jpa是ejb的基礎。現在ejb還沒入門呢，但一定要學下去。從tc中應用ejb的程度來看，這技術在大的應用程序中應用很廣泛。
雖然這次做得很失敗，orm.xml還是沒完全弄明白怎么寫。但還是將自己所了解的寫上來。HelloWorld example來自<java persistence with hibernate>
首先是persistence.xml文件
xml 代碼
接著是orm.xml文件，自己組件的沒做好，但helloworld還是沒問題
xml 代碼
xml version="1.0" encoding="UTF-8"?>
<entity-mappings xmlns="xmlns:xsi="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> |
實體定義，用了繼承，不用繼承的早就解決了。注意@MappedSuperclass ,@entity,@Id的用法。需要jpa實現的包。我用的hibernate,這些包主要來自hibernate core 3.2.0g, hibernate entitymanager 3.2.0 and hibernate annotation 3.2. 具體要用其中的哪些我還沒怎么弄明白，一個個加吧。
java 代碼
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 過程：
java 代碼
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();
}
} |
【編輯推薦】
- 詳解XML與J2EE組合技術的精髓
- J2EE的13種核心技術簡介
- 深入掌握Java技術 EJB調用原理分析
