The following comment has been added to this issue:
Author: James Schopp
Created: Thu, 14 Oct 2004 5:10 PM
Body:
We have implemented data-level pagination by creating a smart lists backed by a
database. While iterating over the list, it can load and unload records as necessary
to save memory.
I have run into the same probelm with DisplayTag since it iterates over the entire
list (causing the entire dataset to be loaded up) even if you only want to view some
small subset on page, say, 100.
I have not yet made any modifications to the source, but I think the changes you
mentioned plus the "lazy list" I have created could solve the problem.
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View this comment:
http://jira.codehaus.org/browse/DISPL-19?page=comments#action_25383
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View the issue:
http://jira.codehaus.org/browse/DISPL-19
Here is an overview of the issue:
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Key: DISPL-19
Summary: Pagination - use subList instead of iterating all records
Type: Improvement
Status: Unassigned
Priority: Major
Original Estimate: Unknown
Time Spent: Unknown
Remaining: Unknown
Project: DisplayTag
Components:
Tag Library
Versions:
1.0 RC2
Assignee:
Reporter: fabrizio giustina
Created: Sat, 25 Sep 2004 5:17 AM
Updated: Thu, 14 Oct 2004 5:10 PM
Description:
====
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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