DATACMNS-58 - Added support for repository populators.

Added RepositoryPopulator abstraction and implementations based on Spring OXM Unmarshallers as well as Jackson. This allows arbitrary repositories being populated with data pulled from XML / JSON, no matter what store they are actually backed. The populator will eventually populate the repositories held in a Repositories instance. It can be used like this:

Repositories repositories = new Repositories(applicationContext);
ResourceReader reader = new JacksonResourceReader();

ResourceReaderRepositoryPopulator populator = new ResourceReaderRepositoryPopulator(reader);
populator.setResourceLocation("classpath*:data.json");
populator.populate(repositories);

The ResourceReader defines what technology shall be used to read the data from the file into objects. The ResourceReaderRepositoryPopulator uses the reader and can either get a set of Resource instances configured or is able to lookup resources using a location string. The actual Repositories instance captures all CrudRepository instances contained inside an ApplicationContext.

The populators can also be used from within XML configuration though the repository namespace elements shown below:

<repository:jackson-populator location="classpath:org/springframework/data/repository/init/data.json" />
		
<repository:unmarshaller-populator location="classpath:org/springframework/data/repository/init/data.xml" unmarshaller-ref="unmarshaller" />

Updated reference documentation accordingly.

src/docbkx/repositories.xml

@@ -1028,5 +1028,88 @@ public class UserController {
 </programlisting>
       </simplesect>
     </section>
+
+    <section>
+      <title>Repository populators</title>
+
+      <para>If you have been working with the JDBC module of Spring you're
+      probably familiar with the support to populate a DataSource using SQL
+      scripts. A similar abstraction is available on the repositories level
+      although we don't use SQL as data definition language as we need to be
+      store independent of course. Thus the populators support XML (through
+      Spring's OXM abstraction) and JSON (through Jackson) to define data for
+      the repositories to be populated with.</para>
+
+      <para>Assume you have a file <filename>data.json</filename> with the
+      following content:</para>
+
+      <example>
+        <title>Data defined in JSON</title>
+
+        <programlisting language="javascript">[ { "_class" : "com.acme.Person",
+ "firstname" : "Dave",
+  "lastname" : "Matthews" },
+  { "_class" : "com.acme.Person",
+ "firstname" : "Carter",
+  "lastname" : "Beauford" } ]</programlisting>
+      </example>
+
+      <para>You can easily populate you repositories by using the populator
+      elements of the repository namespace provided in Spring Data Commons. To
+      get the just shown data be populated to your
+      <interfacename>PersonRepository</interfacename> all you need to do is
+      the following:</para>
+
+      <example>
+        <title>Declaring a Jackson repository populator</title>
+
+        <programlisting language="xml">&lt;?xml version="1.0" encoding="UTF-8"?&gt;
+&lt;beans xmlns="http://www.springframework.org/schema/beans"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xmlns:repository="http://www.springframework.org/schema/data/repository"
+  xsi:schemaLocation="http://www.springframework.org/schema/beans
+    http://www.springframework.org/schema/beans/spring-beans.xsd
+    http://www.springframework.org/schema/data/repository
+    http://www.springframework.org/schema/data/repository/spring-repository.xsd"&gt;
+
+  &lt;repository:jackson-populator location="classpath:data.json" /&gt;
+
+&lt;/beans&gt;</programlisting>
+      </example>
+
+      <para>This declaration causes the data.json file being read,
+      deserialized by a Jackson <classname>ObjectMapper</classname>. The type
+      the JSON object will be unmarshalled to will be determined by inspecting
+      the <code>_class</code> attribute of the JSON document. We will
+      eventually select the appropriate repository being able to handle the
+      object just deserialized.</para>
+
+      <para>To rather use XML to define the repositories shall be populated
+      with you can use the unmarshaller-populator you hand one of the
+      marshaller options Spring OXM provides you with.</para>
+
+      <example>
+        <title>Declaring an unmarshalling repository populator (using
+        JAXB)</title>
+
+        <programlisting language="xml">&lt;?xml version="1.0" encoding="UTF-8"?&gt;
+&lt;beans xmlns="http://www.springframework.org/schema/beans"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xmlns:repository="http://www.springframework.org/schema/data/repository"
+  xmlns:oxm="http://www.springframework.org/schema/oxm"
+  xsi:schemaLocation="http://www.springframework.org/schema/beans
+    http://www.springframework.org/schema/beans/spring-beans.xsd
+    http://www.springframework.org/schema/data/repository
+    http://www.springframework.org/schema/data/repository/spring-repository.xsd
+    http://www.springframework.org/schema/oxm
+    http://www.springframework.org/schema/oxm/spring-oxm.xsd"&gt;
+
+  &lt;repository:unmarshaller-populator location="classpath:data.json" unmarshaller-ref="unmarshaller" /&gt;
+
+  &lt;oxm:jaxb2-marshaller contextPath="com.acme" /&gt;
+
+&lt;/beans&gt;</programlisting>
+      </example>
+    </section>
   </section>
 </chapter>