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5.1.4 Lazy collections
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We ve mentioned lazy collections a few times without going into them in detail. A lazy collection is populated on demand. By on demand, we mean that the collection of entity objects or values is populated only when the application accesses the collection. Populating the collection happens transparently when the collection is first accessed. Why have lazy collections at all A collection of objects can possibly contain hundreds or thousands of objects, and the application may not need to access the collection immediately, or at all. Loading hundreds of persistent objects for no reason will certainly impact application performance. It s better to populate the persistent collection only when it is needed. To populate a lazy collection, the same Session instance used to retrieve the persistent object from the database must be open when the collection is populated. The following code incorrectly attempts to load the collection of attendees for an Event after the Session has been closed:
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Session session = factory.openSession(); Event event = session.get(Event.class, eventId); session.close(); Set attendees = event.getAttendees();
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First, line 1 opens the Session instance. Then, the second line retrieves the Event instance, and line 3 closes the Session. Finally, line 4 attempts to access the collection of Attendees. This line will throw a LazyInitializationException because the Session is closed. If you re getting this exception in your code, it means that you re trying to access
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Persisting collections and arrays
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a lazy collection when the Session is closed. Here is the correct method to populate a lazy collection:
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Session session = factory.openSession(); Event event = session.get(Event.class, eventId); Set attendees = event.getAttendees(); session.close();
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First, the code opens the Session instance and retrieves the Event instance. Then it accesses the collection of Attendees and closes the open Session. Persistent collections are now lazy by default in Hibernate 3. For nonlazy collections, you must explicitly declare them as lazy="false" in the mapping definition. For example:
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<set name="attendees" lazy="false"> <key column="event_id"/> <one-to-many class="Attendee"/> </set>
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Any collection, including collections of values and arrays, can be declared not to be lazy. The obvious problem with lazy collections is populating them in a multitier application, such as a web application, where keeping the Session open can be tricky. We present a few solutions to this problem in chapter 8. In addition to being lazily populated, collections can be sorted. Sorted collections are the topic of the next section.
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5.1.5 Sorted collections
A common requirement when dealing with collections is to sort them according to some criteria. The sorting criteria can be fixed or arbitrary, depending on application requirements. Hibernate supports sorting sets, maps, and bags. If you always want a collection returned in the same order, you can take advantage of the order-by attribute in
Collections and custom types
the <set> element. For instance, to return all Attendees for an Event ordered by their last name, our mapping definition would be
<set name="attendees" order-by="last_name"> <key column="event_id"/> <one-to-many class="Attendee"/> </set>
The ordering value is the name of the SQL column, not the HQL property. The SQL column is given because the ordering takes place in the database. We can expand our ordering clause to sort by the first name as well:
order-by="last_name, first_name"
You can specify the type of sort, ascending or descending, by including the SQL keywords asc or desc, respectively, with desc as the default:
order-by="last_name, first_name asc"
Sorted collections use the LinkedHashMap or LinkedHashSet classes, which are only available in JDK 1.4 or later. If JDK 1.4 is not available in your environment, or you want to order the collections yourself, you may use the sort attribute to specify the type of sort to perform. The sort attribute can take one of three values: unsorted, natural, or the name of a class implementing the java.util.Comparator interface. Unsorted results are returned in the order returned by the database. The natural sorting of elements is determined using the compareTo(Object) method in the java.lang.Comparable interface. A number of objects in the Java API, such as String, Long, and Double, implement Comparable. Here is an example of using compareTo(Object) with Strings:
String a = "a"; String b = "b"; System.out.println(a.compareTo(b));
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