1. JPA规范与ORM框架之间的联系
JPA规范并不属于EJB3规范，它是一套完全独立的规范，不仅可以在基于EJB的JavaEE应用程序中使用，而且完全可以在普通JavaSE应用程序中使用。
JPA事务管理总结：
| 运行环境 类型 | J2EE环境 | J2SE环境 | |
| EJB容器 | Web容器 | ||
| 应用托管的 EntityManager | JTA, RESOURCE_LOCAL | JTA, RESOURCE_LOCAL | RESOURCE_LOCAL |
| 容器托管的 EntityManager | JTA | 不支持 | 不支持 |
常用的3种组件：
1）. 实体：增加了orm.xml映射文件或者Annotation的POJO.
2）. EntityManager：类似于Hibernate框架的Session，使用其对实体进行操作时，JPA规范才可以将这种操作转换为对底层数据库的操作。
3）. JPQL查询：类似于Hibernate提供的HQL查询语言。
#1. TopLink JPA实现(Net Beans创建Java Project, 项目名称：JpaQsTopLink)
实体News.java
package org.crazyit.model;import javax.persistence.*;@Entity@Table(name = "news_table")public class News { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int id; @Column(name = "NEWS_TITLE", length = 50) private String title; @Column(nullable = true) private String content; //构造器 public News() { } //标识属性的setter和getter方法 public void setId(int id) { this.id = id; } public int getId() { return (this.id); } //消息标题的setter方法和getter方法 public void setTitle(String title) { this.title = title; } public String getTitle() { return (this.title); } //消息内容的setter方法和getter方法 public void setContent(String content) { this.content = content; } public String getContent() { return (this.content); }} META-INF/persistence.xml(mysql)
org.hibernate.ejb.HibernatePersistence org.crazyit.model.News
META-INF/persistence.xml(sqlserver)
org.hibernate.ejb.HibernatePersistence org.crazyit.model.News
JpaQs.java
package lee;import java.util.List;import javax.persistence.*;import org.crazyit.model.*;public class JpaQs { //一个持久单元对应一个EntityManagerFactory private static final EntityManagerFactory emfactory = Persistence.createEntityManagerFactory("qs"); public static void main(String[] args) { EntityManager em = emfactory.createEntityManager(); //创建一个News实体 News news = new News(); news.setTitle("Crazy Java Union Created!"); news.setContent("Crazy Java Union Created! Every question will be answered!"); try { System.out.println(news); //开启事务 em.getTransaction().begin(); //保存实体 news = em.merge(news); System.out.println(news); news.setTitle("aaaa"); //提交事务 em.getTransaction().commit(); } finally { em.close(); } }} #2. Hibernate JPA实现(Net Beans创建Java Project, 项目名称：JpaQsHibernate)
News.java(参见#1)
META-INF/persistence.xml(mysql)
org.hibernate.ejb.HibernatePersistence org.crazyit.model.News
META-INF/persistence.xml(sqlserver)
oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider org.crazyit.model.News
JpaQs.java
package lee;import java.util.List;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;//import javax.persistence.*;import org.crazyit.model.*;public class JpaQs { //一个持久单元对应一个EntityManagerFactory private static final EntityManagerFactory emfactory = Persistence.createEntityManagerFactory("qs"); public static void main(String[] args) { final EntityManager em = emfactory.createEntityManager(); News news = new News(); news.setTitle("Crazy Java Union Created!"); news.setContent("Crazy Java Union Created! Every question will be answered!"); try { //开启事务 em.getTransaction().begin(); //保存实体 em.persist(news); //提交事务 em.getTransaction().commit(); } finally { em.close(); } }} #3. Hibernate XML JPA实现(Net Beans创建Java Project, 项目名称：JpaQsHibernateXml)
News.java
package org.crazyit.model;import javax.persistence.*;public class News { private int id; private String title; private String content; public News() { } public void setId(int id) { this.id = id; } public int getId() { return (this.id); } public void setTitle(String title) { this.title = title; } public String getTitle() { return (this.title); } public void setContent(String content) { this.content = content; } public String getContent() { return (this.content); }} News.orm.xml
PROPERTY org.crazyit.model
persistence.xml
org.hibernate.ejb.HibernatePersistence org/crazyit/model/News.orm.xml
JpaQs.java
package lee;import java.util.List;import javax.persistence.*;import org.crazyit.model.*;public class JpaQs { private static final EntityManagerFactory emf = Persistence.createEntityManagerFactory("qs"); public static void main(String[] args) { EntityManager em = emf.createEntityManager(); try { em.getTransaction().begin(); News news = em.find(News.class, 1); news.setContent("疯狂Java联盟的网址：crazyit.org!!"); em.getTransaction().commit(); } finally { em.close(); } }} 2. 实体类的要求和状态迁移
