JPA学习
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2022-03-02 15:23:25
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JPA学习
什么是orm
object-relational-mapper:对象关系映射，就是建立对象与数据库之间的映射关系，就是orm
对象关系映射可以大大的减少重复性的代码，提高开发效率；
常见的orm框架：hibernate
目的：达到操作类一样操作数据库；
什么是JPA
是sum公司提供的一套持久层操作规范；
底层可以使用hibernate进行数据库操作；
JPA入门案例
在META-INF下编写配置文件persistence.xml
<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="2.0">
<!-- 配置持久化单元
name: 持久化单元名称
transaction-type： 事务类型
RESOURCE_LOCAL： 本地事务
JTA： 跨数据库事务【分布式事务】
-->
<persistence-unit name="myJpa" transaction-type="RESOURCE_LOCAL">
<!-- 配置提供商 -->
<provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
<!-- 其他属性 -->
<properties>
<!-- 链接数据库的四要素 -->
<!-- mysql-->
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"></property>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/springjpa?useUnicode=true&characterEncoding=UTF-8"></property>
<property name="javax.persistence.jdbc.user" value="root"></property>
<property name="javax.persistence.jdbc.password" value="root"></property>
<!--Oracle
<property name="javax.persistence.jdbc.driver" value="oracle.jdbc.driver.OracleDriver"></property>
<property name="javax.persistence.jdbc.url" value="jdbc:oracle:thin:@dbnode01:1521:xe"></property>
<property name="javax.persistence.jdbc.user" value="feifei"></property>
<property name="javax.persistence.jdbc.password" value="passw0rd"></property>-->
<!-- 其他属性 : 显示sql语句-->
<property name="hibernate.show_sql" value="true"></property>
<!-- 其他属性：让hibernate维护表结构
create: 先删除表，再创建表
update: 有表，就维护表；没有表就更新表
none: 什么也不干
-->
<property name="hibernate.hbm2ddl.auto" value="none"></property>
</properties>
</persistence-unit>
</persistence>
编写与数据库对应的实体类
package com.itheima.domain;
import javax.persistence.*;
import java.io.Serializable;
/**
* 客户实体类
*
* 导的所有类： javax.persistence
*/
//把一个类标记成实体类
@Entity
//映射表名称
@Table(name = "cst_customer")
public class Customer implements Serializable {
//把一个属性标记成主键属性
@Id
//配置主键生成的策略
@GeneratedValue(strategy = GenerationType.IDENTITY) //适用于： Mysql，主键自动增长
//@GeneratedValue(strategy = GenerationType.SEQUENCE) //适用于： Oracle，使用序列机制
//指定序列
/*@GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "customer_id")
@SequenceGenerator(
//名称，给别人引用
name="customer_id",
//数据库中序列的名称
sequenceName = "SEQ_CUSTOMER_ID",
//指定序列的初始值
initialValue = 1,
//增量：步长
allocationSize = 1
)*/
//@GeneratedValue(strategy = GenerationType.TABLE) //使用表来维护id
//@GeneratedValue(strategy = GenerationType.AUTO) //根据环境选择最优策略
//映射字段
@Column(name="cust_id")
private Long custId;
@Column(name="cust_name")
private String custName;
@Column(name="cust_level")
private String custLevel;
@Column(name="cust_source")
private String custSource;
@Column(name="cust_industry")
private String custIndustry;
@Column(name="cust_address")
private String custAddress;
@Column(name="cust_phone")
private String custPhone;
public Long getCustId() {
return custId;
}
public void setCustId(Long custId) {
this.custId = custId;
}
public String getCustName() {
return custName;
}
public void setCustName(String custName) {
this.custName = custName;
}
public String getCustLevel() {
return custLevel;
}
public void setCustLevel(String custLevel) {
this.custLevel = custLevel;
}
public String getCustSource() {
return custSource;
}
public void setCustSource(String custSource) {
this.custSource = custSource;
}
public String getCustIndustry() {
return custIndustry;
}
public void setCustIndustry(String custIndustry) {
this.custIndustry = custIndustry;
}
public String getCustAddress() {
return custAddress;
}
public void setCustAddress(String custAddress) {
this.custAddress = custAddress;
}
public String getCustPhone() {
return custPhone;
}
public void setCustPhone(String custPhone) {
this.custPhone = custPhone;
}
@Override
public String toString() {
return "Customer{" +
"custId=" + custId +
", custName='" + custName + '\'' +
", custLevel='" + custLevel + '\'' +
", custSource='" + custSource + '\'' +
", custIndustry='" + custIndustry + '\'' +
", custAddress='" + custAddress + '\'' +
", custPhone='" + custPhone + '\'' +
'}';
}
}
测试
import com.itheima.domain.Customer;
import org.junit.Test;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
public class DemoTest {
@Test
public void test01(){
EntityManagerFactory myJpa = Persistence.createEntityManagerFactory("myJpa");
EntityManager entityManager = myJpa.createEntityManager();
EntityTransaction transaction = entityManager.getTransaction();
transaction.begin();
Customer customer = new Customer();
customer.setCustName("nihap");
entityManager.persist(customer);
transaction.commit();
entityManager.close();
myJpa.close();
}
}
主键生成策略有4种
@GenetaredValue(strategy=GenerationType.IDENTITY)这个比较常用
-------------------------------------------------------
@GeneratedValue(strategy = GenerationType.SEQUENCE,generator="payablemoney_seq")
@SequenceGenerator(name="payablemoney_seq", sequenceName="seq_payment")
-------------------------------------------------------
@GeneratedValue(strategy = GenerationType.AUTO)
-------------------------------------------------------
@GeneratedValue(strategy = GenerationType.TABLE, generator="payablemoney_gen")
@TableGenerator(name = "pk_gen",
table="tb_generator",
pkColumnName="gen_name",
valueColumnName="gen_value",
pkColumnValue="PAYABLEMOENY_PK",
allocationSize=1
)
Manager方法
这是简单查询：
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
其中就是find和getReference的区别
find：不管要不要使用都会生成sql语句
getReference:是预编译对象，意思就是要用的时候才会生成sql语句，不用的时候是不会生成的；
复杂查询：
编写jpql语句：
全部查询：
String jpql = "from Customer";
Query query = manager.createQuery(jpql);
List resultList = query.getResultList();
分页查询：
Query query = manager.createQuery(jpql);
query.setFirstResult(0);
query.setMaxResults(2);
List resultList = query.getResultList();
条件查询：
String jpql = "from Customer where custName like ?";
Query query = manager.createQuery(jpql);
query.setParameter(1,"n%");
Object singleResult = query.getSingleResult();会根据你的得到的数据判断，如果 是一条就不会错，如果是多条就会报错；
System.out.println(singleResult);
排序查询：
String jpql = "from Customer order by custId desc";
统计查询：
String jpql = "select count(custId) from Customer"；