Do I need <class> elements in persistence.xml?

Question

I have very simple persistance.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0"
    xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">

    <persistence-unit name="eventractor" transaction-type="RESOURCE_LOCAL">
        <class>pl.michalmech.eventractor.domain.User</class>
        <class>pl.michalmech.eventractor.domain.Address</class>
        <class>pl.michalmech.eventractor.domain.City</class>
        <class>pl.michalmech.eventractor.domain.Country</class>

        <properties>
            <property name="hibernate.hbm2ddl.auto" value="validate" />
            <property name="hibernate.show_sql" value="true" />
        </properties>
    </persistence-unit>

</persistence>

and it works.

But when I remove <class> elements application doesn't see entities (all classes are annotated with @Entity).

Is there any automatic mechanism to scan for @Entity classes?

Answer 1

The persistence.xml has a jar-file that you can use. From the Java EE 5 tutorial:

<persistence>
    <persistence-unit name="OrderManagement">
        <description>This unit manages orders and customers.
            It does not rely on any vendor-specific features and can
            therefore be deployed to any persistence provider.
        </description>
        <jta-data-source>jdbc/MyOrderDB</jta-data-source>
        <jar-file>MyOrderApp.jar</jar-file>
        <class>com.widgets.Order</class>
        <class>com.widgets.Customer</class>
    </persistence-unit>
</persistence>

This file defines a persistence unit named OrderManagement, which uses a JTA-aware data source jdbc/MyOrderDB. The jar-file and class elements specify managed persistence classes: entity classes, embeddable classes, and mapped superclasses. The jar-file element specifies JAR files that are visible to the packaged persistence unit that contain managed persistence classes, while the class element explicitly names managed persistence classes.

In the case of Hibernate, have a look at the Chapter2. Setup and configuration too for more details.

EDIT: Actually, If you don't mind not being spec compliant, Hibernate supports auto-detection even in Java SE. To do so, add the hibernate.archive.autodetection property:

<persistence-unit name="eventractor" transaction-type="RESOURCE_LOCAL">
  <!-- This is required to be spec compliant, Hibernate however supports
       auto-detection even in JSE.
  <class>pl.michalmech.eventractor.domain.User</class>
  <class>pl.michalmech.eventractor.domain.Address</class>
  <class>pl.michalmech.eventractor.domain.City</class>
  <class>pl.michalmech.eventractor.domain.Country</class>
   -->

  <properties>
    <!-- Scan for annotated classes and Hibernate mapping XML files -->
    <property name="hibernate.archive.autodetection" value="class, hbm"/>

    <property name="hibernate.hbm2ddl.auto" value="validate" />
    <property name="hibernate.show_sql" value="true" />
  </properties>
</persistence-unit>