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copied!<p>I just add this answer related to <a href="https://stackoverflow.com/questions/15980473/does-spring-framework-come-with-its-own-beans/15981341#15981341">another question of Koray Tugay</a>.</p> <p>In Java EE a JPA an entity is typically a Bean managed by the JPA Container. This container is provided in any Java EE certified application server. </p> <p>Each entity Object is an in-memory representation of the state of one or more table in a RDBMS instance. Every state modification of a <a href="http://docs.oracle.com/javaee/6/tutorial/doc/bnbqw.html#bnbrc" rel="nofollow noreferrer"><code>managed</code></a> entity within a transaction will be automatically handled by the container and mapped as sql order executed against the database. So you don't have to care about the persistence part of your domain model. Just modify the corresponding Java objects (entities) state and it will be automatically reflected to the database. <em>(Of course this is not magic and comes with its own pitfalls.)</em></p> <p>Every entity is part of a <code>persistence unit</code> associated to a <code>datasource</code> for which a connection pool is provided.</p> <p>A persistence unit is managed by many <code>EntityManager</code> instances. The <code>EntityManager</code> is in charge of managing an in memory representation of all entities of the associated <code>persistence unit</code>; At least the ones that are currently loaded from the database. You usually have one <code>EntityManager</code> instance per thread (~ per http request).</p> <p>When using a <code>container-managed EntityManager</code> (means injected with <code>@PersistenceContext</code>), the container will automatically propagate for you the transaction context between all the beans (controller / services / dao / etc...) implied in the persistence unit manipulation. </p> <p>(last sentence means it will open a transaction when encountering a <code>@transactionnal</code> annotation and every method on any bean executed during the current method call will be part of the same transaction. The transaction will be committed (or rolled back) at the end of the method execution).</p>

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