JPA 技術實作總結：從設定到程式碼範例

前言

JPA 是 EJB 基礎。雖然 EJB 尚未入門，但必須學習。從實務應用程度看，此技術在大型應用程式中廣泛使用。雖然實作過程遇到挫折，orm.xml 仍未完全掌握，但將所理解記錄下來。HelloWorld 範例來自《Java Persistence with Hibernate》。

persistence.xml 設定

<persistence xmlns="https://java.sun.com/xml/ns/persistence"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="https://java.sun.com/xml/ns/persistence https://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>

orm.xml 映射檔

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

實體類別定義（使用繼承）

注意 @MappedSuperclass、@Entity、@Id 用法。需 JPA 實現套件，使用 Hibernate Core 3.2.0 GA、Hibernate EntityManager 3.2.0、Hibernate Annotations 3.2，具體用哪些還在摸索中，先一個個加入。

BaseMessage.java

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; }
}

Message.java

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; }
}

持久化操作流程

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();
    }
}