Writing paging (i.e. next, previous, etc) iterators can be a real pain to do
correctly (at least it is for me).
Looking at the petstore, they seem to wuss out on doing it nice. It occured
to me that there are a few options as far as doing iteration over pages.
1) retrieve all results and keep them in memory (in a session var i guess)
while the user pages through them.
2) go to the db every time.
3) JDBC 2.0 allows scrollable recordsets so you could have time-out-able
recordsets that essentially close themselves
on a time expiration and may reopen themselves if needed. (May be some
nasty concurrency problems with this)
4) create a hybrid that will cache some of the pages and go to the database
if needed. Maybe be able to configure
the number of pages to cache. Really cool would be able to change it
dynamically.
5) somebody elses neat idea
But whatever is used there is the basic principle of getEntities(int index,
int span). So what do you guys think about
a series of paging iterators that looked something like this.
abstract class PagingIterator
abstract List getEntities(int startIndex, int span)
int getPage()
void setPage(int page)
boolean hasNextPage()
void nextPage()
boolean hasPreviousPage()
void previousPage()
class RequeryPagingIterator extends PagingIterator // like the petstore
abstract List getEntities(int startIndex, int span)
class CacheingPagingIterator extends PaginingIterator // good for a lot
of smaller datasets
List _AllEntities = null;
List getEntities(int startIndex, int span){
if(_AllEntities == null){
getAllEntities();
}
int stopIndex = max(startIndex + span, _AllEntities.size() - 1);
return _AllEntites.subList(startIndex, stopIndex);
}
class MultiplePageCachingPagingIterator extends PagingIterator // great
little combination
...
class ResultSetPagingIterator extends PagingIterator { // little new to
JDBC 2 so not sure bout this (suggestions)
... // used with JDBC 2.0
}
The cool thing is the tags on the JSP's all work with PagingIterator so
there is no need for the page to know how
the paging is implemented.
Some other things. Currently i have a messy implementation of this running
inside struts. Works ok. but basically some
of the challenges include
1) knowing when the query is new and when its navigation. currently I check
for the presense of a navigate in
the request params.
2) where and when to stash the iterator. Currently I'm stashing it in the
session using some other custom tags.
3) should the getEntities thing be done as an interface instead of an
abstract method? e.g
new SomePagingIterator(ChunkableRetriever c) where ChunkableRetriever has
a method getEntities(int start, int span)
3) how do you cleanly handle errors (I suck at error handeling in JSPs)
Here's an example how its being used.
ItemsByCategoryPagingIterator extends RequeryPagingIterator {
String _CategoryId = null;
Collection getEntities(int start, int span) throws SomeException {
Collection col = null;
try {
col = itemsHome.findByCategoryId(getCategoryId());
}catch(BusinessException e){
throw new SomeException(e.getMessage());
}
return col;
}
}
Then i have some definition code that initiates sets all the properties
(categoryId in the above example)
and then sends it off to the jsp to do its navigation.
There is also
<list:chunk iterator="wheretheiteratorisstashed" collectionTarget="items"
navigate="<%= navoption from request %>" >
<logic:iterate id="eachitem" name="items">
present the properties of eachitem
</logic:iterate>
<list:previous>something to display if there is a previous
(link)</list:previous>
<list:next>something to display if there is a next (link) </list:next>
</list:chunk>
Disclaimer:
I am not an architecht. Moreover, I am just out of college and still wet
behind the ears, but I would be glad to help
out with this. I would love to hear what you guys think.
Thanks a bunch in advance and keep up the great work.
Brian
--
To unsubscribe, e-mail: <mailto:[EMAIL PROTECTED]>
For additional commands, e-mail: <mailto:[EMAIL PROTECTED]>
