hibernate entitymanager的理解

hibernate EntityManager是围绕提供JPA编程接口的Hibernate Core的一个包装，支持JPA实体实例的生命周期，并允许你用标准的JavaPersistence查询语言编写查询。

1、基本JPA配置(EntityManagerFactory--EMF配置)

persistence.xml，该文件必须放在被部署的持久化单元的META-INF目录下，由于我这里建的是Java project，所以我把META-INF目录放在bin目录下

<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="helloworld">  

        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <!--
        <class>hello.Message</class>
         -->
        <properties>
            <property name="hibernate.archive.autodetection" value="class,hbm"/>  

            <property name="hibernate.connection.username" value="root"/>
            <property name="hibernate.connection.url" value="jdbc:MySQL://localhost:3306/myhibernate"/>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
            <property name="hibernate.hbm2ddl.auto" value="create"/>
            <property name="hibernate.c3p0.min_size" value="5"/>
            <property name="hibernate.c3p0.max_size" value="20"/>
            <property name="hibernate.c3p0.timeout" value="300"/>
            <property name="hibernate.c3p0.max_statements" value="50"/>
            <property name="hibernate.c3p0.idle_test_period" value="3000"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.connection.password" value=""/>
            <property name="myeclipse.connection.profile" value="mysql"/>
        </properties>
   </persistence-unit>
</persistence>

实体类Message.java

    package hello;  

    import javax.persistence.CascadeType;
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.ManyToOne;
    import javax.persistence.Table;  

    @Entity
    @Table(name = "Message")
    public class Message {  

        @Id @GeneratedValue
        @Column(name = "ID")
        private Long id;  

        @Column(name = "MESSAGE_TEXT")
        private String text;  

        @ManyToOne(cascade = CascadeType.ALL)
        @JoinColumn(name = "NEXT_MESSAGE_ID")
        private Message nextMessage;  

        public Long getId() {
            return id;
        }  

        public void setId(Long id) {
            this.id = id;
        }  

        public String getText() {
            return text;
        }  

        public void setText(String text) {
            this.text = text;
        }  

        public Message getNextMessage() {
            return nextMessage;
        }  

        public void setNextMessage(Message nextMessage) {
            this.nextMessage = nextMessage;
        }
    }  

测试类HelloWorld.java

    package hello;  

    import java.util.Iterator;
    import java.util.List;  

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

    public class HelloWorld {  

        @SuppressWarnings("unchecked")
        public static void main(String[] args) {  

            EntityManagerFactory emf = Persistence
                    .createEntityManagerFactory("helloworld");  

            /*----------1------*/
            EntityManager em = emf.createEntityManager();
            EntityTransaction tx = em.getTransaction();
            tx.begin();  

            Message message = new Message();
            message.setText("hello world");
            em.persist(message);  

            tx.commit();
            em.close();
            /*----------2------*/
            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() = " + messages.size());  

            for (Iterator iter = messages.iterator(); iter.hasNext();) {
                Message loadedMsg = (Message) iter.next();
                System.out.println(loadedMsg.getText());
            }  

            newTx.commit();
            newEm.close();  

            emf.close();
        }  

    }  

运行结果

messages.size() = 1
hello world

说明：

javax.persistence.Persistence   给EntityManagerFactory的创建提供一种静态方法的启动类

javax.persistence.EntityManagerFactory 相当于hibernate的SessionFactory

javax.persistence.EntityManager 相当与hibernate的Session

javax.persistence.Query 相当与hibernate的Query,跟hibernate使用hql一样，同样可以使用对象化的查询语言

javax.persistence.EntityTransaction 相当于hibernate的Transaction