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.HibernatePersistence<!---->provider>
        <!---->
            <class>Holiday<!---->class>
        -->
        <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();
    }
}