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