This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
For persistence, you need a JPA deployment descriptor named persistence.xml.
Injecting the EntityManager without specifying the persistence unit works only if a single persistence unit is specified in persistence.xml,
i read the whole article, but i do not understand what is a PERSISTENCE UNIT?