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4.4.1 Retrieving objects by identifier
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The following Hibernate code snippet retrieves a User object from the database:
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User user = (User) session.get(User.class, userID);
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The get() method is special because the identifier uniquely identifies a single instance of a class. Hence it s common for applications to use the identifier as a convenient handle to a persistent object. Retrieval by identifier can use the cache when retrieving an object, avoiding a database hit if the object is already cached. Hibernate also provides a load() method:
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User user = (User) session.load(User.class, userID);
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The load() method is older; get() was added to Hibernate s API due to user request. The difference is trivial:
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If load() can t find the object in the cache or database, an exception is thrown. The load() method never returns null. The get() method returns null if the object can t be found.
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Retrieving objects
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The load() method may return a proxy instead of a real persistent instance. A proxy is a placeholder that triggers the loading of the real object when it s accessed for the first time; we discuss proxies later in this section. On the other hand, get() never returns a proxy.
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Choosing between get() and load() is easy: If you re certain the persistent object exists, and nonexistence would be considered exceptional, load() is a good option. If you aren t certain there is a persistent instance with the given identifier, use get() and test the return value to see if it s null. Using load() has a further implication: The application may retrieve a valid reference (a proxy) to a persistent instance without hitting the database to retrieve its persistent state. So load() might not throw an exception when it doesn t find the persistent object in the cache or database; the exception would be thrown later, when the proxy is accessed. Of course, retrieving an object by identifier isn t as flexible as using arbitrary queries.
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4.4.2 Introducing HQL
The Hibernate Query Language is an object-oriented dialect of the familiar relational query language SQL. HQL bears close resemblances to ODMG OQL and EJB-QL; but unlike OQL, it s adapted for use with SQL databases, and it s much more powerful and elegant than EJB-QL (However, EJB-QL 3.0 will be very similar to HQL.) HQL is easy to learn with basic knowledge of SQL. HQL isn t a data-manipulation language like SQL. It s used only for object retrieval, not for updating, inserting, or deleting data. Object state synchronization is the job of the persistence manager, not the developer. Most of the time, you ll only need to retrieve objects of a particular class and restrict by the properties of that class. For example, the following query retrieves a user by first name:
Query q = session.createQuery("from User u where u.firstname = :fname"); q.setString("fname", "Max"); List result = q.list();
After preparing query q, we bind the identifier value to a named parameter, fname. The result is returned as a List of User objects. HQL is powerful, and even though you may not use the advanced features all the time, you ll need them for some difficult problems. For example, HQL supports the following:
Working with persistent objects
The ability to apply restrictions to properties of associated objects related by reference or held in collections (to navigate the object graph using query language). The ability to retrieve only properties of an entity or entities, without the overhead of loading the entity itself in a transactional scope. This is sometimes called a report query; it s more correctly called projection. The ability to order the results of the query. The ability to paginate the results. Aggregation with group by, having, and aggregate functions like sum, min, and max. Outer joins when retrieving multiple objects per row. The ability to call user-defined SQL functions. Subqueries (nested queries).
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