EntityManager is an interface that describes an API for all basic operations on Enitity, data retrieval and other JPA entities. Essentially the main API for working with JPA. Basic operations :
* For operations on Entity : persist (adding Entity under JPA control), merge (updating), remove (delete), refresh (updating data), detach (removing from management JPA), lock (blocking Enity from changes in other thread),
* data Preparation : find (search and retrieval entity), createQuery, createNamedQuery, createNativeQuery , contains, createNamedStoredProcedureQuery, createStoredProcedureQuery
* Preparation of other entities JPA : getTransaction, getEntityManagerFactory, getCriteriaBuilder, getMetamodel, getDelegate
* Work with EntityGraph : createEntityGraph, getEntityGraph
* General operations on EntityManager or all Entities : close, isOpen, getProperties, setProperty, clear.
