z2-Environment

h1. A plain Hibernate on Z2 sample

Note that Hibernate is used in other samples as well, such as [[Sample-jta-plain]], [[Sample-jta-spring]], and others. This sample shows the minimal things to do to use Hibernate as an implementation of the Java Persistence API ("JPA":http://de.wikipedia.org/wiki/Java_Persistence_API). 

This sample is stored in the repository "z2-samples.hibernate.basic":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic. 

This sample makes use of the [[Hibernate Add-on]] and is most likely the best documentation on how to use its supporting functionality. While some of the possibly complex seeming (but only needed once) glue code below is not required when using Spring (for example), as in [[Sample-spring-hibernate]], [[Sample-springds-hibernate]], this sample is most instructive on how Hibernate, Transaction Management, Z2, and the Jetty Web Container integrate with one another while closely sticking to standard descriptors (JPA) and naming (Java EE JNDI/Servlet).

h2. Prerequisites

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You need to run Java DB as network server on localhost. This is explained next.

The application will create a database "z2-samples"

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h2. Run the sample

If you have the database, the fastest way to verify whether it runs is:

{{include(Install_sample_prefix)}}

Check out the sample

<pre><code class="ruby">

git clone -b v2.6 http://git.z2-environment.net/z2-samples.hibernate-basic

</code></pre>

{{include(Install_sample_postfix)}}

When started, go to http://localhost:8080/hibernate-basic. You should see this:

!hibernate-basic.png!

h2. Details

A lot of the things happening here relate to what is explained in [[How to transaction management]].

The assumption of this example is that of a re-use domain module *com.zfabrik.samples.hibernate-basic.domain* that implements a "Thingy Repository" and is used from a web application that is in another module *com.zfabrik.samples.hibernate-basic.web*. The domain module exposes the Thingy Repository as a Z2 component that is bound by the Web app as an environment (ENC) variable and injected into the controller filter by the Web container.

The domain module makes use of Hibernate's JPA implementation and integrates with the transaction management provided by *com.zfabrik.jta*.

Now, step-by-step.

h3. The domain module and its persistence context

The domain module *com.zfabrik.samples.hibernate-basic.domain* defines a persistence unit "thingies" in "java/src.impl/META-INF/persistence.xml":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.domain/java/src.impl/META-INF/persistence.xml, i.e. in its implementation. That makes sense, as the XML file will be looked up with a class loader and we do not intent to retrieve from another module. Or, put differently, the persistence unit is not part of the module's API.

In order to integrate with the built-in transaction management the <code>persistence.xml</code> declares the JTA data source 

<pre><code class="xml">

<jta-data-source>components:com.zfabrik.samples.hibernate-basic.domain/DB</jta-data-source>

</code></pre>

and the _JTA Platform_ (Hibernate's transaction management abstraction since v4.3) 

<pre><code class="xml">

<property name="hibernate.transaction.jta.platform" value="com.zfabrik.hibernate.Z2JtaPlatform"/>

</code></pre>

The former points to the data source component "com.zfabrik.samples.hibernate-basic.domain/DB":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.domain/DB.properties, while the latter makes sure Hibernate can register with the transaction manager implementation that comes built-in with Z2 (other samples, such as [[Sample-jta-plain]], [[Sample-springds-hibernate]] show alternative approaches).

The persistence unit defines only one entity. The Thingy as in "Thingy.java":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.domain/java/src.api/com/zfabrik/samples/hibernate_basic/thingies/Thingy.java. That is an API-exposed type. We use the simplified pattern of exposing persistent objects in the API rather than using Data Transfer Objects (DTOs).

Also, the domain module exposes the interface of the "Thingy Repository":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.domain/java/src.api/com/zfabrik/samples/hibernate_basic/thingies/ThingyRepository.java. This interface is used by the Web application to retrieve, store, and delete thingies.

The implementation of the Thingy Repository, "ThingyRepositoryImpl":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.domain/java/src.impl/com/zfabrik/samples/hibernate_basic/impl/thingies/ThingyRepositoryImpl.java is not a public type. Instead, it is instantiated and held on to via a Z2 component lookup from the Web app on the component "com.zfabrik.samples.hibernate-basic.domain/repository":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.domain/repository.properties.

In ThingyRepositoryImpl, in order to access and re-use the JPA Entity Manager for the persistence unit "thingies" we use EntityManagerUtil from *org.hibernate* with a Entity Manager Factory that we create upon service instantiation and using the user transaction implementation of *com.zfabrik.jta*:

<pre><code class="java">

public class ThingyRepositoryImpl implements ThingyRepository {

	private EntityManagerFactory emf;

	/**

	 * Create the repo

	 */ 

	public ThingyRepositoryImpl() {

		/*

		 * We switch the context class loader so that Hibernate finds our persistence.xml.

		 * We use the EntityManagerUtil so that Hibernate doesn't freak out in a 

		 * no-Initial-Context-Factory (but URL for lookup) naming environment.

		 */

		this.emf = ThreadUtil.cleanContextExecute(

			this.getClass().getClassLoader(),

			new Callable<EntityManagerFactory>() {

				@Override

				public EntityManagerFactory call() throws Exception {

					return EntityManagerUtil.createEntityManagerFactory("thingies");

				}

			}

		);

	}

	@Override

	public void store(Thingy thingy) {

		this.em().persist(thingy);

	}

	@SuppressWarnings("unchecked")

	@Override

	public Collection<Thingy> findAll() {

		return this.em().createQuery("select t from Thingy t").getResultList();

	}

	@Override

	public void delete(int id) {

		Thingy t = this.em().find(Thingy.class, id);

		if (t != null) {

			this.em().remove(t);

		}

	}

	//

	// Uses the Entity Manager Util that holds on to EMs created from the passed on EMF

	// while the transaction is still open.

	//

	private EntityManager em() {

		return EntityManagerUtil.getEntityManager(

			IComponentsLookup.INSTANCE.lookup(

				"com.zfabrik.jta/userTransaction", 

				TransactionManager.class

			),

			this.emf

		);

	}

}

</code></pre>

Note that when creating the entity manager factory, we have to make sure the right context class loader is set so that the persistence unit definition will be picked up by Hibernate. This is a general pattern when initializing services in a modular application: You need to distinguish when the service's context matters vs. when the caller's context matters (check out "a blog article on that":http://www.z2-environment.net/blog/2012/07/for-techies-protecting-java-in-a-modular-world-context-classloaders/).

We would normally use the JPA class "Persistence":http://docs.oracle.com/javaee/6/api/javax/persistence/Persistence.html to create an entity manager factory. Due to "HHH-8818":https://hibernate.atlassian.net/browse/HHH-8818 we do need to work around some JNDI issue by using the EntityManagerUtil (see its "source code":https://redmine.z2-environment.net/projects/z2-addons/repository/z2-addons-hibernate/revisions/master/entry/org.hibernate/java/src.api/com/zfabrik/hibernate/EntityManagerUtil.java here).

h3. The web module, transaction boundaries, and service re-use

Let's turn to the Web application in *com.zfabrik.samples.hibernate-basic.web/web*. And let's start with how the Thingy Repository is accessed from the Web app. In this example we do not use Spring or direct lookups, instead we use the Web container provided dependency injection mechanisms. In "WebContent/WEB-INF/jetty-env.xml":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.web/web/WebContent/WEB-INF/jetty-env.xml we bind the result of a JNDI lookup for the repository implementation component to the _Environment Naming Context_ (ENC) variable "repos/thingies". This is java EE mechanics. It means that from within the Web app, the repository is available under the JNDI name "java:comp/env/repos/thingies":

<pre><code class="xml">

<Configure class="org.eclipse.jetty.webapp.WebAppContext">

  <New id="thingyRepository" class="org.eclipse.jetty.plus.jndi.Resource">

    <Arg>repos/thingies</Arg>

    <Arg>

      <New class="javax.naming.LinkRef">

        <Arg>components:com.zfabrik.samples.hibernate-basic.domain/repository?type=com.zfabrik.samples.hibernate_basic.thingies.ThingyRepository</Arg>

      </New>

    </Arg>

  </New>

</Configure>

</code></pre>

See also "Jetty JNDI":http://docs.codehaus.org/display/JETTY/JNDI.

Anyway. Now, in the "ControllerFilter":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.web/java/src.impl/com/zfabrik/samples/hibernate_basic/impl/web/ControllerFilter.java we inject the Thingy Repository like this:

<pre><code class="java">

public class ControllerFilter implements Filter {

	// inject thingies repository (see WEB-INF/jetty-env.xml)

	@Resource(name="repos/thingies")

	private ThingyRepository thingyRepository;

...

}

</code></pre>

Alternatively, if you think this is a little overkill, you might as well use a direct lookup, either with JNDI using the name in the XML or via Z2's component lookup

<pre><code class="java">

IComponentsLookup.INSTANCE.lookup("com.zfabrik.samples.hibernate-basic.domain/repository",ThingyRepository.class);

</code></pre>

Finally a word on transaction management. Transaction boundaries are controlled by the "TransactionFilter":http://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-hibernate-basic/revisions/master/entry/com.zfabrik.samples.hibernate-basic.web/java/src.impl/com/zfabrik/samples/hibernate_basic/impl/web/TransactionFilter.java contained in the sample. We make use of *TransactionUtil* from *com.zabrik.jta* to wrap the actual web app request in a transaction:

<pre><code class="java">

@Override

public void doFilter(final ServletRequest sreq, final ServletResponse sres,	final FilterChain chain) throws IOException, ServletException {

  HttpServletRequest req = (HttpServletRequest) sreq;

  if (req.getDispatcherType()==DispatcherType.REQUEST || req.getDispatcherType()==DispatcherType.ASYNC) {

    try {

      TransactionUtil.run(

        (UserTransaction) new InitialContext().lookup("java:comp/UserTransaction"),

        new Callable<Void>() {

          public Void call() throws Exception {

            chain.doFilter(sreq, sres);

            return null;

          }

        }

      );

    } catch (Exception e) {

      throw new ServletException(e);

    }

  } else {

    chain.doFilter(sreq, sres);

  }

}

</code></pre>

Finally note that we retrieved the transaction manager via the standard Java EE JNDI name @java:comp/UserTransaction@. Z2 uses Jetty as its Web container implementation. In order to have Jetty bind the transaction manager, we configure the Web server in "environment/webServer/jetty.xml":https://redmine.z2-environment.net/projects/z2-samples/repository/revisions/master/entry/environment/webServer/jetty.xml to retrieve the built-in transaction manager:

<pre><code class="xml">

...

	<New id="tx" class="org.eclipse.jetty.plus.jndi.Transaction">

		<Arg>

			<New class="com.zfabrik.tx.UserTransaction">

			</New>

		</Arg>

	</New>

...

</code></pre>

h2. How this sample makes use of MVNCR

This sample in conjunction with the [[Hibernate add-on]] is good example on how to use the Maven component repository (see [[How to Access a Maven Repository]]).

The situation is this:

* The add-on (the [[Hibernate add-on]] in this case) defines the artifacts required but may not define the actual artifact repository to use. After all it may be used with a locally hosted artifact repository.

* The solution environment (in this case this sample) may define the actual artifact repository but it should not need to repeat all the details on the actual artifact names and versions

To resolve this we use Maven component repository declarations and Maven component repository fragment declarations. 

* The repository declared as part of the sample in "environment/mavenDefault":https://redmine.z2-environment.net/projects/z2-samples/repository/revisions/master/entry/environment/mavenDefault has all the information on where to get artifacts from, while

* the fragment declared in the add-on at "org.hibernate/mvnFragment":https://redmine.z2-environment.net/projects/z2-addons/repository/z2-addons-hibernate/revisions/master/entry/org.hibernate/mvnFragment.properties declares the dependency roots it needs to have resolved from the (yet unknown - from the perspective of the add-on) MVNCR *environment/mavenDefault*.