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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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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, JPA 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 align="center" fileref="images/DataAccessException.gif" />
      </imageobject>
    </mediaobject>
  </section>

  <section id="dao-annotations">
    <title>Annotations used for configuring DAO or Repository classes</title>

    <para>The best way to guarantee that your Data Access Objects (DAOs) or
    repositories provide exception translation is to use the
    <interfacename>@Repository</interfacename> annotation. This annotation
    also allows the component scanning support to find and configure your DAOs
    and repositories without having to provide XML configuration entries for
    them.</para>

    <programlisting language="java"><emphasis role="bold">@Repository</emphasis>
public class SomeMovieFinder implements MovieFinder {

    // ...

}</programlisting>

    <para>Any DAO or repository need to access to a persistence resource,
    depending on the persistence technology used. The easiest way to
    accomplish this is to have this resource dependency injected using one of
    the <interfacename>@Autowired,</interfacename>
    <interfacename>@Resource</interfacename> or
    <interfacename>@PersistenceContext</interfacename> annotations. Here is an
    example for a JPA repository:</para>

    <programlisting language="java">@Repository
public class JpaMovieFinder implements MovieFinder {

    @PersistenceContext
    private EntityManager entityManager;
    
    // ...

}</programlisting>

    <para>If you are using the classic Hibernate APIs than you can inject the
    SessionFactory:</para>

    <programlisting language="java">@Repository
public class HibernateMovieFinder implements MovieFinder {

    private SessionFactory sessionFactory;
    
    @Autowired
    public void setSessionFactory(SessionFactory sessionFactory) {
        this.sessionFactory = sessionFactory;
    }
    
    // ...

}</programlisting>

    <para>Last example we will show here is for typical JDBC support. You
    would have the <classname>DataSource</classname> injected into an
    initialization method where you would create a
    <classname>JdbcTemplate</classname> and other data access support classes
    like <classname>SimpleJdbcCall</classname> etc using this
    <classname>DataSource</classname>.</para>

    <programlisting language="java">@Repository
public class JdbcMovieFinder implements MovieFinder {

    private JdbcTemplate jdbcTemplate;
    
    @Autowired
    public void init(DataSource dataSource) {
        this.jdbcTemplate = new JdbcTemplate(dataSource);
    }
    
    // ...

}</programlisting>

    <note>
      <para>Please see the specific coverage of each persistence technology
      for details on how to configure the application context to take
      advantage of these annotations.</para>
    </note>

    <para></para>
  </section>
</chapter>
