第一个JPA程序

JPA是最新的Java ORM规范。在EJB3中，JPA用来实现持久化，取代原来的Entity Bean。
关于实现的版本，现在主要有OpenJPA和Hibernate，个人感觉这两个实现版本没有什么太大的区别，
因为我打算在JBoss下练习EJB3，所以采用了Hibernate的实现。
目录结构如果下：
src
|
|--divinemind.......
|
|--META-INF
        |-MANIFEST.MF
        |-orm.xml
        |-persistence.xml
1.persistence.xml
a.我用的Hibernate的实现
<provider>org.hibernate.ejb.HibernatePersistence</provider>
b.用Annotation而不是xml
<class>divinemind.onlyfun.jpa.entity.User</class>
  ※如果想用xml进行映射，可以删除<class>...</class>之后，添加<mapping-file>/META-INF/orm.xml</mapping-file>
c.剩下的就是数据库的设置了
具体代码如下：

<?xml version="1.0" encoding="UTF-8"?><persistence version="1.0"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"><persistence-unit name="JPADemo" transaction-type="RESOURCE_LOCAL"><provider>org.hibernate.ejb.HibernatePersistence</provider><class>divinemind.onlyfun.jpa.entity.User</class><!--<mapping-file>/META-INF/orm.xml</mapping-file>--><properties><property name="hibernate.connection.driver_class" value="org.apache.derby.jdbc.EmbeddedDriver" /><property name="hibernate.connection.url"value="jdbc:derby:D:/Project/002/JavaDB/T11111T;bootPassword=aLNwHOVoVGZ9N9zOsWf3Bw==" /><property name="hibernate.connection.username" value="T11111T" /><property name="hibernate.connection.password" value="aLNwHOVoVGZ9N9zOsWf3Bw==" /><property name="hibernate.dialect" value="org.hibernate.dialect.DerbyDialect" /><property name="hibernate.show_sql" value="false" /><property name="minPoolSize" value="5" /><property name="initialPoolSize" value="10" /><property name="maxPoolSize" value="200" /><property name="idleConnectionTestPeriod" value="120" /><property name="acquireIncrement" value="10" /><property name="checkoutTimeout" value="3600" /><property name="numHelperThreads" value="4" /><property name="maxStatements" value="400" /><property name="maxStatementsPerConnection" value="20" /><property name="maxIdleTime" value="180" /><property name="acquireRetryAttempts" value="30" /></properties></persistence-unit></persistence>

 

 2.orm.xml
这个很简单，没有什么好说的。

<?xml version="1.0" encoding="UTF-8"?><entity-mappings version="1.0"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 http://java.sun.com/xml/ns/persistence/orm_1_0.xsd"><entity class="divinemind.onlyfun.jpa.entity.User"><table name="USER_INFO"></table><attributes><id name="userId"><column name="USER_ID" /></id><basic name="userName"><column name="USER_NAME" /></basic><basic name="password"><column name="USER_PASSWORD" /></basic></attributes></entity></entity-mappings>

3.User.java

很明显，这里的annotation和上面的orm.xml的效果是一样的。但是个人感觉如果能用annotation就用annotation，因为我感觉代码总是比xml看起来要舒服点。

package divinemind.onlyfun.jpa.entity;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.Id;import javax.persistence.Table;@Entity@Table(name = "USER_INFO")public class User { @Id @Column(name = "USER_ID") private String userId; @Column(name = "USER_NAME") private String userName; @Column(name = "USER_PASSWORD") private String password; public String getUserId() { return userId; } public void setUserId(String userId) { this.userId = userId; } public String getUserName() { return userName; } public void setUserName(String userName) { this.userName = userName; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; }}

 4.测试代码

package divinemind.onlyfun.jpa.dao.impl;import java.util.List;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import javax.persistence.Query;import divinemind.onlyfun.jpa.dao.UserDAO;import divinemind.onlyfun.jpa.entity.User;public class UserDAOImpl implements UserDAO { public List<User> getUserByJPQL(String jpql) { EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPADemo"); EntityManager entityManager = factory.createEntityManager(); Query query = entityManager.createQuery(jpql); List<User> userList = query.getResultList(); return userList; } public User getUserByPrimaryKey(String userId) { EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPADemo"); EntityManager entityManager = factory.createEntityManager(); return entityManager.find(User.class, userId); } public void persistUser(User user) { EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPADemo"); EntityManager entityManager = factory.createEntityManager(); entityManager.getTransaction().begin(); entityManager.persist(user); entityManager.getTransaction().commit(); } public void removeUser(String userId) { EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPADemo"); EntityManager entityManager = factory.createEntityManager(); entityManager.getTransaction().begin(); User userFromDB = entityManager.find(User.class, userId); if (userFromDB != null) { entityManager.remove(userFromDB); } entityManager.getTransaction().commit(); } public void updateUser(User user) { EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPADemo"); EntityManager entityManager = factory.createEntityManager(); entityManager.getTransaction().begin(); entityManager.merge(user); entityManager.getTransaction().commit(); }}

上面的代码其实问题还是蛮多的，每个方法都需要自己创建EntityManager，自己控制事务，这些最好是托管给IOC容器。
后面再讨论这个问题