是否提供`EntityManagerFactory` 解决`Unable to retrieve EntityManagerFactory for unitName xyz`

Question

This is the (bottom) error from the stack trace:

Caused by: java.lang.IllegalStateException: Unable to retrieve EntityManagerFactory for unitName null
at com.sun.enterprise.container.common.impl.EntityManagerWrapper.init(EntityManagerWrapper.java:138)
at com.sun.enterprise.container.common.impl.EntityManagerWrapper._getDelegate(EntityManagerWrapper.java:171)
at com.sun.enterprise.container.common.impl.EntityManagerWrapper.createQuery(EntityManagerWrapper.java:477)
...

The code is as follows (inspired by a @BalusC 's example I found on SO today):

@Stateless
public class GradService {

    @PersistenceContext
    private EntityManager em;

    public List<Grad> listAll() {
        return em.createQuery("SELECT g FROM Grad g", Grad.class).getResultList();
    }
}

As reference, this is my persistence.xml (webmodule-root:Configuration files: WEB-INFMETA-INF:persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" 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">
    <persistence-unit name="MojaPU" transaction-type="JTA">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <jta-data-source>jdbc/ekstraResource</jta-data-source>
        <exclude-unlisted-classes>false</exclude-unlisted-classes>
        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
            <property name="hibernate.hbm2ddl.auto" value="update"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
        </properties>
    </persistence-unit>
</persistence>

Answer 1

It seems, that you placed your persistence.xml file in wrong location. It should by located in /META-INF/ folder and not in WEB-INF.