This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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?