Adam
> > > >
> > > > On 3/16/06, Mike Snare <mikesnare@gmail.com> wrote:
> > > > > Try this...
> > > > >
> > > > > In hivemodule.xml:
> > > > >
> > > > > <service-point id="daoprovider" interface="com.yourcompany.ISessionDAOProvider">
> > > > >    <invoke-factory>
> > > > >       <construct class="com.yourcompany.impl.SessionDAOProviderImpl"
/>
> > > > >    </invoke-factory>
> > > > > </service-point>
> > > > >
> > > > > ISessionDAOProvider.java:
> > > > >
> > > > > public interface ISessionDAOProvider {
> > > > >    public ISessionDAO getDAO();
> > > > >    public boolean getDAOExists();
> > > > > }
> > > > >
> > > > > SessionDAOProviderImpl.java:
> > > > >
> > > > > public class SessionDAOProviderImpl implements ISessionDAOProvider
{
> > > > >
> > > > >    private static final String DAO_KEY = "DAO_KEY";
> > > > >
> > > > >    /**
> > > > >     * HiveMind provides you with a proxy for the real request so
that every time
> > > > >     * you call a method on it it gets routed to the request that
is associated
> > > > >     * with the current thread.  magic...
> > > > >     */
> > > > >    private HttpServletRequest req;
> > > > >
> > > > >    public SessionDAOProviderImpl(HttpServletRequest req) {
> > > > >       this._req = req;
> > > > >    }
> > > > >
> > > > >    /**
> > > > >     * retrieves the DAO from the session, creating it if it doesn't
exist yet.
> > > > >     */
> > > > >    public ISessionDAO getDAO() {
> > > > >       if (!exists()) {
> > > > >          // ideally the implementation class would be specified externally...
> > > > >          req.getSession().addAttribute(DAO_KEY, new SessionDAOImpl());
> > > > >       } else {
> > > > >          return (ISessionDAO) req.getSession().getAttribute(DAO_KEY);
> > > > >       }
> > > > >    }
> > > > >
> > > > >    public boolean getDAOExists() {
> > > > >       return req.getSession().getAttribute(DAO_KEY) != null;
> > > > >    }
> > > > > }
> > > > >
> > > > > Wherever you need the ISessionDAO, just inject the daoprovider service
> > > > > and call getDAO...
> > > > >
> > > > > It's not ideal because you are sort of rolling your own persistence,
> > > > > but it achieves the goal of hiding the implementation of the dao,
and
> > > > > I don't know of another way to do that.
> > > > >
> > > > > -Mike
> > > > >
> > > > > On 3/16/06, Adam Zimowski <zimowski74@gmail.com> wrote:
> > > > > > How could I do this? I don't know Hivemind very well, yet because
it's
> > > > > > integrated with Tapestry I highly prefer it over Spring IOC.
Any
> > > > > > chance you may have an example?
> > > > > >
> > > > > > On 3/16/06, Mike Snare <mikesnare@gmail.com> wrote:
> > > > > > > By the way, if any tap developers are reading this, it
would be great
> > > > > > > if you could declare an interface for an ASO similar to
the way you
> > > > > > > can for a service...
> > > > > > >
> > > > > > > -Mike
> > > > > > >
> > > > > > > On 3/16/06, Mike Snare <mikesnare@gmail.com> wrote:
> > > > > > > > That will work, but doesn't enforce your intent on
other developers
> > > > > > > > (they would be free to inject the ASO as a SessionDAO
and not an
> > > > > > > > ISessionDAO).  Perhaps a better way would be to create
a service whose
> > > > > > > > sole purpose would be to retrieve an instance of the
ISessionDAO from
> > > > > > > > the ApplicationStateManager, which can be auto-wired
to your
> > > > > > > > ISessionDAORetriever service.  Noone would then know
the type.
> > > > > > > >
> > > > > > > > -Mike
> > > > > > > >
> > > > > > > > On 3/16/06, Adam Zimowski <zimowski74@gmail.com>
wrote:
> > > > > > > > > Silly me :-)  How simple and elegant !  I've
been thinking in the
> > > > > > > > > spring context, yet Tap/Hivemind make it so simple..
> > > > > > > > >
> > > > > > > > > Thanks!
> > > > > > > > >
> > > > > > > > > On 3/16/06, Kristian Marinkovic <kristian.marinkovic@porsche.co.at>
wrote:
> > > > > > > > > > hi Adam,
> > > > > > > > > >
> > > > > > > > > > @InjectState("sessionDAO")
> > > > > > > > > > public abstract ISessionDAO getSessionDAO();
> > > > > > > > > >
> > > > > > > > > > works fine too; i'm using it with tapestry-spring
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >              "Adam Zimowski"
> > > > > > > > > >              <zimowski74@gmail
> > > > > > > > > >              .com>                  
                                   An
> > > > > > > > > >                                        
"Tapestry users"
> > > > > > > > > >              16.03.2006 14:11          
<tapestry-user@jakarta.apache.org>
> > > > > > > > > >                                        
                             Kopie
> > > > > > > > > >
> > > > > > > > > >               Bitte antworten          
                             Thema
> > > > > > > > > >                     an                 
POJO dependency injection (with
> > > > > > > > > >              "Tapestry users"          
interface) into TAP4 application
> > > > > > > > > >              <tapestry-user@ja
> > > > > > > > > >              karta.apache.org>
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Hi there,
> > > > > > > > > >
> > > > > > > > > > I'd like to inject my DAOs from Hivemind
as an interface such that my
> > > > > > > > > > app is not aware of implementation. I only
know I can do this:
> > > > > > > > > >
> > > > > > > > > > <contribution configuration-id="tapestry.state.ApplicationObjects">
> > > > > > > > > >  <state-object name="sessionDAO" scope="application">
> > > > > > > > > >   <create-instance class="data.dao.SessionDAO"/>
> > > > > > > > > >  </state-object>
> > > > > > > > > > </contribution>
> > > > > > > > > >
> > > > > > > > > > Then, in my class I'd do:
> > > > > > > > > >
> > > > > > > > > > @InjectState("sessionDAO")
> > > > > > > > > > public abstract SessionDAO getSessionDAO();
> > > > > > > > > >
> > > > > > > > > > I have a few problems with this:
> > > > > > > > > >
> > > > > > > > > > 1) I'd like to inject an interface ISessionDAO,
not the concrete
> > > > > > > > > > implementation.
> > > > > > > > > >
> > > > > > > > > > 2) Question: will Hivemind give me a singleton?
I don't want my DAO's
> > > > > > > > > > be a bunch of short lived objects. I'd like
to be sure they are
> > > > > > > > > > singletons. I think they are because the
scope is application, but I'm
> > > > > > > > > > not sure.
> > > > > > > > > >
> > > > > > > > > > 3) I'd like to be able to inject it to other
POJOs, not just Tapestry
> > > > > > > > > > derived objects (pages, components, etc).
I probably could use
> > > > > > > > > > Registry object, but I really prefer to
do this with annotations? They
> > > > > > > > > > are so elegant.. Does Hivemind has annotation
support ?
> > > > > > > > > >
> > > > > > > > > > As always, I appreciate your help up front.
> > > > > > > > > >
> > > > > > > > > > Regards,
> > > > > > > > > > Adam
> > > > > > > > > >
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