[JPA] Pro JPA2 study - Chapter 2

	1. Obtaining an Entity Manager and Persisting an Entity
	EntityManagerFactory emf = Persistence.createEntityManagerFactory("EmployeeService");
	EntityManager em = emf.createEntityManager();
	Employee emp = new Employee(130);
	em.persist(emp);
	persist가 완료되면 emp 객체는 entity manager의 persistence context에의해 관리된다.
	2. Finding an Entity
	Employee emp = em.find(Employee.class, 130);
	못찾으면 null return.
	3. Removing an Entity
	Employee emp = em.find(Employee.class, 130);
	em.remove(emp);
	emp가 managed instance여야함.
	emp가 null이면 IllegalArgumentException 발생.
	4. Updating an Entity
	Employee emp = em.find(Employy.class, 130);
	emp.setSalary(emp.getSalary() + 1000);
	emp가 managed instance여야함. 아니면 persistance provider가 값의 변경을 인지하지 못 함.
	5. Transaction
	em.getTransaction().begin();
	Employee emp = new Employee(130);
	em.persist(emp);
	em.getTransaction().commit();
	6. Queries
	TypedQuery<Employee> query = em.createQuery("SELECT e FROM Employee e", Employee.class); //JPQL임
	List<Employee> emps = query.getResultList();
	7. persistence.xml Example
	<persistence>
	<persistence-unit name="EmployeeService" transaction-type="RESOURCE-LOCAL">
	<class>examples.model.Employee</class>
	<properties>
	<property name="javax.persistence.jdbc.driver" value="org..." />
	...
	</properties>
	</persistence>
	in the META-INF directory

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