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"&gt;

    <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>

Answer 2

Out-of-the-box JPA does have auto-detection of entities, but it's extremely limited. The JavaEE tutorial says:

If you package the persistent unit as a set of classes in an EJB JAR file, persistence.xml should be put in the EJB JAR’s META-INF directory.

If you package the persistence unit as a set of classes in a WAR file, persistence.xml should be located in the WAR file’s WEB-INF/classes/META-INF directory.

In other words, as long as you put your classes exactly where the JPA spec expects to find them, it should auto-detect them without you having to list them individually in persistence.xml

JPA becomes much more palatable when you use Spring, which uses its own JPA integration to bring much more powerful and flexible classpath scanning to the party.

Answer 3

In Java SE enviroment, by specification you have to specify all classes as you have done: "A list of all named managed persistence classes must be specified in Java SE environments to insure portability" and "If it is not intended that the annotated persistence classes contained in the root of the persistence unit be included in the persistence unit, the exclude-unlisted-classes element should be used. The exclude-unlisted-classes element is not intended for use in Java SE environments." (JSR-000220 6.2.1.6). In Java EE enviroments, you do not have to do this as the provider scans for annotations for you.

Unofficially, you can try to set <exclude-unlisted-classes>false</exclude-unlisted-classes> in your persistence.xml. This parameter defaults to false in EE and truein SE. Both EclipseLink and Toplink supports this as far I can tell. But you should not rely on it working in SE, according to spec, as stated above.

You can TRY the following (may or may not work in SE-enviroments):

<persistence-unit name="eventractor" transaction-type="RESOURCE_LOCAL">
     <exclude-unlisted-classes>false</exclude-unlisted-classes>

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

Answer 4

Hibernate doesn't support <exclude-unlisted-classes>false</exclude-unlisted-classes> under SE, (another poster mentioned this works with TopLink and EclipseLink).

There are tools that will auto-generate the list of classes to persistence.xml e.g. the Import Database Schema wizard in IntelliJ. Once you've got your project's initial classes in persistence.xml it should be simple to add/remove single classes by hand as your project progresses.