Implementing dynamic paged queries - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 426

implementations such as Kodo JDO let you specify whether it should navigate the
result set or use a SELECT statement with a ROWNUM -like feature. However, there is
no guarantee that all JDO implementations provide this capability. As a result, you
might need to use i BATIS or JDBC instead.
In this section, we provide an overview of how to implement efficient paged
queries with Hibernate and JDO object queries. You will learn about what support,
if any, Hibernate and JDO provide for dynamically generating object queries. We
describe how to persuade the persistence framework to execute an object query
using a SELECT statement similar to those that a JDBC or i BATIS application would
use. You'll learn how JDO and Hibernate applications can implement pagination.
In sections 11.4 and 11.5 you will see how to apply the techniques described here
in more detail. In those sections we also describe how to use JDO and Hibernate
native SQL queries.

11.3.1 Generating Hibernate and JDO queries dynamically

One important aspect of implementing a search screen is generating queries
dynamically from the search criteria entered by the user. Ideally, you want to avoid
writing messy repository code that generates object queries by concatenating
query fragments. Unfortunately, JDO lacks support for dynamically generating
queries. Only Hibernate has API s for dynamically generating object queries. Let's
see how JDO and Hibernate repositories dynamically generate queries.

Dynamically generating JDO queries
A JDO application must generate an object query by concatenating fragments of
the JDO query language ( JDOQL ), which makes the code messy and error-prone.
Here is an example of a code fragment that constructs the WHERE clause of a query
from an OrderSearchCriteria :

…
public class JDOOrderRepositoryImpl … {

public PagedQueryResult findOrders(int startIndex,
int pageSize, OrderSearchCriteria criteria) {
…
StringBuffer where = new StringBuffer();
Map parameters = new HashMap();
if (criteria.isDeliveryCitySpecified()) {
if (where.length() != 0)
where.append(" && ");
where.append("deliveryAddress.city == :pDeliveryCity");
}
if (criteria.isRestaurantSpecified()) {
if (where.length() != 0)