JPA技术总结

    jpa是ejb的基础。现在ejb还没入门呢，但一定要学下去。从tc中应用ejb的程度来看，这技术在大的应用程序中应用很广泛。
虽然这次做得很失败，orm.xml还是没完全弄明白怎么写。但还是将自己所了解的写上来。HelloWorld example来自<java persistence with hibernate>
首先是persistence.xml文件

    xml 代码

<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="events_event">   
<provider>org.hibernate.ejb.HibernatePersistenceprovider>   

<class>Holidayclass>   
-->   
<properties>   
<property name="hibernate.show_sql" value="true" />   
<property name="hibernate.format_sql" value="true" />   
<property name="hibernate.connection.driver_class"   
value="oracle.jdbc.driver.OracleDriver" />   
<property name="hibernate.connection.url"   
value="jdbc:oracle:thin:@localhost:1521:tctest" />   
<property name="hibernate.connection.username" value="root" />   
<property name="hibernate.connection.password" value="gg" />   
<property name="hibernate.dialect"   
value="org.hibernate.dialect.OracleDialect" />   
properties>   
persistence-unit>   

persistence>

    接着是orm.xml文件，自己组件的没做好，但helloworld还是没问题

    xml 代码

xml version="1.0" encoding="UTF-8"?>   
<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm"   
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"   
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd" version="1.0">   

<mapped-superclass class="hello.BaseMessage">   
<attributes>   
<id name="id">   
<column name="id" column-definition="number(38)" precision="38" scale="0" nullable="false" />   
<generated-value strategy="SEQUENCE" generator="SEQ"/>   
id>   
attributes>   
mapped-superclass>   

<entity class="hello.Message" name="Message">   
<table name="MESSAGES">table>   

<sequence-generator name="SEQ" sequence-name="SQ_messages" allocation-size="1" />   

<attribute-override name="id">   
<column name="message_id" column-definition="number(18)" nullable="false" precision="18" scale="0"/>   
attribute-override>   

<attributes>   
<basic name="text">   
<column name="MESSAGE_TEXT" nullable="false" />   
basic>   
attributes>   
entity>   
entity-mappings>

    实体定义，用了继承，不用继承的早就解决了。注意@MappedSuperclass ,@entity,@Id的用法。需要jpa实现的包。我用的hibernate,这些包主要来自hibernate core 3.2.0g, hibernate entitymanager 3.2.0 and hibernate annotation 3.2. 具体要用其中的哪些我还没怎么弄明白，一个个加吧。

    java 代码

package hello;
import java.io.Serializable;

import javax.persistence.Id;
import javax.persistence.MappedSuperclass;
@MappedSuperclass
public class BaseMessage implements Serializable {
private static final long serialVersionUID = 4585624359812256628L;
private Long id;
public BaseMessage() {
super();
}
@Id
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
}

package hello;
import javax.persistence.Entity;
@Entity
public class Message extends BaseMessage {
private static final long serialVersionUID = -7660981805652763488L;
private String text;
Message() {
}
public Message(String text) {
this.text = text;
}
public String getText() {
return text;
}
public void setText(String text) {
this.text = text;
}
}

    persistence 过程：

    java 代码

package hello;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class HelloWorldEM {
public static void main(String[] args) {
// Start EntityManagerFactory
EntityManagerFactory emf = Persistence.createEntityManagerFactory("helloworld");
// First unit of work
EntityManager em = emf.createEntityManager();
EntityTransaction tx = em.getTransaction();
tx.begin();
Message message = new Message("Hello World");
em.persist(message);
Long msgId = message.getId();
tx.commit();
em.close();

// Second unit of work
EntityManager newEm = emf.createEntityManager();
EntityTransaction newTx = newEm.getTransaction();
newTx.begin();
List messages = newEm.createQuery("select m from Message m order by m.text asc").getResultList();
System.out.println(messages.size() + " message(s) found");
for (Object m : messages) {
Message loadedMsg = (Message) m;
System.out.println(msgId.longValue());
System.out.println(loadedMsg.getText());
}
newTx.commit();
newEm.close();
emf.close();
}
}