Just a followup idea- define the following method in TableDecorator:
public Collection sort(String columnPropertyName, int direction, int page)
{
// call default internal implementation and return result
}
Developers can override this with their own custom TableDecorator which
can hook into their persistence layers, or have a custom middle tier
sort/page capability, and only return the sorted rows which displaytag
should render.
I didn't look at feasibility, but this may be the simplest solution.
> iBATIS (http://www.ibatis.com) has a similar concept already implemented
> called com.ibatis.common.util.PaginatedArrayList. It implements
> java.util.Collection and java.util.List as well as
> com.ibatis.common.util.PaginatedList, which defines these methods:
>
> nextPage(), previousPage(), gotoPage(), isFirstPage(), isMiddlePage(),
> isLastPage(), etc.
>
> You define the number of elements on a page and it will give you an
> iterator which only iterates across the current page.
>
> Also, if you'd prefer to get pages from the persistence layer as they are
> requested (instead of returning all rows at once) you can use
> com.ibatis.db.sqlmap.PaginatedDataList which implements the same
> interfaces.
>
> Not that displaytag should be tied to iBATIS, but on the other hand it
> seems to me that the type of feature being requested is more of a
> persistence layer issue than a display issue. In other words, by the time
> the Collection gets exposed to displaytag, it should already be in a
> manageable form. The business logic / objects which perform this
> management, in my opinion, fall outside the scope of a tag library.
>
> The tricky part is what to do about auto-sorting by column. Perhaps
> displaytag could offer a stub interface method which gets called when the
> user asks to sort by a different column (to override the internal sorting
> logic), which developers could implement with calls into the persistence
> layer. This would allow a well designed application to get only 1 page at
> a time from the database, allowing the database to do the sorting and
> cutting down on overhead in the middle tier.
>
> So, in conclusion, I think iBATIS can already do alot of what is being
> asked for.
>
> Chris.
>
>> This sounds like a good idea. I've been thinking about various ways to
>> implement this as well. I think I agree with most of this. I think that
>> if
>> we made the DataSet object implement the Iterator interface, we could
>> encapsulate all the database paging/sorting code in that object, and the
>> TableTag still just treats it as an Iterator, which would mean very
>> minimal changes to the existing code.
>>
>> The only problem I see with this approach is that the implementation
>> will
>> be tied to the database you are using. Each database may handle
>> paging/sorting differently, so that will need to be taken into account.
>> Maybe what you end up doing is creating a set of DatabaseDriver classes,
>> that could control any of the per-database oddities. You could try
>> sticking to the SQL spec, but there will always be more efficient ways
>> to
>> perform these operations with database-specific addons.
>>
>> I'd still like to split out the different functionalites of this tag
>> into
>> at least 3 or 4 groups:
>>
>> 1. displaying data
>> 2. paging data
>> 3. sorting data
>> 4. decorating data
>>
>> If we could do this, then each piece could be extended for user-specific
>> things, and we could provide all the common things with the binary
>> distribution.
>>
>> Just my $.02 as always...
>>
>> John
>>
>>
>> On Fri, 16 Apr 2004, Alex Burgel wrote:
>>
>>> ok, i see.
>>>
>>> this design makes it almost impossible to handle large data sets.
>>> especially
>>> things coming out of a database. i've been thinking about some ideas to
>>> get
>>> around this. i'll just throw them out here to get some comments. my
>>> apologies if these have been brought up before, i'm new to the list.
>>>
>>> the trouble with dealing with large datasets is that many of the
>>> features
>>> that the tag provides are more efficiently handled outside. like
>>> sorting
>>> and
>>> paging are better handled inside a database. but thats only for things
>>> that
>>> comes from a database, for everything else, its useful that the tag
>>> does
>>> it
>>> for you. so any solution would mean that you have to support both ways,
>>> having the db do these things and having the tag do it.
>>>
>>> the idea is to factor out all the handling of the data into a separate
>>> interface, maybe called DataSet. users could provide different
>>> implementations of the interface that could interact with their
>>> database
>>> to
>>> do the sorting, paging, etc.
>>>
>>> the TableTag would be modified to accept either a list or an object
>>> implementing the above interface. if it gets a list, then the TableTag
>>> would
>>> use a default implemenation of the interface, otherwise it would just
>>> use
>>> the object. you would move all the TableModel stuff inside the default
>>> implementation.
>>>
>>> so the TableTag methods would look something like this:
>>>
>>> doStartTag() {
>>>
>>> Object data = ... get from jsp tag attribute 'name'
>>> dataSet = null;
>>>
>>> if (data instanceof DataSet)
>>> dataSet = data
>>> else if (data instanceof List)
>>> dataSet = new DataSetListImpl(list)
>>>
>>> dataSet.setSort(sort)
>>> dataSet.setPage(page)
>>> dataSet.setPageSize(pagesize)
>>>
>>> return doIteration();
>>> }
>>>
>>> doIteration() {
>>> # very similar to current impl
>>> # except it gets the next object from the dataSet
>>> # but all the TableModel stuff is internal to DataSet
>>>
>>> # the dataSet can choose to have all the rows iterated over
>>> # or it can have only the ones that i knows will be displayed
>>> # since it already knows the paging information
>>> }
>>>
>>> doEndTag() {
>>> out.print(dataSet.getHtmlData())
>>>
>>> # or export it in another format
>>> }
>>>
>>> since dataSet already knows what the page size and sorting, etc. is, it
>>> knows which html data to display.
>>>
>>> i'm sure there are tons of things that i've missed. but my point is to
>>> move
>>> a lot of these things to a user provided object so people with larger
>>> datasets can do optimizations, while providing a default implementation
>>> for
>>> people who just have a simple list.
>>>
>>>
>>> btw, i'm happy to implement this, if people think it would be of some
>>> use.
>>>
>>> --alex
>>>
>>> > Date: Thu, 15 Apr 2004 12:50:26 -0400 (EDT)
>>> > From: John York <[EMAIL PROTECTED]>
>>> > To: "[EMAIL PROTECTED]"
>>> > <[EMAIL PROTECTED]>
>>> > Subject: Re: [displaytag-devel] why iterator over undisplayed rows?
>>> > Reply-To: [EMAIL PROTECTED]
>>> >
>>> > sorting. You need to look at all the data otherwise you can't sort
>>> the
>>> > data or provide the sorting links in the table headers.
>>> >
>>> > We run into the problem all the time at my company, because we'd
>>> really
>>> > like a more efficient way to handle large sets of data. It may be
>>> > possible
>>> > to optimize the tag when sorting isn't used, Fabrizio, do you have
>>> any
>>> > idea if that would work?
>>> >
>>> >
>>> > On Thu, 15 Apr 2004, Alex Burgel wrote:
>>> >
>>> > > hi,
>>> > >
>>> > > i've been playing around with displaytag, trying to get it to
>>> > work with my
>>> > > set up. i have a question about its design.
>>> > >
>>> > > in TableTag, it seems to iterate over all the items in the list
>>> > to populate
>>> > > the TableModel. once its populated, in doEndTag() it finds out
>>> > which ones
>>> > > are being displayed and then writes them out.
>>> > >
>>> > > it seems a bit unnecessary to do all this extra processing for
>>> > the rows that
>>> > > won't be displayed, especially since at the point of doStartTag
>>> > you already
>>> > > know which rows will be displayed because you know the page
>>> > size and which
>>> > > page you're on.
>>> > >
>>> > > is there a reason for this that i'm not seeing?
>>> > >
>>> > > the reason i ask is because i'm trying to come up with a way to
>>> > allow the
>>> > > tag to accept partial lists but still provide all the features
>>> > like paging,
>>> > > etc.
>>> > >
>>> > > thanks.
>>> > >
>>> > > --alex
>>> > >
>>> > >
>>> > >
>>> > >
>>> > > -------------------------------------------------------
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>>> of
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>>> > >
>>> >
>>> > --
>>> > John York
>>> > Software Engineer
>>> > CareerSite Corporation
>>> >
>>>
>>>
>>>
>>>
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>>
>> --
>> John York
>> Software Engineer
>> CareerSite Corporation
>>
>>
>>
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>>
>
>
>
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