How to retrieve the datasource used by a persistence unit programmatically

You need to:

1. cast the EntityManager to EntityManagerImpl (the Hibernate implementation)
2. call getFactory()
3. cast the EntityManagerFactory to HibernateEntityManagerFactory
4. call getSessionFactory() and cast it to SessionFactoryImpl
5. call getConnectionProvider() and cast it to the correct implementation. You can see the implementations here. I'll assume that it's a DatasourceConnectionProvider
6. call getDataSource() and you're done.

Unfortunately, you must use the Hibernate API, as there's no way to retrieve the DataSource using the JPA API.

In a Spring environment you can use this:

import org.springframework.orm.jpa.EntityManagerFactoryInfo;
...

@PersistenceContext
EntityManager entityManager;

public DataSource getDataSourceFromHibernateEntityManager() {
   EntityManagerFactoryInfo info = (EntityManagerFactoryInfo) entityManager.getEntityManagerFactory();
   return info.getDataSource();
}

I am using hibernate 5.2.10.Final and the following worked for me:

    import org.hibernate.SessionFactory;
    import org.hibernate.engine.jdbc.connections.spi.ConnectionProvider;
    import javax.persistence.EntityManagerFactory;
    import javax.sql.DataSource;
    //...
    public static DataSource getDataSource(EntityManagerFactory entityManagerFactory) {
    ConnectionProvider cp = ((SessionFactory) entityManagerFactory).getSessionFactoryOptions()
            .getServiceRegistry()
            .getService(ConnectionProvider.class);
    return cp.unwrap(DataSource.class);
    }

What you need is just to pass entityManager.getEntityManagerFactory() to this method (For my case, I have multiple factories. Then I can use this method to get the datasource for any of them when needed).

If you just want the name of the datasource and that datasource name was supplied per JPA means, you should be able to get that information via:

entityManager.getEntityManagerFactory().getProperties().get( "javax.persistence.jtaDataSource" );

or

entityManager.getEntityManagerFactory().getProperties().get( "javax.persistence.nonJtaDataSource" );

depending on how you defined the datasource.