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Querying persistent objects
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Hibernate stores objects in a second-level cache based on the object s class type and id. The iterate methods can be more efficient if the object is already cached, since only the matching object ids are returned on the initial query and the remainder of the object is retrieved from the cache. The find methods in the Session interface are ideal for simple queries. However, most applications typically require at least a handful of complex queries. The Query interface provides a rich interface for retrieving persistent objects with complicated queries. If you re using Hibernate 3, you must use the Query interface since the find methods in the Session interface have been deprecated (although they have been moved and are still available in the org.hibernate.class.Session subinterface). Hibernate 3 applications should use createQuery() and get NamedQuery() for all query execution.
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6.1.2 The Query interface
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Instances of the Query interface are created by the Session. The Query interface gives you more control over the returned objects, such as limiting the number of returned objects, setting a timeout for the query, and creating scrollable result sets. Let s execute our previous query using the Query interface:
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Query q = session.createQuery("from Event"); List results = q.list();
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If you want to set bounds on the number of Event objects to return, set the maxResults property:
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Query q = session.createQuery("from Event"); q.setMaxResults(15); List results = q.list();
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The Query interface also provides an iterate() method, which behaves identically to Session.iterate( ). Another feature of the Query
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Using HQL
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interface is the ability to return query results as a ScrollableResults object. The ScrollableResults object allows you to move through the returned objects arbitrarily, and is typically useful for returning paged collections of objects, commonly found in web applications. Of course, our static queries aren t very useful in real applications. Real applications need to populate query parameters dynamically. JDBC s PreparedStatement interface supports setting positional query parameters dynamically, but populating queries can be cumbersome. Developers must know the type of each parameter in order to call the correct setter method in the interface. They also have to keep track of which positional parameter they are setting. Hibernate expands and improves on this notion by providing both positional and named query parameters.
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Positional parameters
Positional parameters in Hibernate are very similar to their PreparedStatement counterparts. The only significant difference is that the position index starts at 0 instead of 1, as in the PreparedStatement. Suppose you want to return all of the Event instances with a certain name. Using a positional parameter, your code would look like
Query q = session.createQuery("from Event where name = "); q.setParameter(0, "Opening Plenary"); List results = q.list();
Note that you didn t need to set the type of parameter; the Query object will attempt to determine the type using reflection. It s also possible to set a parameter to a specific type using the org.hibernate.Hibernate class:
q.setParameter(0, "Opening Plenary", Hibernate.STRING);
Named parameters are a more interesting, and more powerful, way to populate queries. Rather than using question marks for parameter placement, you can use distinctive names.
Querying persistent objects
Named parameters
The easiest way to explain named parameters is with an example. Here s our earlier query with a named parameter:
from Event where name = :name
Instead of the to denote a parameter, you can use :name to populate the query:
Query q = session.createQuery("from Event where name = :name"); q.setParameter("name", "Opening Plenary"); List results = q.list();
With named parameters, you don t need to know the index position of each parameter. Named parameters may seem like a minor feature, but they can save time when populating a query instead of counting question marks and making sure you re populating the query correctly, you simply match the named parameters with your code. If your query has a named parameter that occurs more than once, it will be set in the query each time. For instance, if a query has the parameter startDate twice, it will be set to the same value:
Query q = session.createQuery("from Event where "+ "startDate = :startDate or endDate < :startDate"); q.setParameter("startDate", eventStartDate); List results = q.list();
We ve covered the two styles of query parameters supported by Hibernate. For the purpose of our examples, we ve been displaying the queries as hardcoded in application code. Anyone who s built an application will know that embedding your queries can quickly create a maintenance nightmare. Ideally, you would store the queries in a text file, and the most natural place to do that with Hibernate is in the mapping definition file.
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