spring-framework/spring-framework-reference/src/dao.xml

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<chapter id="dao">
	<title>DAO support</title>
	<section id="dao-introduction">
		<title>Introduction</title>
		<para>
			The Data Access Object (DAO) support in Spring is aimed at
			making it easy to work with data access technologies like
			JDBC, Hibernate or JDO in a consistent way. This allows one
			to switch between the aforementioned persistence technologies
			fairly easily and it also allows one to code without worrying
			about catching exceptions that are specific to each technology.
		</para>
	</section>
	<section id="dao-exceptions">
		<title>Consistent exception hierarchy</title>
		<para>
			Spring provides a convenient translation from technology-specific
			exceptions like <classname>SQLException</classname> to its own
			exception class hierarchy with the
			<classname>DataAccessException</classname> as the root exception.
			These exceptions wrap the original exception so there is never
			any risk that one might lose any information as to what might
			have gone wrong.
		</para>
		<para>
			In addition to JDBC exceptions, Spring can also wrap Hibernate-specific
			exceptions, converting them from proprietary, checked exceptions
			(in the case of versions of Hibernate prior to Hibernate 3.0), to
			a set of focused runtime exceptions (the same is true for JDO and
			JPA exceptions). This allows one to handle most persistence exceptions,
			which are non-recoverable, only in the appropriate layers, without
			having annoying boilerplate catch-and-throw blocks and exception
			declarations in one's DAOs. (One can still trap and handle exceptions
			anywhere one needs to though.) As mentioned above, JDBC exceptions
			(including database-specific dialects) are also converted to the
			same hierarchy, meaning that one can perform some operations with
			JDBC within a consistent programming model.
		</para>
		<para>
			The above holds true for the various template classes in Springs
			support for various ORM frameworks. If one uses the interceptor-based
			classes then the application must care about handling
			<classname>HibernateExceptions</classname> and
			<classname>JDOExceptions</classname> itself, preferably via delegating
			to <classname>SessionFactoryUtils</classname>'
			<methodname>convertHibernateAccessException(..)</methodname> or
			<methodname>convertJdoAccessException</methodname> methods respectively.
			These methods convert the exceptions to ones that are compatible
			with the exceptions in the <literal>org.springframework.dao</literal>
			exception hierarchy. As <classname>JDOExceptions</classname> are
			unchecked, they can simply get thrown too, sacrificing generic DAO
			abstraction in terms of exceptions though.
		</para>
		<para>
			The exception hierarchy that Spring provides can be seen below.
			(Please note that the class hierarchy detailed in the image
			shows only a subset of the entire
			<classname>DataAccessException</classname> hierarchy.)
		</para>
		<mediaobject>
			<imageobject>
				<imagedata fileref="images/DataAccessException.gif" align="center" />
			</imageobject>
		</mediaobject>
	</section>
	<section id="dao-abstract-superclasses">
		<title>Consistent abstract classes for DAO support</title>
		<para>
			To make it easier to work with a variety of data access technologies
			such as JDBC, JDO and Hibernate in a consistent way, Spring provides
			a set of <literal>abstract</literal> DAO classes that one can extend.
			These abstract classes have methods for providing the data source and
			any other configuration settings that are specific to the relevant
			data-access technology.
		</para>
		<itemizedlist>
			<listitem>
				<para>
					<classname>JdbcDaoSupport</classname> - superclass for JDBC data
					access objects. Requires a <interfacename>DataSource</interfacename>
					to be provided; in turn, this class provides a 
					<classname>JdbcTemplate</classname> instance initialized from the
					supplied <interfacename>DataSource</interfacename> to subclasses.
				</para>
			</listitem>
			<listitem>
				<para>
					<classname>HibernateDaoSupport</classname> - superclass for
					Hibernate data access objects. Requires a
					<interfacename>SessionFactory</interfacename> to be provided;
					in turn, this class provides a
					<classname>HibernateTemplate</classname> instance initialized
					from the supplied <interfacename>SessionFactory</interfacename>
					to subclasses. Can alternatively be initialized directly via a
					<classname>HibernateTemplate</classname>, to reuse the latters
					settings like <interfacename>SessionFactory</interfacename>,
					flush mode, exception translator, and so forth.
				</para>
			</listitem>
			<listitem>
				<para>
					<classname>JdoDaoSupport</classname> - super class for JDO data
					access objects. Requires a
					<interfacename>PersistenceManagerFactory</interfacename>
					to be provided; in turn, this class provides a
					<classname>JdoTemplate</classname> instance initialized from the
					supplied <interfacename>PersistenceManagerFactory</interfacename>
					to subclasses.
				</para>
			</listitem>
			<listitem>
				<para>
					<classname>JpaDaoSupport</classname> - super class for JPA data
					access objects. Requires a
					<interfacename>EntityManagerFactory</interfacename> to be provided;
					in turn, this class provides a <classname>JpaTemplate</classname>
					instance initialized from the supplied
					<interfacename>EntityManagerFactory</interfacename> to subclasses.
				</para>
			</listitem>
		</itemizedlist>
	</section>

</chapter>