The JPA compatible EntityManager is configured through the
persistence.xml file located in the META-INF directory of the classpath. The
persistence.xml file starts with the XML declaration and the
persistence element as root. The root defines the schema, determining the version of JPA according to which the document is created. Inside the root element, an unlimited number of
persistence-unit elements can be present. In most situations, only a single
persistence-unit element is used. However, in rare situations when you are using two or more persistence strategies together in your application (for example, Hibernate for persisting some classes and iBatis for others), you can distinct them with individual
persistence-unit elements. ...