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Collections and custom types
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While similar to components, custom user types insulate your application from changes made to the persistent object. If a component changes, you are forced to update the mapping files using the component. If changes are made to a custom user type, you only need to update the implementing user type class.
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Querying persistent objects
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Querying persistent objects using Hibernate The Hibernate Query Language
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ith a solid grasp of Hibernate basics, we need to move on to querying our persistent objects. Instead of SQL, Hibernate has its own, objectoriented (OO) query language called Hibernate Query Language (HQL). HQL is intentionally similar to SQL so that it leverages existing developer knowledge and thus minimizes the learning curve. It supports the commonly used SQL features, wrapped into an OO query language. In some ways, HQL is easier to write than SQL because of its OO foundation.
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Why do we need HQL While SQL is more common and has been standardized, vendor-dependent features limit the portability of SQL statements between different databases. HQL provides an abstraction between the application and the database, and so improves portability. Another problem with SQL is that it is designed to work with relational tables, not objects. HQL is optimized to query object graphs. This chapter introduces you to HQL gradually and quickly moves on to more complicated features and queries. First we ll cover the major
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concepts important to using HQL, such as executing queries using a few different classes. After the introductory material is covered, we ll spend the rest of the chapter with HQL examples. If you have a solid grasp of SQL, you shouldn t have any problem picking up the key concepts.
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We have three primary goals in this chapter:
Exploring the basics of HQL, including two query mechanisms Identifying variations in HQL queries, including named and positional parameters Understanding how to query objects, associations, and collections
Assumptions
Since HQL is based on SQL, we anticipate that you
Understand SQL basics, including knowledge of joins, subselects, and functions. Have a firm grasp of JDBC, including the PreparedStatement and ResultSet interfaces.
6.1 Using HQL
Hibernate queries are structured similar to their SQL counterparts, with SELECT, FROM, and WHERE clauses. HQL also supports subselects if they are supported by the underlying database. Let s jump in with the most basic query we can create:
from Event
This query will return all of the Event instances in the database, as well as the associated objects and non-lazy collections. (You ll recall from chapter 5 that, by default, persistent collections are populated only when initially accessed.) The first thing you probably noticed was the lack of the SELECT clause in front of the FROM clause. Because we
Using HQL
want to return complete objects, the SELECT clause is implied and doesn t need to be explicitly stated. How can we execute this query Two methods are provided in the Hibernate API to execute queries. The Session interface has three find( ) methods that can be used for simple queries. The Query interface can be used for more complex queries.
6.1.1 session.find( )
In Hibernate 2, the Session interface has three overloaded find( ) methods, two of which support parameter binding. Each of the methods returns a java.util.List with the query results. For instance, let s execute our earlier query:
List results = session.find("from Event");
The Session interface also has a set of methods, named iterate( ), which are similar to the find( ) methods. Although they appear to be the same, each of methods functions differently. The find methods return all of the query results in a List, which is what you d expect. The objects in the list are populated when the query is executed. However, the iterate methods first select all of the object ids matching a query and populate the objects on demand as they are retrieved from the Iterator. Here s an example:
Iterator results = session.iterate("from Event"); while (results.hasNext()) { Event myEvent = (Event) results.next(); // }
When the Iterator is returned, only the id values for the Event instances have been retrieved. Calling results.next() causes the next Event instance to be retrieved from the database. The iterate methods are typically less efficient than their find counterparts, except when dealing with objects stored in a second-level cache.
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