Persisting a domain model with Hibernate 3 - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 209

6.2 Other Hibernate issues

In addition to these O/R mapping issues, there are other issues with Hibernate
that can make writing applications difficult.

6.2.1 Exception handling

One potential problem with how Hibernate handles errors is that when it encoun-
ters an error, it throws a HibernateException and leaves the Session in a poten-
tially inconsistent state. According to the Hibernate documentation, the
application must close the Session immediately, which can sometimes make
recovering from exceptions unnecessarily complicated if the application wants to
continue using the Session . For more information, see chapter 8, which describes
how to recover from errors in an application that uses an exposed domain model.
Fortunately, this isn't a problem when using a POJO façade because the applica-
tion uses a new Session each time the façade is called.

6.2.2 Lazy loading and inheritance hierarchies

As we saw in chapter 4, lazy loading is an important feature of an ORM frame-
work. Hibernate provides a couple of ways to do lazy loading. The simpler
approach is to use Hibernate's proxy-based mechanism, which is enabled by
default in Hibernate 3. When using proxies, a reference to a lazily loaded object
is actually a reference to a proxy that will load the real object the first time one of
its methods is called. The trouble with proxies is that, as we describe a bit later,
they break code that uses instanceof or downcasting, which are two important
features of the Java language.
Alternatively, if your application needs objects to use instanceof and down-
casting, then you can use lazy property loading. This mechanism works by loading
objects only when the property that references them is first accessed. The draw-
back of lazy property loading is that it is less convenient because you must run a
bytecode enhancer that modifies the classes to intercept references to properties.
In addition, Hibernate can generate suboptimal SQL to load an object referenced
by a lazily loaded property. Let's look at the details of these two mechanisms.

Using proxies with instanceof and downcasting
Let's imagine that you are writing some presentation tier code to display a pend-
ing order and that how you display a coupon depends on its actual class. You will
probably write some code similar to this: