I understand that a no-arg constructor is needed to allow the JSP engine to
construct an instance if it's not yet present, but why is it mandatory
(spec. footnote on page 56) if you make sure that an instance will be
available for the page to use? You can do this either by using the 'model
2' approach of jsp 0.92 (ie. a servlet creates the objects) or using the
redirect or include application models of the jsp 1.0 spec.
In fact, why not allow an interface to be used in the 'class' attribute of
the usebean tag?
This would decouple the data generation/presentation even more since you
could build more generic JSPs. As it is now, a JSP is locked to the
inheritance tree of a certain bean class, while it might otherwise be made
generic enough to be used for all sorts of beans (especially if you want to
start building a library of JSPs that can be included in other pages)
An simple example would be a JSP that's specialised in displaying lists of
names (perhaps using pretty colours or formatted as an option list in a
form). It only requires that your bean has the multivalued property "name",
so it can get the lines by calling String getName(int index).
Instead of requiring that all beans that use this page derive from a common
base class (and requiring changes everywhere if the hierarchy changes for
whatever reason), you could specify that the bean must implement a certain
*interface*. It doesn't concern the JSP builder what other properties the
bean has, only that it has the ones declared in the interface.
This would also reduce the need to create 'wrapper' beans for your data (in
the worst case the programmer can then even pass his complete servlet to do
some quick and dirty testing of the JSP page). You can have a bean
representing your data implementing multiple interfaces to suit the JSPs it
will be used in.
In fact, some of the implicit objects in a JSP are declared through
interfaces (like javax.servlet.http.HttpServletRequest (btw. that's a type
in the spec, they left out the .http bit))
I haven't tried it yet, but I would be surprised if some of the reference
implementations don't already allow this. It probably just depends on what
tests they do while introspecting and if they generate a "new classname();"
instruction for the 'not-found' case or the more generic (and at least
compilable if classname is an interface)
"Class.forName("classname").newInstance();".
Luc
> -----Original Message-----
> From: LAURENCE CABLE [mailto:[EMAIL PROTECTED]]
> Sent: Friday, May 07, 1999 10:51 PM
> To: [EMAIL PROTECTED]
> Subject: Re: usebean
>
>
> "Kirkdorffer, Daniel" wrote:
> >
> > Which is why Integer objects for instance can't be JSP
> beans - they lack a
> > no-arg constructor.
> >
>
> Well, firstly, they are not Beans ... they are
> objects, there is a
> difference!
> Secondly, I dont really think that an Integer is a
> valuable abstraction
> as a
> bean anyway. Then finally, if it is of value, it
> would'nt be hard to
> work around
> this ...
>
> 8^)
>
> > Dan
> > --
> > Daniel Kirkdorffer
> > NACN IS: 425-580-6225
> > Sr. Consultant, Syllogistics LLC
> > Email: [EMAIL PROTECTED]
> > Web: http://www.syllogistics.com/
> >
> > > ----------
> > > From: LAURENCE CABLE[SMTP:[EMAIL PROTECTED]]
> > > Reply To: LAURENCE CABLE
> > > Sent: Friday, May 07, 1999 11:57 AM
> > > To: [EMAIL PROTECTED]
> > > Subject: Re: usebean
> > >
> > > <<File: Larry.Cable.vcf>>
> > > > Hi,
> > > >
> > > > I am new to JSP and have been trying to go through the
> JSP Version 1
> > > Public
> > > > Draft to get an idea of the concept behind JSP. While having
> > > declaratives,
> > > > actions, expressions etc. were all making sense, the
> one thing which I
> > > could
> > > > not be sure about was this - "When you define a class
> in USEBEAN, is it
> > > just
> > > > any java class or is it some kind of a special class
> implementing a
> > > certain
> > > > interface etc."
> > > >
> > > > I did not see anything in the documentation that talked
> about how to
> > > write
> > > > the beans referred to in the "USEBEAN" tag.
> > > >
> > >
> > > The "beans" used by the usebean tag do not need
> to implement any
> > > particular JSP dependent interface or derive from
> a particular
> > > class. The only constraint is that they implement a public
> > > "no-args"
> > > constructor:
> > >
> > > public class CanonicalButUselessBean { public
> > > CanonicalButUselessBean()
> > > { } }
> > >
> > > Rgds
> > >
> > > - Larry Cable
> > >
>
===========================================================================
To unsubscribe, send email to [EMAIL PROTECTED] and include in the body
of the message "signoff JSP-INTEREST". For general help, send email to
[EMAIL PROTECTED] and include in the body of the message "help".
