code: @Override
      public final <E> E find(Class<E> entityClass, Object primaryKey)
      {
          checkClosed();
          checkTransactionNeeded();
          return getPersistenceDelegator().findById(entityClass, primaryKey);
      }
docs: Find by primary key. Search for an entity of the specified class and
  primary key. If the entity instance is contained in the persistence
  context it is returned from there.
  
  @param entityClass
  @param primaryKey
  @return the found entity instance or null if the entity does not exist
  @throws IllegalArgumentException
  if the first argument does not denote an entity type or the
  second argument is is not a valid type for that entity’s
  primary key or is null
  @see javax.persistence.EntityManager#find(java.lang.Class,
  java.lang.Object)