Eclipselink:你如何获得每个包中的EntityManager？

             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
             version="2.1">

   <persistence-unit name="example-unit" transaction-type="JTA">
       <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
        <jta-data-source>jdbc/exampleDataSource</jta-data-source>
        <exclude-unlisted-classes>false</exclude-unlisted-classes>
       <shared-cache-mode>ENABLE_SELECTIVE</shared-cache-mode>
       <properties>
           <property name="eclipselink.logging.level" value="FINE"/>
        </properties>
    </persistence-unit>
</persistence>

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

public class UserService {

    @PersistenceContext(unitName = "example-unit")
    private EntityManager entityManager;

    public void createUser(User user) {
        entityManager.persist(user);
    }

    public User getUserById(Long id) {
        return entityManager.find(User.class, id);
    }
}