Just had an interesting discussion on ejb3-feedback@sun.com, started by David Cherryhomes, which saw me stupidly insistingthat something can't be done when in fact, now that I think about it, /I realize I've actually done it before/, and that even the Hibernate AdminApp example uses this pattern!
So, just so I don't forget this pattern again, I'm going to write it down, and also write a reuseable class implementing it.
The basic problem is pagination. I want to display next
and previous
buttons to the user, but disable them if there are
no more, or no previous query results. But I don't want to retrieve all the query results in each request, or execute a
separate query to count them. So, here's the correct approach:
public class Page {
private List results;
private int pageSize;
private int page;
public Page(Query query, int page, int pageSize) {
this.page = page;
this.pageSize = pageSize;
results = query.setFirstResult(page * pageSize)
.setMaxResults(pageSize+1)
.list();
}
public boolean isNextPage() {
return results.size() > pageSize;
}
public boolean isPreviousPage() {
return page > 0;
}
public List getList() {
return isNextPage() ?
results.subList(0, pageSize-1) :
results;
}
}
You can return this object to your JSP, and use it in Struts, WebWork or JSTL tags. Getting a page in your persistence logic is as simple as:
public Page getPosts(int page) {
return new Page(
session.createQuery("from Posts p order by p.date desc")
page,
40
);
}
The Page class works in both Hibernate and EJB 3.0.
