类很简单，但对于注释则有很多内容需要解释：
@Column：将属性标记为数据库内的一个列，可含有列名，不管它是否唯一且是否可为空
@Entity：将类作为一个实体 bean 声明，表明它是一个要留存的 POJO
@GeneratedValue：指定生成主键的策略;包括 AUTO、IDENTITY、SEQUENCE 和 TABLE
@Id：指明属性为每个 Java 对象的唯一标识符(即主键)
@NamedQueries：列出一组命名查询
@NamedQuery：将预定义查询声明为一个字符串常量，以供执行时引用。
@Table：将 Java 类指定为数据库内的一个表
每次需要留存一个内存中的 Java 对象时，Hibernate 将任何 Java 对象的状态信息转换为一个 SQL 更新。同样地，带结果集的 SQL 语句用于填充 Java 对象。因此，可将所有对象保存为数据库内的记录，且可检索所有记录并将其转换回 Java 对象。
注释告知 Hibernate 一个类中的哪些内容可以考虑留存。但它们只是类的一部分。
业务服务：数据库连接
执行 ORM 时需要有一个服务类来执行对 Hibernate 的调用。清单 2 显示了 ContactsService 类，它充当应用程序服务。
清单 2. ContactsService 类
public class ContactsService {
private static Logger logger = Logger.getLogger(ContactsService.class);
private static final String PERSISTENCE_UNIT = "contacts";
private static EntityManagerFactory emf = null;
static {
logger.info("LOADING CONTACTSSERVICE CLASS.");
emf = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT);
}
public ContactsService() {
super();
}
public void addContact(Contact c) {
if(c == null) {
return;
}
EntityManager em = emf.createEntityManager();
logger.info("PERSISTENCE ENTITYMANAGER ACQUIRED.");
logger.info("ABOUT TO ADD CONTACT: fName: " + c.getFirstName()
+ ", lName: " + c.getLastName() + ", email:" + c.getEmailAddress()
+ ", phone: " + c.getPhoneNumber());
EntityTransaction tx = em.getTransaction();
try {
tx.begin();
em.merge(c);
tx.commit();
} catch (Exception e) {
logger.error("CONTACT APP PERSISTING ERROR: " + e.getMessage());
tx.rollback();
} finally {
logger.info("CONTACT APP CLOSING ENTITY MANAGER.");
em.close();
}
}
public void editContact(Contact c) {
logger.info("CONTACT TO UPDATE: " + c);
addContact(c);
}
public void deleteContact(Long id) {
logger.info("ABOUT TO DELETE CONTACT");
EntityManager em = emf.createEntityManager();
logger.info("PERSISTENCE ENTITYMANAGER ACQUIRED.");
Query contactByIdQuery = em.createNamedQuery("contact.getById");
contactByIdQuery.setParameter("id", id);
Contact c = (Contact) contactByIdQuery.getSingleResult();
EntityTransaction tx = em.getTransaction();
try {
tx.begin();
em.remove(c);
tx.commit();
} catch (Exception e) {
logger.error("CONTACT APP PERSISTING ERROR: " + e.getMessage());
tx.rollback();
} finally {
logger.info("CONTACT APP CLOSING ENTITY MANAGER.");
em.close();
}
}
public List<Contact> getContacts() {
logger.info("ABOUT TO RETRIEVE CONTACTS");
EntityManager em = emf.createEntityManager();
logger.info("PERSISTENCE ENTITYMANAGER ACQUIRED.");
Query findAllContactsQuery =
em.createNamedQuery("contact.findAll");
List<Contact> contacts = findAllContactsQuery.getResultList();
if (contacts != null) {
logger.debug("CONTACT APP RETRIEVED: " + contacts.size()
+ " CONTACT(S)");
}
return contacts;
}
}
private static Logger logger = Logger.getLogger(ContactsService.class);
private static final String PERSISTENCE_UNIT = "contacts";
private static EntityManagerFactory emf = null;
static {
logger.info("LOADING CONTACTSSERVICE CLASS.");
emf = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT);
}
public ContactsService() {
super();
}
public void addContact(Contact c) {
if(c == null) {
return;
}
EntityManager em = emf.createEntityManager();
logger.info("PERSISTENCE ENTITYMANAGER ACQUIRED.");
logger.info("ABOUT TO ADD CONTACT: fName: " + c.getFirstName()
+ ", lName: " + c.getLastName() + ", email:" + c.getEmailAddress()
+ ", phone: " + c.getPhoneNumber());
EntityTransaction tx = em.getTransaction();
try {
tx.begin();
em.merge(c);
tx.commit();
} catch (Exception e) {
logger.error("CONTACT APP PERSISTING ERROR: " + e.getMessage());
tx.rollback();
} finally {
logger.info("CONTACT APP CLOSING ENTITY MANAGER.");
em.close();
}
}
public void editContact(Contact c) {
logger.info("CONTACT TO UPDATE: " + c);
addContact(c);
}
public void deleteContact(Long id) {
logger.info("ABOUT TO DELETE CONTACT");
EntityManager em = emf.createEntityManager();
logger.info("PERSISTENCE ENTITYMANAGER ACQUIRED.");
Query contactByIdQuery = em.createNamedQuery("contact.getById");
contactByIdQuery.setParameter("id", id);
Contact c = (Contact) contactByIdQuery.getSingleResult();
EntityTransaction tx = em.getTransaction();
try {
tx.begin();
em.remove(c);
tx.commit();
} catch (Exception e) {
logger.error("CONTACT APP PERSISTING ERROR: " + e.getMessage());
tx.rollback();
} finally {
logger.info("CONTACT APP CLOSING ENTITY MANAGER.");
em.close();
}
}
public List<Contact> getContacts() {
logger.info("ABOUT TO RETRIEVE CONTACTS");
EntityManager em = emf.createEntityManager();
logger.info("PERSISTENCE ENTITYMANAGER ACQUIRED.");
Query findAllContactsQuery =
em.createNamedQuery("contact.findAll");
List<Contact> contacts = findAllContactsQuery.getResultList();
if (contacts != null) {
logger.debug("CONTACT APP RETRIEVED: " + contacts.size()
+ " CONTACT(S)");
}
return contacts;
}
}
