回滚后发现JPA / Hibernate实体，实体管理器清除和缓存逐出

Question

我有这种测试方法：

public void myMethod()
{

    transactionManager.begin();
    entityManager.joinTransaction();
    MyEntity e = MyEntity();

    entityManager.persist(e);
    entityManager.flush();

    Assert.assertNotNull(e);
    Assert.assertNotNull(e.getId());

    Long id = e.getId();

    entityManager.detach(e);

    Assert.assertFalse(entityManager.contains(e));
    Assert.assertFalse(entityManager.getEntityManagerFactory().getCache().
        contains(MyEntity.class, id));

    e = null;

    entityManager.getEntityManagerFactory().getCache().evictAll();
    transactionManager.rollback();
    entityManager.clear();

    e = entityManager.find(MyEntity.class, id);

    Assert.assertNull(e);
}

在相关的DB中没有任何MyEntity（每次运行测试时都会创建数据库），因为没有显式提交，DB在执行方法后保持清洁但是最后一个Assert（一个在最后一次查找之后）仍然继续失败，因为找到了先前的持久（但回滚）实例。

这是我的persistence.xml文件：

<persistence 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_2_0.xsd"
version="2.0">
<persistence-unit transaction-type="JTA" name="persistenceUnit">

    <jar-file>myjar.jar</jar-file>  

    <properties>
        <!-- validate | update | create | create-drop -->
        <property name="hibernate.hbm2ddl.auto" value="create-drop"/>
        <property name="hibernate.show_sql" value="true"/>
        <property name="hibernate.validator.apply_to_ddl" value="false"/>
        <property name="hibernate.id.new_generator_mappings" value="true"/>
        <property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/>
        <property name="hibernate.connection.url" value="jdbc:hsqldb:mem:persistenceUnitDB"/>
        <!--<property name="hibernate.connection.url" value="jdbc:hsqldb:file:persistenceUnitDB/persistenceUnitDB"/>-->
        <property name="hibernate.connection.autocommit" value="false"/>
        <property name="hibernate.connection.username" value="sa"/>
        <property name="hibernate.connection.password" value=""/>
        <property name="hibernate.transaction.manager_lookup_class" value="org.hibernate.transaction.BTMTransactionManagerLookup"/>
    </properties>
</persistence-unit>
</persistence>

我无法理解为什么这段代码有这种行为。