JPA 基础
JPA 基础
数据库驱动 ==> JDBC 规范 ==> ORM 框架 ==> JPA 规范 ==> spring-data-jpa
ORM 思想
JPA 的使用步骤
jpa 依赖
<properties>
<project.hibernate.version>5.4.2.Final</project.hibernate.version>
</properties>
<!-- hibernate对jpa的支持包 -->
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-entitymanager</artifactId>
<version>${project.hibernate.version}</version>
</dependency>
<!-- c3p0 -->
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-c3p0</artifactId>
<version>${project.hibernate.version}</version>
</dependency>
<!-- Mysql and MariaDB -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.6</version>
</dependency>
jpa 的持久化配置
<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">
<!-- 持久化单元 -->
<persistence-unit name="myJPA" transaction-type="RESOURCE_LOCAL">
<!-- JPA 的实现者 -->
<provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
<properties>
<!-- 数据库信息
驱动 javax.persistence.jdbc.driver
数据库地址 javax.persistence.jdbc.url
用户名 javax.persistence.jdbc.user
密码 javax.persistence.jdbc.password
-->
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:mysql:///jpa_db" />
<property name="javax.persistence.jdbc.user" value="root" />
<property name="javax.persistence.jdbc.password" value="root" />
<!-- jpa 实现者的配置
显示 sql : hibernate.show_sql
格式化 sql : hibernate.format_sql
创建表: hibernate.hbm2ddl.auto 库必须存在,hibernate 不会自动创建库
create: 运行时创建,有则先删除再创建
update: 运行时创建,有则不创建
none: 不创建
-->
<property name="hibernate.show_sql" value="true" />
<property name="hibernate.format_sql" value="true" />
<property name="hibernate.hbm2ddl.auto" value="update" />
</properties>
</persistence-unit>
</persistence>
实体类
package com.mozq.jpa.domain;
import org.hibernate.annotations.GeneratorType;
import javax.persistence.*;
@Entity
@Table
public class Customer {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long custId;
@Column
private String custName;
@Column
private String custSource;
@Column
private String custLevel;
@Column
private String custIndustry;
@Column
private String custPhone;
@Column
private String custAddress;
//省略构造器和 get/set
}
使用 jpa 的 API 进行操作
JPA实体的四种状态 https://www.jianshu.com/p/636954880af8
JPA中的实体对象拥有四种状态:
- 瞬时状态(transient)
- 持久状态(persistent)
- 游离状态(detached)
- 删除状态 (deleted)
瞬时状态
瞬时状态的实体就是一个普通的java对象,和持久化上下文无关联,数据库中也没有数据与之对应。
托管状态
使用 EntityManager 进行 find 或者 persist 操作返回的对象即处于托管状态,此时该对象已经处于持久化上下文中,因此任何对于该实体的更新都会同步到数据库中。
游离状态
当事务提交后,处于托管状态的对象就转变为了游离状态。此时该对象已经不处于持久化上下文中,因此任何对于该对象的修改都不会同步到数据库中。但是数据库中有这个对象对应的记录。
删除状态
当调用EntityManger对实体进行delete后,该实体对象就处于删除状态。其本质也就是一个瞬时状态的对象。
持久:处于持久的对象被 EntityManager 管理,当事务被提交时,则对持久对象的任何修改都将同步到数据库中。
public interface EntityManager {
/*
瞬时对象
游离对象将报异常
*/
void persist(Object var1); // 对象持久化,交给 EntityManager 管理。数据库中必须不存在。瞬时 ==> 持久
<T> T merge(T var1);// 根据 id 是否存在,插入新对象或更新现有对象。瞬时 ==> 持久 / 游离 ==> 持久
void remove(Object var1);
<T> T find(Class<T> entityClass, Object id);// 立即加载,返回真实对象
<T> T getReference(Class<T> entityClass, Object id);// 延迟加载,返回代理对象
void flush();
/* obj 必须是受管对象,用于将数据库数据同步到受管对象,如果是其他状态的对象将抛异常。*/
void refresh(Object var1);
void clear();
void detach(Object var1);
void close();//关闭
EntityTransaction getTransaction();// 获取事务对象
}
# void refresh(Object var1);
java.lang.IllegalArgumentException: Entity not managed
# persist 方法异常
Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: com.mozq.jpa.domain.Customer
Customer customer = new Customer();
customer.setCustId(1L);
customer.setCustName("刘备");
em.persist(customer);//此时数据库中存在 1L 记录,报错。
@Test
public void testSave(){
// Persistence 创建实体类管理器工厂
EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJPA");
// 实体类管理器
EntityManager em = factory.createEntityManager();
// 事务对象
EntityTransaction tx = em.getTransaction();
tx.begin();
// CRUD
Customer customer = new Customer();
customer.setCustName("刘备");
em.persist(customer); // 保存
tx.commit();
// 释放资源
em.close();
factory.close();
}
JPQL 查询
// 查询全部
Query query = em.createQuery("from Customer");
List resultList = query.getResultList();
System.out.println(resultList);
// 排序
Query query = em.createQuery("from Customer order by custAddress desc ,custName asc, custId");
List resultList = query.getResultList();
/*
order by
customer0_.custAddress desc,
customer0_.custName asc,
customer0_.custId
*/
// 分页
Query query = em.createQuery("from Customer order by custId");
query.setFirstResult(2);
query.setMaxResults(3);
/*
order by
customer0_.custId limit ?, ?
*/
Hibernate 主键生成策略和创建表策略
public enum GenerationType {
TABLE,
SEQUENCE,
IDENTITY,
AUTO;
private GenerationType() {
}
}
hibernate.hbm2ddl.auto create/update/none
# <property name="hibernate.hbm2ddl.auto" value="create" /> # 先删除表,再创建表
# @GeneratedValue(strategy = GenerationType.IDENTITY)
drop table if exists Customer
create table Customer (
custId bigint not null auto_increment, # 使用自增主键,生成的 sql 不会有主键字段。需要数据库支持自增。
custAddress varchar(255),
custIndustry varchar(255),
custLevel varchar(255),
custName varchar(255),
custPhone varchar(255),
custSource varchar(255),
primary key (custId)
) engine=InnoDB
insert
into
Customer
(custAddress, custIndustry, custLevel, custName, custPhone, custSource)
values
(?, ?, ?, ?, ?, ?)
# <property name="hibernate.hbm2ddl.auto" value="update" />
# @GeneratedValue(strategy = GenerationType.IDENTITY) # 使用自增主键,生成的 sql 不会有主键字段。需要数据库支持自增。
create table Customer ( # 如果表不存在则会发送创建表的语句。
custId bigint not null auto_increment,
custAddress varchar(255),
custIndustry varchar(255),
custLevel varchar(255),
custName varchar(255),
custPhone varchar(255),
custSource varchar(255),
primary key (custId)
) engine=InnoDB
insert
into
Customer
(custAddress, custIndustry, custLevel, custName, custPhone, custSource)
values
(?, ?, ?, ?, ?, ?)
GenerationType.AUTO + create
drop table if exists Customer
drop table if exists hibernate_sequence
create table Customer (
custId bigint not null,
custAddress varchar(255),
custIndustry varchar(255),
custLevel varchar(255),
custName varchar(255),
custPhone varchar(255),
custSource varchar(255),
primary key (custId)
) engine=InnoDB
create table hibernate_sequence (
next_val bigint
) engine=InnoDB
insert into hibernate_sequence values ( 1 )
select
next_val as id_val
from
hibernate_sequence for update 锁
update
hibernate_sequence
set
next_val= ?
where
next_val=?
insert into
Customer
(custAddress, custIndustry, custLevel, custName, custPhone, custSource, custId)
values
(?, ?, ?, ?, ?, ?, ?)
JPA
Caused by: org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): com.mozq.jpa.domain.Customer