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               <h1>JPA Concepts</h1>
             </div>
           </div>
         </div>
         <div class="row">

             <div class="col-md-12">
                 <h1 id="_jpa_101" class="sect0">JPA 101</h1>
 <div class="paragraph">
 <p>If there&#8217;s one thing you have to understand to successfully use JPA
 (Java Persistence API) it&#8217;s the concept of a <em>Cache</em>. Almost everything
 boils down to the Cache at one point or another. Unfortunately the Cache
 is an internal thing and not exposed via the JPA API classes, so it not
 easy to touch or feel from a coding perspective.</p>
 </div>
 <div class="paragraph">
 <p>Here&#8217;s a quick cheat sheet of the JPA world:</p>
 </div>
 <div class="ulist">
 <ul>
 <li>
 <p>A <strong>Cache</strong> is a <strong>copy of data</strong>, copy meaning pulled from but living
 outside the database.</p>
 </li>
 <li>
 <p><strong>Flushing</strong> a Cache is the act of putting modified data back into the
 database.</p>
 </li>
 <li>
 <p>A <strong>PersistenceContext</strong> is essentially a Cache. It also tends to have
 it&#8217;s own non-shared database connection.</p>
 </li>
 <li>
 <p>An <strong>EntityManager</strong> represents a PersistenceContext (and therefore a
 Cache)</p>
 </li>
 <li>
 <p>An <strong>EntityManagerFactory</strong> creates an EntityManager (and therefore a
 PersistenceContext/Cache)</p>
 </li>
 </ul>
 </div>
 <div class="paragraph">
 <p>Comparing <code>RESOURCE_LOCAL</code> and <code>JTA</code> persistence contexts</p>
 </div>
 <div class="paragraph">
 <p>With &lt;persistence-unit transaction-type="<strong>RESOURCE_LOCAL</strong>"&gt; <strong>you</strong> are
 responsible for EntityManager (PersistenceContext/Cache) creating and
 tracking&#8230;&#8203;</p>
 </div>
 <div class="ulist">
 <ul>
 <li>
 <p>You <strong>must</strong> use the <strong>EntityManagerFactory</strong> to get an EntityManager</p>
 </li>
 <li>
 <p>The resulting <strong>EntityManager</strong> instance <strong>is</strong> a PersistenceContext/Cache</p>
 </li>
 <li>
 <p>An <strong>EntityManagerFactory</strong> can be injected via the <strong>@PersistenceUnit</strong>
 annotation only (not <code>@PersistenceContext</code>)</p>
 </li>
 <li>
 <p>You are <strong>not</strong> allowed to use @PersistenceContext to refer to a unit of
 type RESOURCE_LOCAL</p>
 </li>
 <li>
 <p>You <strong>must</strong> use the <strong>EntityTransaction</strong> API to begin/commit around
 <strong>every</strong> call to your EntityManger</p>
 </li>
 <li>
 <p>Calling entityManagerFactory.createEntityManager() twice results in
 <strong>two</strong> separate EntityManager instances and therefor <strong>two</strong> separate
 PersistenceContexts/Caches.</p>
 </li>
 <li>
 <p>It is <strong>almost never</strong> a good idea to have more than one <strong>instance</strong> of
 an EntityManager in use (don&#8217;t create a second one unless you&#8217;ve
 destroyed the first)</p>
 </li>
 </ul>
 </div>
 <div class="paragraph">
 <p>With &lt;persistence-unit transaction-type="<strong>JTA</strong>"&gt; the <strong>container</strong> will do
 EntityManager (PersistenceContext/Cache) creating and tracking&#8230;&#8203;</p>
 </div>
 <div class="ulist">
 <ul>
 <li>
 <p>You <strong>cannot</strong> use the <strong>EntityManagerFactory</strong> to get an EntityManager</p>
 </li>
 <li>
 <p>You can only get an <strong>EntityManager</strong> supplied by the <strong>container</strong></p>
 </li>
 <li>
 <p>An <strong>EntityManager</strong> can be injected via the <strong>@PersistenceContext</strong>
 annotation only (not <code>@PersistenceUnit</code>)</p>
 </li>
 <li>
 <p>You are <strong>not</strong> allowed to use @PersistenceUnit to refer to a unit of
 type JTA</p>
 </li>
 <li>
 <p>The <strong>EntityManager</strong> given by the container is a <strong>reference</strong> to the
 PersistenceContext/Cache associated with a JTA Transaction.</p>
 </li>
 <li>
 <p>If no JTA transaction is in progress, the EntityManager <strong>cannot be
 used</strong> because there is no PersistenceContext/Cache.</p>
 </li>
 <li>
 <p>Everyone with an EntityManager reference to the <strong>same unit</strong> in the
 <strong>same transaction</strong> will automatically have a reference to the <strong>same
 PersistenceContext/Cache</strong></p>
 </li>
 <li>
 <p>The PersistenceContext/Cache is <strong>flushed</strong> and cleared at JTA <strong>commit</strong>
 time</p>
 </li>
 </ul>
 </div>
 <h1 id="_cache_persistencecontext" class="sect0">Cache == PersistenceContext</h1>
 <div class="paragraph">
 <p>The concept of a database cache is an extremely important concept to be
 aware of. Without a copy of the data in memory (i.e. a cache) when you
 call account.getBalance() the persistence provider would have to go read
 the value from the database. Calling account.getBalance() several times
 would cause several trips to the database. This would obviously be a big
 waste of resources. The other side of having a cache is that when you
 call account.setBalance(5000) it also doesn&#8217;t hit the database
 (usually). When the cache is "flushed" the data in it is sent to the
 database via as many SQL updates, inserts and deletes as are required.
 That is the basics of java persistence of any kind all wrapped in a
 nutshell. If you can understand that, you&#8217;re good to go in nearly any
 persistence technology java has to offer.</p>
 </div>
 <div class="paragraph">
 <p>Complications can arise when there is more than one
 PersistenceContext/Cache relating the same data in the same transaction.
 In any given transaction you want exactly one PersistenceContext/Cache
 for a given set of data. Using a JTA unit with an EntityManager created
 by the container will always guarantee that this is the case. With a
 RESOURCE_LOCAL unit and an EntityManagerFactory you should create and
 use exactly one EntityManager instance in your transaction to ensure
 there is only one active PersistenceContext/Cache for the given set of
 data active against the current transaction.</p>
 </div>
 <h1 id="_caches_and_detaching" class="sect0">Caches and Detaching</h1>
 <div class="paragraph">
 <p>Detaching is the concept of a persistent object <strong>leaving</strong> the
 PersistenceContext/Cache. Leaving means that any updates made to the
 object are <strong>not</strong> reflected in the PersistenceContext/Cache. An object
 will become Detached if it somehow <strong>lives longer</strong> or is <strong>used outside</strong>
 the scope of the PersistenceContext/Cache.</p>
 </div>
 <div class="paragraph">
 <p>For a JTA unit, the PersistenceContext/Cache will live as long as the
 transaction does. When a transaction completes (commits or rollsback)
 all objects that were in the PersistenceContext/Cache are Detached. You
 can still use them, but they are no longer associated with a
 PersistenceContext/Cache and modifications on them will <strong>not</strong> be
 reflected in a PersistenceContext/Cache and therefore not the database
 either.</p>
 </div>
 <div class="paragraph">
 <p>Serializing objects that are currently in a PersistenceContext/Cache
 will also cause them to Detach.</p>
 </div>
 <div class="paragraph">
 <p>In some cases objects or collections of objects that become Detached may
 not have all the data you need. This can be because of lazy loading.
 With lazy loading, data isn&#8217;t pulled from the database and into the
 PersistenceContext/Cache until it is requested in code. In many cases
 the Collections of persistent objects returned from an
 javax.persistence.Query.getResultList() call are completely empty until
 you iterate over them. A side effect of this is that if the Collection
 becomes Detached before it&#8217;s been fully read it will be permanently
 empty and of no use and calling methods on the Detached Collection can
 cause strange errors and exceptions to be thrown. If you wish to Detach
 a Collection of persistent objects it is always a good idea to iterate
 over the Collection at least once.</p>
 </div>
 <div class="paragraph">
 <p>You <strong>cannot</strong> call EntityManager.persist() or EntityManager.remove() on a
 Detached object.</p>
 </div>
 <div class="paragraph">
 <p>Calling EntityManager.merge() will re-attach a Detached object.</p>
 </div>
 <h1 id="_valid_resource_local_unit_usage" class="sect0">Valid RESOURCE_LOCAL Unit usage</h1>
 <div class="paragraph">
 <p>Servlets and EJBs can use RESOURCE_LOCAL persistence units through the
 EntityManagerFactory as follows:</p>
 </div>
 <div class="listingblock">
 <div class="content">
 <pre class="highlight"><code class="language-xml" data-lang="xml">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;
 &lt;persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0"&gt;

   &lt;!-- Tutorial "unit" --&gt;
   &lt;persistence-unit name="Tutorial" transaction-type="RESOURCE_LOCAL"&gt;
     &lt;non-jta-data-source&gt;myNonJtaDataSource&lt;/non-jta-data-source&gt;
     &lt;class&gt;org.superbiz.jpa.Account&lt;/class&gt;
   &lt;/persistence-unit&gt;

 &lt;/persistence&gt;</code></pre>
 </div>
 </div>
 <div class="paragraph">
 <p>And referenced as follows</p>
 </div>
 <div class="listingblock">
 <div class="content">
 <pre class="highlight"><code class="language-java" data-lang="java">import javax.persistence.EntityManagerFactory;
 import javax.persistence.EntityManager;
 import javax.persistence.EntityTransaction;
 import javax.persistence.PersistenceUnit;

 public class MyEjbOrServlet ... {

     @PersistenceUnit(unitName="Tutorial")
     private EntityManagerFactory factory;

     // Proper exception handling left out for simplicity
     public void ejbMethodOrServletServiceMethod() throws Exception {
         EntityManager entityManager = factory.createEntityManager();

         EntityTransaction entityTransaction = entityManager.getTransaction();

         entityTransaction.begin();

         Account account = entityManager.find(Account.class, 12345);

         account.setBalance(5000);

         entityTransaction.commit();
     }

     ...
 }</code></pre>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_valid_jta_unit_usage">Valid JTA Unit usage</h2>
 <div class="sectionbody">
 <div class="paragraph">
 <p>EJBs can use JTA persistence units through the EntityManager as follows:</p>
 </div>
 <div class="listingblock">
 <div class="content">
 <pre class="highlight"><code class="language-xml" data-lang="xml">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;
 &lt;persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0"&gt;

   &lt;!-- Tutorial "unit" --&gt;
   &lt;persistence-unit name="Tutorial" transaction-type="JTA"&gt;
     &lt;jta-data-source&gt;myJtaDataSource&lt;/jta-data-source&gt;
     &lt;non-jta-data-source&gt;myNonJtaDataSource&lt;/non-jta-data-source&gt;
     &lt;class&gt;org.superbiz.jpa.Account&lt;/class&gt;
   &lt;/persistence-unit&gt;

 &lt;/persistence&gt;</code></pre>
 </div>
 </div>
 <div class="paragraph">
 <p>And referenced as follows</p>
 </div>
 <div class="listingblock">
 <div class="content">
 <pre class="highlight"><code class="language-java" data-lang="java">import javax.ejb.Stateless;
 import javax.ejb.TransactionAttribute;
 import javax.ejb.TransactionAttributeType;
 import javax.persistence.EntityManager;
 import javax.persistence.PersistenceContext;

 @Stateless
 public class MyEjb implements MyEjbInterface {

     @PersistenceContext(unitName = "Tutorial")
     private EntityManager entityManager;

     // Proper exception handling left out for simplicity
     @TransactionAttribute(TransactionAttributeType.REQUIRED)
     public void ejbMethod() throws Exception {

     Account account = entityManager.find(Account.class, 12345);

     account.setBalance(5000);

     }
 }</code></pre>
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	<div id="main-block" class="container main-block">
	<div class="row title">
	<div class="col-md-12">
	<div class='page-header'>

	<h1>JPA Concepts</h1>
	</div>
	</div>
	</div>
	<div class="row">

	<div class="col-md-12">
	<h1 id="_jpa_101" class="sect0">JPA 101</h1>
	<div class="paragraph">
	<p>If there’s one thing you have to understand to successfully use JPA
	(Java Persistence API) it’s the concept of a <em>Cache</em>. Almost everything
	boils down to the Cache at one point or another. Unfortunately the Cache
	is an internal thing and not exposed via the JPA API classes, so it not
	easy to touch or feel from a coding perspective.</p>
	</div>
	<div class="paragraph">
	<p>Here’s a quick cheat sheet of the JPA world:</p>
	</div>
	<div class="ulist">
	<ul>
	<li>
	<p>A <strong>Cache</strong> is a <strong>copy of data</strong>, copy meaning pulled from but living
	outside the database.</p>
	</li>
	<li>
	<p><strong>Flushing</strong> a Cache is the act of putting modified data back into the
	database.</p>
	</li>
	<li>
	<p>A <strong>PersistenceContext</strong> is essentially a Cache. It also tends to have
	it’s own non-shared database connection.</p>
	</li>
	<li>
	<p>An <strong>EntityManager</strong> represents a PersistenceContext (and therefore a
	Cache)</p>
	</li>
	<li>
	<p>An <strong>EntityManagerFactory</strong> creates an EntityManager (and therefore a
	PersistenceContext/Cache)</p>
	</li>
	</ul>
	</div>
	<div class="paragraph">
	<p>Comparing <code>RESOURCE_LOCAL</code> and <code>JTA</code> persistence contexts</p>
	</div>
	<div class="paragraph">
	<p>With <persistence-unit transaction-type="<strong>RESOURCE_LOCAL</strong>"> <strong>you</strong> are
	responsible for EntityManager (PersistenceContext/Cache) creating and
	tracking…</p>
	</div>
	<div class="ulist">
	<ul>
	<li>
	<p>You <strong>must</strong> use the <strong>EntityManagerFactory</strong> to get an EntityManager</p>
	</li>
	<li>
	<p>The resulting <strong>EntityManager</strong> instance <strong>is</strong> a PersistenceContext/Cache</p>
	</li>
	<li>
	<p>An <strong>EntityManagerFactory</strong> can be injected via the <strong>@PersistenceUnit</strong>
	annotation only (not <code>@PersistenceContext</code>)</p>
	</li>
	<li>
	<p>You are <strong>not</strong> allowed to use @PersistenceContext to refer to a unit of
	type RESOURCE_LOCAL</p>
	</li>
	<li>
	<p>You <strong>must</strong> use the <strong>EntityTransaction</strong> API to begin/commit around
	<strong>every</strong> call to your EntityManger</p>
	</li>
	<li>
	<p>Calling entityManagerFactory.createEntityManager() twice results in
	<strong>two</strong> separate EntityManager instances and therefor <strong>two</strong> separate
	PersistenceContexts/Caches.</p>
	</li>
	<li>
	<p>It is <strong>almost never</strong> a good idea to have more than one <strong>instance</strong> of
	an EntityManager in use (don’t create a second one unless you’ve
	destroyed the first)</p>
	</li>
	</ul>
	</div>
	<div class="paragraph">
	<p>With <persistence-unit transaction-type="<strong>JTA</strong>"> the <strong>container</strong> will do
	EntityManager (PersistenceContext/Cache) creating and tracking…</p>
	</div>
	<div class="ulist">
	<ul>
	<li>
	<p>You <strong>cannot</strong> use the <strong>EntityManagerFactory</strong> to get an EntityManager</p>
	</li>
	<li>
	<p>You can only get an <strong>EntityManager</strong> supplied by the <strong>container</strong></p>
	</li>
	<li>
	<p>An <strong>EntityManager</strong> can be injected via the <strong>@PersistenceContext</strong>
	annotation only (not <code>@PersistenceUnit</code>)</p>
	</li>
	<li>
	<p>You are <strong>not</strong> allowed to use @PersistenceUnit to refer to a unit of
	type JTA</p>
	</li>
	<li>
	<p>The <strong>EntityManager</strong> given by the container is a <strong>reference</strong> to the
	PersistenceContext/Cache associated with a JTA Transaction.</p>
	</li>
	<li>
	<p>If no JTA transaction is in progress, the EntityManager <strong>cannot be
	used</strong> because there is no PersistenceContext/Cache.</p>
	</li>
	<li>
	<p>Everyone with an EntityManager reference to the <strong>same unit</strong> in the
	<strong>same transaction</strong> will automatically have a reference to the <strong>same
	PersistenceContext/Cache</strong></p>
	</li>
	<li>
	<p>The PersistenceContext/Cache is <strong>flushed</strong> and cleared at JTA <strong>commit</strong>
	time</p>
	</li>
	</ul>
	</div>
	<h1 id="_cache_persistencecontext" class="sect0">Cache == PersistenceContext</h1>
	<div class="paragraph">
	<p>The concept of a database cache is an extremely important concept to be
	aware of. Without a copy of the data in memory (i.e. a cache) when you
	call account.getBalance() the persistence provider would have to go read
	the value from the database. Calling account.getBalance() several times
	would cause several trips to the database. This would obviously be a big
	waste of resources. The other side of having a cache is that when you
	call account.setBalance(5000) it also doesn’t hit the database
	(usually). When the cache is "flushed" the data in it is sent to the
	database via as many SQL updates, inserts and deletes as are required.
	That is the basics of java persistence of any kind all wrapped in a
	nutshell. If you can understand that, you’re good to go in nearly any
	persistence technology java has to offer.</p>
	</div>
	<div class="paragraph">
	<p>Complications can arise when there is more than one
	PersistenceContext/Cache relating the same data in the same transaction.
	In any given transaction you want exactly one PersistenceContext/Cache
	for a given set of data. Using a JTA unit with an EntityManager created
	by the container will always guarantee that this is the case. With a
	RESOURCE_LOCAL unit and an EntityManagerFactory you should create and
	use exactly one EntityManager instance in your transaction to ensure
	there is only one active PersistenceContext/Cache for the given set of
	data active against the current transaction.</p>
	</div>
	<h1 id="_caches_and_detaching" class="sect0">Caches and Detaching</h1>
	<div class="paragraph">
	<p>Detaching is the concept of a persistent object <strong>leaving</strong> the
	PersistenceContext/Cache. Leaving means that any updates made to the
	object are <strong>not</strong> reflected in the PersistenceContext/Cache. An object
	will become Detached if it somehow <strong>lives longer</strong> or is <strong>used outside</strong>
	the scope of the PersistenceContext/Cache.</p>
	</div>
	<div class="paragraph">
	<p>For a JTA unit, the PersistenceContext/Cache will live as long as the
	transaction does. When a transaction completes (commits or rollsback)
	all objects that were in the PersistenceContext/Cache are Detached. You
	can still use them, but they are no longer associated with a
	PersistenceContext/Cache and modifications on them will <strong>not</strong> be
	reflected in a PersistenceContext/Cache and therefore not the database
	either.</p>
	</div>
	<div class="paragraph">
	<p>Serializing objects that are currently in a PersistenceContext/Cache
	will also cause them to Detach.</p>
	</div>
	<div class="paragraph">
	<p>In some cases objects or collections of objects that become Detached may
	not have all the data you need. This can be because of lazy loading.
	With lazy loading, data isn’t pulled from the database and into the
	PersistenceContext/Cache until it is requested in code. In many cases
	the Collections of persistent objects returned from an
	javax.persistence.Query.getResultList() call are completely empty until
	you iterate over them. A side effect of this is that if the Collection
	becomes Detached before it’s been fully read it will be permanently
	empty and of no use and calling methods on the Detached Collection can
	cause strange errors and exceptions to be thrown. If you wish to Detach
	a Collection of persistent objects it is always a good idea to iterate
	over the Collection at least once.</p>
	</div>
	<div class="paragraph">
	<p>You <strong>cannot</strong> call EntityManager.persist() or EntityManager.remove() on a
	Detached object.</p>
	</div>
	<div class="paragraph">
	<p>Calling EntityManager.merge() will re-attach a Detached object.</p>
	</div>
	<h1 id="_valid_resource_local_unit_usage" class="sect0">Valid RESOURCE_LOCAL Unit usage</h1>
	<div class="paragraph">
	<p>Servlets and EJBs can use RESOURCE_LOCAL persistence units through the
	EntityManagerFactory as follows:</p>
	</div>
	<div class="listingblock">
	<div class="content">
	<pre class="highlight"><code class="language-xml" data-lang="xml"><?xml version="1.0" encoding="UTF-8" ?>
	<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">

	<!-- Tutorial "unit" -->
	<persistence-unit name="Tutorial" transaction-type="RESOURCE_LOCAL">
	<non-jta-data-source>myNonJtaDataSource</non-jta-data-source>
	<class>org.superbiz.jpa.Account</class>
	</persistence-unit>

	</persistence></code></pre>
	</div>
	</div>
	<div class="paragraph">
	<p>And referenced as follows</p>
	</div>
	<div class="listingblock">
	<div class="content">
	<pre class="highlight"><code class="language-java" data-lang="java">import javax.persistence.EntityManagerFactory;
	import javax.persistence.EntityManager;
	import javax.persistence.EntityTransaction;
	import javax.persistence.PersistenceUnit;

	public class MyEjbOrServlet ... {

	@PersistenceUnit(unitName="Tutorial")
	private EntityManagerFactory factory;

	// Proper exception handling left out for simplicity
	public void ejbMethodOrServletServiceMethod() throws Exception {
	EntityManager entityManager = factory.createEntityManager();

	EntityTransaction entityTransaction = entityManager.getTransaction();

	entityTransaction.begin();

	Account account = entityManager.find(Account.class, 12345);

	account.setBalance(5000);

	entityTransaction.commit();
	}

	...
	}</code></pre>
	</div>
	</div>
	<div class="sect1">
	<h2 id="_valid_jta_unit_usage">Valid JTA Unit usage</h2>
	<div class="sectionbody">
	<div class="paragraph">
	<p>EJBs can use JTA persistence units through the EntityManager as follows:</p>
	</div>
	<div class="listingblock">
	<div class="content">
	<pre class="highlight"><code class="language-xml" data-lang="xml"><?xml version="1.0" encoding="UTF-8" ?>
	<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">

	<!-- Tutorial "unit" -->
	<persistence-unit name="Tutorial" transaction-type="JTA">
	<jta-data-source>myJtaDataSource</jta-data-source>
	<non-jta-data-source>myNonJtaDataSource</non-jta-data-source>
	<class>org.superbiz.jpa.Account</class>
	</persistence-unit>

	</persistence></code></pre>
	</div>
	</div>
	<div class="paragraph">
	<p>And referenced as follows</p>
	</div>
	<div class="listingblock">
	<div class="content">
	<pre class="highlight"><code class="language-java" data-lang="java">import javax.ejb.Stateless;
	import javax.ejb.TransactionAttribute;
	import javax.ejb.TransactionAttributeType;
	import javax.persistence.EntityManager;
	import javax.persistence.PersistenceContext;

	@Stateless
	public class MyEjb implements MyEjbInterface {

	@PersistenceContext(unitName = "Tutorial")
	private EntityManager entityManager;

	// Proper exception handling left out for simplicity
	@TransactionAttribute(TransactionAttributeType.REQUIRED)
	public void ejbMethod() throws Exception {

	Account account = entityManager.find(Account.class, 12345);

	account.setBalance(5000);

	}
	}</code></pre>
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