JPA详解

柔光的暖阳◎ 2023-10-13 11:26 18阅读 0赞

#### 一、JPA是什么 ####

1.  PA（Java Persistence API）是Java持久化规范，是ORM框架的标准，主流ORM框架都实现了这个标准。
2.  ORM是一种思想，是在应用程序与JDBC`API`之间插入的一个中间层
3.  JDBC并不能很好地支持面向对象的程序设计，ORM解决了这个问题，通过JDBC将字段高效的与对象进行映射

![1fb2044e31e04bceab8279aab886d246.png][]

#### 二、使用步骤 ####

1.创建maven项目、导入Maven坐标

<properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.hibernate.version>5.0.7.Final</project.hibernate.version>
      </properties>
    
      <dependencies>
    
        <dependency>
          <groupId>org.projectlombok</groupId>
          <artifactId>lombok</artifactId>
          <version>1.18.26</version>
        </dependency>
        
        <dependency>
          <groupId>junit</groupId>
          <artifactId>junit</artifactId>
          <version>4.12</version>
          <scope>test</scope>
        </dependency>
    
        
        <dependency>
          <groupId>org.hibernate</groupId>
          <artifactId>hibernate-entitymanager</artifactId>
          <version>${project.hibernate.version}</version>
        </dependency>
    
        
        <dependency>
          <groupId>org.hibernate</groupId>
          <artifactId>hibernate-c3p0</artifactId>
          <version>${project.hibernate.version}</version>
        </dependency>
    
        
        <dependency>
          <groupId>log4j</groupId>
          <artifactId>log4j</artifactId>
          <version>1.2.17</version>
        </dependency>
    
        
        <dependency>
          <groupId>mysql</groupId>
          <artifactId>mysql-connector-java</artifactId>
          <version>8.0.31</version>
        </dependency>
      </dependencies>

2.创建配置文件，在META-INF文件夹中创建persistence.xml

![0ff1805e74c84e828a91b39687f9dc3b.png][]

3.在persistence.xml配置持久化单元，底层供应商，数据库信息（驱动、url、用户名、密码）， Hibernate个性化配置（显示sql、格式化sql、自动建表）

<?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">
        
        <persistence-unit name="myJpa" transaction-type="RESOURCE_LOCAL">
            
            <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
            <properties>
                
                <property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver" />
                
                <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpa" />
                
                <property name="javax.persistence.jdbc.user" value="root" />
                
                <property name="javax.persistence.jdbc.password" value="123sa" />
    
                
                <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>

#### 4.数据库、数据表创建，实体类的创建 ####

package com.yy.entity;
    import lombok.AllArgsConstructor;
    import lombok.Data;
    import lombok.NoArgsConstructor;
    
    import javax.persistence.*;
    
    /**
     * @Author 杨 
     * @CreateDate 2023-07-04 11:20
     **/
    @Data
    @NoArgsConstructor
    @AllArgsConstructor
    @Entity
    @Table(name = "t_users")
    public class User {
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private int id;
        private String username;
        private String pwd;
        private int age;
    
        public User(String username, String pwd, int age) {
            this.username = username;
            this.pwd = pwd;
            this.age = age;
        }
    }

#### 5.测试 ####

@Test
        public void aa(){
            EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
            //创建实体管理类
            EntityManager em = factory.createEntityManager();
            //获取事务对象
            EntityTransaction tx = em.getTransaction();
            //开启事务
            tx.begin();
            User u = new User("杨","456",24);
            //保存操作
            em.persist(u);
    
            //提交事务
            tx.commit();
            //释放资源
            em.close();
            factory.close();
        }

#### 三、基本增删改查操作 ####

1.  增加：persist(临时对象)
    
        public void aa(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                User u = new User("杨滢","456",24);
                //保存操作
                em.persist(u);
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }
2.  删除：remove(持久化对象)
    
        public void del(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                User u = em.find(User.class, 1);
                //删除操作
                em.remove(u);
        
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }
3.  修改：merge(持久化对象)  
    通过查询方法先查询对象，修改属性，调用merge方法  
    通过查询方法先查询对象，修改属性，entityManager提交之后，发送修改语句  
    通过new对象数据，没有id的对象执行添加操作，有id的对象  
    如果id存在，属性有所变动，执行修改语句  
    如果id不存在，执行添加操作
    
        public void upd(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                User u = em.find(User.class, 1);
                u.setAge(18);
                //修改操作
                em.merge(u);
        
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }
4.  查询：find(反射类型,主键) 及时加载，如果主键对象不存在，得到null  
    getReference(反射类型,主键)，懒加载，如果主键对象不存在，报错空指针
    
        public void sel(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                User user = em.find(User.class, 1);
                System.out.println(user);
                
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }

#### 四、复杂查询操作 ####

1.  查询全部
    
        public void sel(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                String jpa1="from User";
                Query query = em.createQuery(jpa1);
                // 查询并得到返回结果
                List list = query.getResultList(); // 得到集合返回类型
                for (Object object : list) {
                    System.out.println(object);
                }
        
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }
2.  分页查询
    
        public void limit(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                String jpa1="from User";
                Query query = em.createQuery(jpa1);
                query.setFirstResult(0);
                query.setMaxResults(2);
        
                // 查询并得到返回结果
                List list = query.getResultList(); // 得到集合返回类型
                for (Object object : list) {
                    System.out.println(object);
                }
        
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }
3.  条件查询
    
        public void where(){
                EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
                //创建实体管理类
                EntityManager em = factory.createEntityManager();
                //获取事务对象
                EntityTransaction tx = em.getTransaction();
                //开启事务
                tx.begin();
                String jpa1="from User where username like ?1";
                Query query = em.createQuery(jpa1);
                query.setParameter(1, "%杨%");
        
                // 查询并得到返回结果
                List list = query.getResultList(); // 得到集合返回类型
                for (Object object : list) {
                    System.out.println(object);
                }
        
                //提交事务
                tx.commit();
                //释放资源
                em.close();
                factory.close();
            }

#### 五、对象的状态转换 ####

![bb4885bffce37e6b073724c2baa9e5c4.png][]

[1fb2044e31e04bceab8279aab886d246.png]: https://img-blog.csdnimg.cn/1fb2044e31e04bceab8279aab886d246.png
[0ff1805e74c84e828a91b39687f9dc3b.png]: https://img-blog.csdnimg.cn/0ff1805e74c84e828a91b39687f9dc3b.png
[bb4885bffce37e6b073724c2baa9e5c4.png]: https://img-blog.csdnimg.cn/img_convert/bb4885bffce37e6b073724c2baa9e5c4.png