我需要<class>persistence.xml 中的元素?

<persistence><persistence-unit name=订单管理"><描述>这个单元管理订单和客户.它不依赖于任何供应商特定的功能，并且可以因此可以部署到任何持久性提供程序.</描述><jta-data-source>jdbc/MyOrderDB</jta-data-source><jar-file>MyOrderApp.jar</jar-file><class>com.widgets.Order</class><class>com.widgets.Customer</class></持久化单元></持久性>

这个文件定义了一个持久化单元命名为 OrderManagement，它使用JTA 感知数据源 jdbc/MyOrderDB.jar-file 和 class 元素指定托管持久性类:实体类、可嵌入类和映射超类.jar-file 元素指定对包含托管持久性类的打包持久性单元可见的 JAR 文件，而 class 元素显式命名托管持久性类.

如果是 Hibernate，请查看 第二章.设置和配置了解更多详情.

实际上，如果您不介意不符合规范，Hibernate 甚至在 Java SE 中也支持自动检测.为此，请添加 hibernate.archive.autodetection 属性:

<persistence-unit name="eventtractor"事务类型=RESOURCE_LOCAL"><!-- 这需要符合规范，但 Hibernate 支持即使在 JSE 中也可以自动检测.<class>pl.michalmech.eventtractor.domain.User</class><class>pl.michalmech.eventtractor.domain.Address</class><class>pl.michalmech.eventtractor.domain.City</class><class>pl.michalmech.eventtractor.domain.Country</class>--><属性><!-- 扫描带注释的类和 Hibernate 映射 XML 文件 --><属性名称=hibernate.archive.autodetection"值=类，hbm"/><属性名称=hibernate.hbm2ddl.auto"值=验证"；/><属性名称=hibernate.show_sql";值=真"；/></属性></持久化单元>

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?

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>

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