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The Session interface allows you to create Query objects to retrieve persistent objects. (In Hibernate 2, the Session interface supported a number of overloaded find methods. They were deprecated in Hibernate 3.) HQL statements are object-oriented, meaning that you query on object properties instead of database table and column names. Let s look at some examples using the Query interface. This example returns a collection of all Event instances. Notice that you don t need to provide a select ... clause when returning entire objects:
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Query query = session.createQuery("from Event"); List events = query.list();
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In chapter 6, you ll see how the SELECT clause works with HQL. This query is a little more interesting since we re querying on a property of the Event class:
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Query query = session.createQuery("from Event where name = "+ "'Opening Presentation'"); List events = query.list();
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We ve hardcoded the name value in the query, which isn t optimal. Let s rewrite it:
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Query query = session.createQuery("from Event where name = ", "Opening Presentation"); query.setParameter(0, "Opening Presentation", Hibernate.STRING); List events = query.list();
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The question mark in the query string represents the variable, which is similar to the JDBC PreparedStatement interface. The second method parameter is the value bound to the variable, and the third parameter tells Hibernate the type of the value. (The Hibernate class provides constants for the built-in types, such as STRING, INTEGER, and LONG, so they can be referenced programmatically.)
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Hibernate basics
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One topic we haven t touched on yet is the cache maintained by the Session. The Session cache tends to cause problems for developers new to Hibernate, so we ll talk about it next.
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3.6 The Session cache
One easy way to improve performance within the Hibernate service, as well as your applications, is to cache objects. By caching objects in memory, Hibernate avoids the overhead of retrieving them from the database each time. Other than saving overhead when retrieving objects, the Session cache also impacts saving and updating objects. Let s look at a short code listing:
Session session = factory.openSession(); Event e = (Event) session.load(Event.class, myEventId); e.setName("New Event Name"); session.saveOrUpdate(e); // later, with the same Session instance Event e = (Event) session.load(Event.class, myEventId); e.setDuration(180); session.saveOrUpdate(e); session.flush();
This code first retrieves an Event instance, which the Session caches internally. It then does the following: updates the Event name, saves or updates the Event instance, retrieves the same Event instance (which is stored in the Session cache), updates the duration of the Event, and saves or updates the Event instance. Finally, you flush the Session. All the updates made to the Event instance are combined into a single update when you flush the Session. This is made possible in part by the Session cache. The Session interface supports a simple instance cache for each object that is loaded or saved during the lifetime of a given Session. Each object placed into the cache is keyed on the class type, such as
The Session cache
and the primary key value. However, this cache presents some interesting problems for unwary developers. A common problem new developers run into is associating two instances of the same object with the same Session instance, resulting in a NonUniqueObjectException. The following code generates this exception:
Session session = factory.openSession(); Event firstEvent = (Event) session.load(Event.class, myEventId); // ... perform some operation on firstEvent Event secondEvent = new Event(); secondEvent.setId(myEventId); session.save(secondEvent);
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This code opens the Session instance, loads an Event instance with a given ID, creates a second Event instance with the same ID, and then attempts to save the second Event instance, resulting in the NonUniqueObjectException. Any time an object passes through the Session instance, it s added to the Session s cache. By passes through, we re referring to saving or retrieving the object to and from the database. To see whether an object is contained in the cache, call the Session.contains() method. Objects can be evicted from the cache by calling the Session.evict() method. Let s revisit the previous code, this time evicting the first Event instance:
Session session = factory.openSession(); Event firstEvent = (Event) session.load(Event.class, myEventId); // ... perform some operation on firstEvent if (session.contains(firstEvent)) { session.evict(firstEvent); } Event secondEvent = new Event(); secondEvent.setId(myEventId); session.save(secondEvent);
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