JPA的全称是Java Persistence API， 即Java 持久化API，是SUN公司推出的一套基于ORM的规范，内部是由一系列的接口和抽象类构成

Persistence对象

Persistence对象主要作用是用于获取EntityManagerFactory对象的

EntityManagerFactory

EntityManagerFactory 接口主要用来创建 EntityManager 实例

EntityManager

在 JPA 规范中, EntityManager是完成持久化操作的核心对象

getTransaction : 获取事务对象persist ： 保存操作merge ： 更新操作remove ： 删除操作find/getReference ： 根据id查询

EntityTransaction

begin,commit,rollback

使用

创建classpath:META-INF/persistence.xml

<?xml version="1.0" encoding="UTF-8"?><persistence xmlns="http://java.sun.com/xml/ns/persistence" version="2.0">    <!--持久化单元名称-->  <!--事务类型-->    <persistence-unit  name="jpa" transaction-type="RESOURCE_LOCAL">        <!--实现方式-->        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>        <properties>            <!--数据库信息-->            <property name="javax.persistence.jdbc.user" value="root"/>            <property name="javax.persistence.jdbc.password" value="123"/>            <property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>            <property name="javax.persistence.jdbc.url" value="jdbc:mysql:///ssm"/>            <!--实现方信息-->            <property name="hibernate.show_sql" value="true"/>            <!--create:每次启动都会创建（存在则删除） update:不存在表则创建 none:不操作-->            <property name="hibernate.hbm2ddl.auto" value="update"/>        </properties>    </persistence-unit></persistence>

创建实体类

@Entity@Table(name = "cst_customer")@Datapublic class Customer {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    @Column(name = "cust_id")    /*    * IDENTITY：底层数据库必须支持自动增长    * SEQUENCE: 底层数据库必须支持序列    * TABLE:jpa提供的机制    * AUTO:由程序自动选择策略    * */    private Long custId;    @Column(name = "cust_name")    private String custName;    @Column(name = "cust_source")    private String custSource;    @Column(name = "cust_level")    private String custLevel;    @Column(name = "cust_industry")    private String custIndustry;    @Column(name = "cust_phone")    private String custPhone;    @Column(name = "cust_address")    private String custAddress;    }

操作

        EntityManager manager = Persistence        .createEntityManagerFactory("jpa").createEntityManager();        EntityTransaction tx = manager.getTransaction();        tx.begin();        Customer customer = new Customer();        customer.setCustName("老王");        customer.setCustIndustry("隔壁");        manager.persist(customer);        tx.commit();        manager.close();
Customer customer = entityManager.find(Customer.class, 1L); // 立即加载Customer customer = entityManager.getReference(Customer.class, 1L); // 延迟加载
Customer customer = entityManager.find(Customer.class, 1L);entityManager.remove(customer);
Customer customer = entityManager.find(Customer.class, 2L);customer.setCustName("老王八");entityManager.merge(customer);

JPQL

基于首次在EJB2.0中引入的EJB查询语言(EJB QL),Java持久化查询语言(JPQL)是一种可移植的查询语言，旨在以面向对象表达式语言的表达式，将SQL语法和简单查询语义绑定在一起·使用这种语言编写的查询是可移植的，可以被编译成所有主流数据库服务器上的SQL

String jpql = "FROM Customer ";List list = entityManager.createQuery(jpql).getResultList();for (Object o : list) {    System.out.println(o);}
FROM Customer ORDER BY custId DESC
SELECT COUNT(custId) FROM Customer
String jpql = "FROM Customer";List list = entityManager.createQuery(jpql)                .setFirstResult(0)                .setMaxResults(2).getResultList();
String jpql = "FROM Customer WHERE custName LIKE ?1";List list = entityManager.createQuery(jpql)                .setParameter(1,"%李%")                .getResultList();