JPA开发入门实例

一. 什么是JPA

JPA是sun官方提出的Java持久化规范, 它为Java开发人员提供了一种对象/关系映射工具来管理Java应用中的关系数据, 

它的出现主要是为了简化现有的持久化开发工作和整合ORM技术.

JPA总体思想和现有的Hibernate、TopLink等ORM框架大体一致. 总的来说, JPA包括以下3方面的技术:

1. ORM映射元数据(JPA支持XML和注解两种元数据的形式) - 元数据描述对象和表之间的映射关系.

2. Java持久化API: 用来操作实体对象, 执行CRUD操作,框架在后台替我们完成所有的事情, 开发者可以从繁琐的JDBC和SQL代码中解脱出来.

3. 查询语句: 通过面向对象而非面向数据库的查询语句查询数据, 避免程序和SQL语句紧密耦合.

二. JPA实例:

1. 代码结构图:

2. 配置persistence.xml

<?xml version="1.0"?><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="test" transaction-type="RESOURCE_LOCAL">     <provider>org.hibernate.ejb.HibernatePersistence</provider>      <properties>         <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>         <property name="hibernate.connection.driver_class" value="org.gjt.mm.mysql.Driver"/>         <property name="hibernate.connection.username" value="root"/>         <property name="hibernate.connection.password" value="root"/>         <property name="hibernate.connection.url"          value="jdbc:mysql://localhost:3306/jpa?useUnicode=true&characterEncoding=UTF-8"/>         <property name="hibernate.max_fetch_depth" value="3"/>         <property name="hibernate.hbm2ddl.auto" value="update"/>      </properties>   </persistence-unit></persistence>

JPA规范要求在类路径的META-INF目录下放置persistence.xml，文件的名称是固定的

3. 实体类

(1) Person:

@Entity@Table(name = "person")public class Person {private Integer id;private String name;private Date birthday;private Gender gender = Gender.MAN;private String info;private Byte[] file;private String imagepath;public Person() {}public Person(String name, Date d) {this.name = name;this.birthday = d;}@Id@GeneratedValue(strategy = GenerationType.AUTO)public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}@Column(length = 20, nullable = false, name = "personName")public String getName() {return name;}public void setName(String name) {this.name = name;}@Temporal(TemporalType.DATE)public Date getBirthday() {return birthday;}public void setBirthday(Date birthday) {this.birthday = birthday;}@Enumerated(EnumType.STRING)@Column(length = 5, nullable = false)public Gender getGender() {return gender;}public void setGender(Gender gender) {this.gender = gender;}@Lobpublic String getInfo() {return info;}public void setInfo(String info) {this.info = info;}@Lob@Basic(fetch = FetchType.LAZY)public Byte[] getFile() {return file;}public void setFile(Byte[] file) {this.file = file;}@Transientpublic String getImagepath() {return imagepath;}public void setImagepath(String imagepath) {this.imagepath = imagepath;}}

(2) Gender

public enum Gender {MAN, WOMAN}

4. 测试代码

public class PersonTest {private static EntityManager em;private static EntityManagerFactory factory;@BeforeClasspublic static void before() throws Exception {factory = Persistence.createEntityManagerFactory("test");em = factory.createEntityManager();}@AfterClasspublic static void after() throws Exception {em.close();factory.close();}@Testpublic void save() {em.getTransaction().begin();em.persist(new Person("wangwu", new Date()));em.getTransaction().commit();}@Testpublic void getPerson1() {Person person = em.find(Person.class, 1); // get()System.out.println(person.getName());}@Testpublic void getPerson2() {Person person = em.getReference(Person.class, 2); // load()System.out.println(person.getName());}@Testpublic void updatePerson1() {em.getTransaction().begin();Person person = em.find(Person.class, 1);person.setName("老张");em.getTransaction().commit();}@Testpublic void updatePerson2() {em.getTransaction().begin();Person person = em.find(Person.class, 1);em.clear(); // 把实体变成游离状态person.setName("老王");em.merge(person); // 用于把游离状态的对象更新同步到数据库em.getTransaction().commit();}@Testpublic void delete() {em.getTransaction().begin();Person person = em.find(Person.class, 1);em.remove(person); // 把托管状态的实体删掉em.getTransaction().commit();}@Testpublic void query() {Query query = em.createQuery("select o from Person o where o.id=?1");query.setParameter(1, 2);Person person = (Person) query.getSingleResult();System.out.println(person.getName());}@Testpublic void deletequery() {em.getTransaction().begin();Query query = em.createQuery("delete from Person o where o.id=?1");query.setParameter(1, 2); // id为2的删除query.executeUpdate();em.getTransaction().commit();}@Testpublic void updatequery() {em.getTransaction().begin();Query query = em.createQuery("update Person o set o.name=:name where o.id=:id");query.setParameter("name", "xxx");query.setParameter("id", 3);query.executeUpdate();em.getTransaction().commit();}}

三. 总结:

JPA不是一种新的ORM框架，他的出现只是用于规范现有的ORM技术，他不能取代现有的Hibernate、TopLink等ORM框架。相反，在采用JPA开发时，我们仍将使用到这些ORM框架，只是此时开发出来的应用不再依赖于某个持久化提供商。应用可以在不修改代码的情况下在任何JPA环境下运行，真正做到低耦合，可扩展的程序设计。