[ http://jira.codehaus.org/browse/DISPL-19?page=all ]
fabrizio giustina closed DISPL-19:
----------------------------------
Resolution: Duplicate
Fix Version: 1.1
see DISPL-122 and DISPL-129
> Pagination - use subList instead of iterating all records
> ---------------------------------------------------------
>
> Key: DISPL-19
> URL: http://jira.codehaus.org/browse/DISPL-19
> Project: DisplayTag
> Type: Improvement
> Components: Paging/Sorting
> Versions: 1.0 RC2
> Reporter: fabrizio giustina
> Fix For: 1.1
> Attachments: LongListPagingDraftChanges.zip
>
>
> ====
> imported from sf tracker
> id 1023387
> submitted by Ian Barnett - ianbdev
> http://sourceforge.net/tracker/index.php?func=detail&group_id=73068&atid=536613&aid=1023387
>
> ====
> The code that obtains a subset of records from the List
> object (which is the subject of the display table)
> iterates through all the records in the list until it finds
> the ones it requires.
> This is no problem for small lists where the subset might
> be page 2 as 21-40 for example as only 20 records are
> skipped during iteration.
> But when the lists are very large (for example 20,000
> records) and the page is a high numbered page (e.g.
> page 9001, records 18001-18021) the iteration is very
> expensive as it must iterate through and skip over
> 18,000 records just to get to the first record.
> We have a simple modified verision of SmartListHelp.java
> (based on the 1.0rc-1 library) that does not iterate
> through unwanted records but rather uses the List
> interface's subList method to retrieve only the records
> required.
> Then it is up to the application developer to either
> burden the list with all (for example 20,000) records or
> implement a List class that provides a subList method
> that intelligently retrieves only the records required from
> the List's datasource (e.g. may only grab 20 records at
> a time for the database).
> This is a very lightweight solution to the calls for
> improved long list pagination that has a low impact on
> the Display Tag code base.
> Below is the current code for paginated record retrieval:
> /**
> * Returns a subsection of the list that contains just
> the elements that are supposed to be shown on the
> given page.
> * @param pageNumber page number
> * @return List subsection of the list that contains
> just the elements that are supposed to be shown on the
> given
> * page
> */
> protected List getListForPage(int pageNumber) {
> log.debug("getListForPage page=" + pageNumber);
> List list = new ArrayList(this.pageSize + 1);
> int firstIndex = getFirstIndexForPage(pageNumber);
> int lastIndex = getLastIndexForPage(pageNumber);
> Iterator iterator = this.fullList.iterator();
> int j = 0;
> while (iterator.hasNext()) {
> Object object = iterator.next();
> if (j > lastIndex) {
> break;
> } else if (j >= firstIndex) {
> list.add(object);
> }
> j++;
> }
> return list;
> }
> The recommended fix is:
> protected List getListForPage(int pageNumber) {
> log.debug("getListForPage page=" + pageNumber);
> int firstIndex = getFirstIndexForPage(pageNumber);
> int lastIndex = getLastIndexForPage(pageNumber);
> return fullList.subList(firstIndex, lastIndex+1);
> }
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