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Organizing with Spring and data access objects
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some of the problems this implementation has, and what we can do to improve it.
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7.2 Analyzing the DAO
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The simple DAO implementation we examined in the previous section has a few problems, some of which you may already have already picked up on. Let s take a look.
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7.2.1 Boilerplate code
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Listing 7.1 includes lots of resource management and exception-handling code. Each method has to open a session, start a transaction, do its business operation, commit a transaction, handle rollbacks, and finally close the session. Each of the methods looks basically like this:
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try { startOperation(); session.save(event); tx.commit(); } catch (HibernateException e) { handleException(e); } finally { HibernateFactory.close(session); }
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The line session.save(event); is essentially the only one that changes between methods. Even refactoring out a few convenience methods, such as startOperation() and handleException(), doesn t completely rid you of boilerplate code. One potential solution is the Layer Supertype pattern, discussed in section 7.3.
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7.2.2 Potential duplication
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Adding new DAOs could easily become a rat s nest of copy-and-paste reuse. If we need another DAO, say LocationDao, we would have to copy and paste EventDao and change a relatively few lines of code in
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Analyzing the DAO
each method. Since we know duplication is the root of all programming evil, clearly something must be done. This too can be helped by the Layer Supertype pattern.
7.2.3 Detached objects only
Since each method is working with a single session and transaction, all of the Events the DAO works with are strictly detached objects. This behavior might be fine, but it doesn t take advantage of Hibernate s automatic dirty object checking or the session-level object cache. For example, suppose a client writes the following:
Event foundEvent = eventDao.find(event.getId()); foundEvent.setDuration(30); eventDao.update(foundEvent);
Here the find() occurs in one session and the update() occurs in another. This will certainly work, but it would be really nice if the find and update methods could somehow share a single session. Also, it would be preferable to avoid cluttering up the method signatures passing around a session. While this works, it s ugly, so we don t want to see something like this:
Session session = HibernateFactory.openSession(); Event foundEvent = eventDao.find(event.getId(), session); foundEvent.setDuration(30); eventDao.update(foundEvent, session);
Adding Session parameters to the methods forces the responsibility, management, and sharing of sessions onto the client code, which increases coupling, complexity, and potential for errors. One of the potential solutions to this problem is a pattern known as the Thread Local Session. This pattern is covered in chapter 8 so we aren t going to directly cover it here. Instead, in a moment, we examine another framework, Spring, which uses the Thread Local Session pattern under the covers.
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7.3 The Layer Supertype pattern
Conventional J2EE wisdom says that an application should be divided into layers. Which layers your application is supposed to have does, of course, depend on which book you are reading. Some popular choices for layers are as follows: The Presentation layer, where all user interaction and presentation code goes The Domain layer, where all the business logic goes The Persistence layer, where our data storage access code goes Regardless of which layers your application has, it s very common that each object in that layer have some common code that could be consolidated into a single class. This notion gives rise to the Layer Supertype pattern, where each layer has a type that acts as the supertype for all types in its layer. 2 You can use the Layer Supertype pattern to simplify your DAO.
The sample hierarchy in figure 7.2 shows the layer supertype AbstractDao, which provides the protected methods that the subclasses override and make public.
AbstractDao #saveOrUpdate() #find() #delete()
EventDao +saveOrUpdate() +find() +delete()
LocationDao +saveOrUpdate() +find() +delete()
Figure 7.2 A diagram of the layer supertype AbstractDao
From Patterns of Enterprise Application Architecture, by Martin Fowler (Addison-Wesley Professional, 2003).
The Layer Supertype pattern
The next step is to actually create the AbstractDao, which you ll do in the next section.
7.3.1 Creating an AbstractDao
The first step to creating your supertype is writing an AbstractDao, which all the DAOs will ultimately extend. Listing 7.2 shows how that class might look. Listing 7.2 A layer supertype implementation, AbstractDao, which has the common operations all DAOs need
package com.manning.hq.ch07; import org.hibernate.HibernateException; import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.Transaction; import java.util.List; /** * A layer supertype that handles the common operations for all * Data Access Objects. */ public abstract class AbstractDao { private Session session; private Transaction tx; public AbstractDao() { HibernateFactory.buildIfNeeded(); } protected void saveOrUpdate(Object obj) { try { startOperation(); session.saveOrUpdate(obj); tx.commit(); } catch (HibernateException e) { handleException(e); } finally { HibernateFactory.close(session); } }
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