A TomEE 1.5 persistence unit example using EclipseLink as the JPA provider. The data source is defined in a resources.xml file, an example of which can be found here... https://gist.github.com/4629327.

persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" 
             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_2_0.xsd">
  <persistence-unit name="CustomerDBSpringPU" transaction-type="JTA">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <jta-data-source>jdbc/sample</jta-data-source>
    <class>customerdb.Customer</class>
    <class>customerdb.DiscountCode</class>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
    <properties/>
  </persistence-unit>
</persistence>