jpa入门

1.与表对应的实体类

 

@Entity //声明实体类
@Table(name = "cst_customer") //实体类与表建立映射关系
public class Customer  {
    @Id //主键
    @GeneratedValue(strategy = GenerationType.IDENTITY)//主键自增
    @Column(name = "cust_id") //字段名与属性名
    private Long custId;
    @Column(name = "cust_name")
    private String custName;
    @Column(name = "cust_source")
    private String custSource;
    @Column(name = "cust_industry")
    private String custIndustry;
    @Column(name ="cust_level")
    private String custLevel;
    @Column(name = "cust_address")
    private String custAddress;
    @Column(name = "cust_phone")
    private String custPhone;
   //getter setter方法

2.配置文件在resources下META-INF下的persistence.xml中(固定位置名称)

<?xml version="1.0" encoding="UTF-8"?>
<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_2_0.xsd"
             version="2.0">
    <!--resource_local本地事务管理-->
    <persistence-unit name="myJpa" transaction-type="RESOURCE_LOCAL">
        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
        <properties>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql:///jpa"/>
            <property name="javax.persistence.jdbc.user" value="root"/>
            <property name="javax.persistence.jdbc.password" value="root"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <!--update有表就不创建表了,没有就创建表 create无论有没有都创建新表,只会存最新的数据-->
            <property name="hibernate.hb2ddl.auto" value="update"/>
        </properties>
    </persistence-unit>

</persistence>

3.工具类

/**
 * jpa工具类
 */
public class JpaUtil {
    private static EntityManagerFactory factory;
    static{ //加载配置文件,创建出工厂对象
         factory = Persistence.createEntityManagerFactory("myJpa");
    }

    public static EntityManager getEntityManager(){
        return factory.createEntityManager();
    }
}

4.测试

 

    @Test
    public void testPersist(){
        Customer customer = new Customer();
        customer.setCustName("传智播客");
        customer.setCustAddress("江苏");
        customer.setCustLevel("high");
        customer.setCustIndustry("IT教育");
        EntityManager em =null;
        EntityTransaction tx=null;
        try {
            em=JpaUtil.getEntityManager();
            tx=em.getTransaction();
            tx.begin(); //开启
            em.persist(customer);
            tx.commit();//提交
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();//回滚
        } finally {
            em.close();//释放资源
        }
    }

    @Test
    public void find(){ //立即加载
        EntityManager em =null;
        EntityTransaction tx=null;
        try {
            em=JpaUtil.getEntityManager();
            tx=em.getTransaction();
            tx.begin();
            Customer customer = em.find(Customer.class, 2L);
            Customer customer1 = em.find(Customer.class, 2L);
            System.out.println(customer==customer1); //true EntityManager有缓存存在
            tx.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();
        } finally {
            em.close();
        }
    }

    @Test
    public void getReference(){//延迟加载
        EntityManager em =null;
        EntityTransaction tx=null;
        try {
            em=JpaUtil.getEntityManager();
            tx=em.getTransaction();
            tx.begin();
            Customer customer = em.getReference(Customer.class, 3L);
            Customer customer1 = em.getReference(Customer.class, 3L);
            tx.commit();
            System.out.println(customer==customer1);//true 有缓存
            System.out.println(customer);
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();
        } finally {
            em.close();
        }
    }

    @Test
    public void remove(){
        EntityManager em =null;
        EntityTransaction tx=null;
        try {
            em=JpaUtil.getEntityManager();
            tx=em.getTransaction();
            tx.begin();
            Customer customer = em.find(Customer.class, 1L);
            em.remove(customer);
            tx.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();
        } finally {
            em.close();
        }
    }

    @Test
    public void merge(){
        EntityManager em =null;
        EntityTransaction tx=null;
        try {
            em=JpaUtil.getEntityManager();
            tx=em.getTransaction();
            tx.begin();
            Customer c1 = em.find(Customer.class, 3L);
            c1.setCustName("黑马来了");
            em.merge(c1);
            tx.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();
        } finally {
            em.close();
        }
    }

 find与getReference区别

   1.find 是立即加载,getReference是延迟加载

  2.find返回的是真实的实体对象,getReference返回的是代理对象

  3.如果查询对象不存在,find返回null,getReference报错