App engine java在数据存储方面不仅支持JDO，还支持JPA。第4，5课我们存储数据使用的是JDO，这一课看一下如果使用JPA。
一，Using JPA
打开gapp_flexblog项目，在src/META-INF/目录下，添加persistence.xml文件，内容如下：
<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">
<persistence-unit name="transactions-optional">
<provider>org.datanucleus.store.appengine.jpa.DatastorePersistenceProvider</provider>
<properties>
<property name="datanucleus.NontransactionalRead" value="true"/>
<property name="datanucleus.NontransactionalWrite" value="true"/>
<property name="datanucleus.ConnectionURL" value="appengine"/>
</properties>
</persistence-unit>
</persistence>
其中persistence-unit的name在实例化EntityManagerFactory时会用到。
在sban.flexblog.managers目录下，添加类EMFactory：
package sban.flexblog.managers;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
public final class EMFactory {
private static EntityManagerFactory _instance = Persistence
.createEntityManagerFactory("transactions-optional");
private EMFactory() {
}
public static EntityManagerFactory getInstance() {
return _instance;
}
}
该类用于返回EntityManagerFactory一个实例，其中参数”transactions-optional”为上文中定义。
在sban.flexblog目录下添加实体类GreetingEntity：
package sban.flexblog;
import java.util.Date;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
@Entity
public class GreetingEntity {
public GreetingEntity(String user, String content, Date date)
{
this.user = user;
this.greetingContent=content;
this.date = date;
}
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String user;
private String greetingContent;
private Date date;
public Long getId()
{
return this.id;
}
public String getGreetingContent()
{
return this.greetingContent;
}
public void setGreetingContent(String v)
{
this.greetingContent = v;
}
public Date getDate()
{
return this.date;
}
}