JPA进阶-持久化状态

冷不防 2023-10-09 13:06 27阅读 0赞

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## JPA进阶 ##

### 主键生成策略 ###

**主键：数据的唯一标识，必须是非空唯一的**

JPA主键生成的标准策略有4种：

auto：默认，根据方言自动选择生成策略

identity：mysql的主键自增长策略

sequence：主键的创建效率最高，Oracle的序列对象

table：其他的数据库使用，创建一个表，专门用来存储主键

一般使用默认的生成策略就行了

### JPA持久对象的状态 ###

#### 临时状态 ####

在刚用new语句创建，没有和entityManager发生关系

#### 持久化状态(托管状态) ####

事务开始，和JPA发生关系了，被加入到了一级缓存当中

#### 游离状态(脱管状态) ####

事务提交之后，但是已经和JPA脱离关系了

#### 删除状态：从JPA开始才有 ####

调用了remove()方法，删除对象后，变味了删除状态

#### 脏数据更新 ####

一个持久状态对象在事务管理内,如果改变原来的数据(非主键)，此时出现脏数据,在事务提交的时候自动发出update语句去修改

#### Demo ####

package com.ifueen.sm;
    
    import com.ifueen.util.JPAUtil;
    import org.junit.Test;
    
    import javax.persistence.EntityManager;
    import javax.persistence.EntityTransaction;
    
    /**
     *
     * 测试持久化状态
     *
     */
    public class StateTest {
        
        @Test
        public void test(){
        
            EntityManager jpa = JPAUtil.getJpa();
            Student student = new Student();
            student.setName("希区柯克");   //临时状态
            EntityTransaction transaction = jpa.getTransaction();
            transaction.begin();    //事务开启
            jpa.persist(student);   //脱管状态，与JPA发生关系
            transaction.commit();   //事务提交，对象处于游离状态
            jpa.close();
    
        }
    
        @Test
        public void test1(){
        
            EntityManager jpa = JPAUtil.getJpa();
    
            EntityTransaction transaction = jpa.getTransaction();
            transaction.begin();    //事务开启
            Student student = jpa.find(Student.class, 12L);//脱管状态，与JPA发生关系
            jpa.remove(student);    //删除状态
            transaction.commit();   //事务提交，对象处于游离状态
            jpa.close();
    
        }
    
        @Test
        public void test2(){
        
            EntityManager jpa = JPAUtil.getJpa();
    
            EntityTransaction transaction = jpa.getTransaction();
            transaction.begin();    //事务开启
            Student student = jpa.find(Student.class, 13L);//脱管状态，与JPA发生关系
            System.out.println(student);
            student.setName("库布里克");    //脏数据更新
            transaction.commit();   //事务提交，对象处于游离状态
            jpa.close();
    
        }
    
    
    }

### 域对象之间关系 ###

域对象之间的关系分为一对一，一对多(多对一)，多对多，现主要讲一下多对一的关系

#### 单向多对一关系 ####

说直白点，就是多个学生类可以对应一个老师类

从表Student类

package com.ifueen.sm;
    
    import javax.persistence.*;
    
    @Entity
    @Table(name = "t_student")
    public class Student {
        
    
        @Id
        @GeneratedValue
        private Long id;
        private String name;
    
        @ManyToOne(fetch = FetchType.LAZY)  //懒加载
        private Tearcher tearcher;
    
        public Student() {
        
        }
    
        @Override
        public String toString() {
        
            return "Student{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", tearcher=" + tearcher +
                    '}';
        }
    
        public Long getId() {
        
            return id;
        }
    
        public void setId(Long id) {
        
            this.id = id;
        }
    
        public String getName() {
        
            return name;
        }
    
        public void setName(String name) {
        
            this.name = name;
        }
    
        public Tearcher getTearcher() {
        
            return tearcher;
        }
    
        public void setTearcher(Tearcher tearcher) {
        
            this.tearcher = tearcher;
        }
    }

主表Tearch类设计

package com.ifueen.sm;
    
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.Table;
    
    @Entity
    @Table(name = "t_tearch")
    public class Tearcher {
        
        @Id
        @GeneratedValue
        private Long id;
        private String name;
    
        public Tearcher() {
        
        }
    
        @Override
        public String toString() {
        
            return "Tearcher{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    '}';
        }
    
        public Long getId() {
        
            return id;
        }
    
        public void setId(Long id) {
        
            this.id = id;
        }
    
        public String getName() {
        
            return name;
        }
    
        public void setName(String name) {
        
            this.name = name;
        }
    }

测试

package com.ifueen.sm;
    
    import com.ifueen.util.JPAUtil;
    import org.junit.Test;
    
    import javax.persistence.EntityManager;
    import javax.persistence.EntityTransaction;
    
    public class StudentTest {
        
    
    
        @Test
        public void create(){
        
    
            EntityManager jpa = JPAUtil.getJpa();
    
        }
    
        @Test
        public void test1(){
        
            //一方
            Tearcher tearcher = new Tearcher();
            tearcher.setName("高级算法");
            //多方
            Student student = new Student();
            student.setName("山泥若");
            student.setTearcher(tearcher);
    
    
    
            EntityManager jpa = JPAUtil.getJpa();
            EntityTransaction transaction = jpa.getTransaction();
            transaction.begin();
            jpa.persist(student);
            jpa.persist(tearcher);
            transaction.commit();
            jpa.close();
        }
    
        @Test
        public void test2(){
        
            //一方
            Tearcher tearcher = new Tearcher();
            tearcher.setName("物理大牛");
            //多方
            Student student = new Student();
            student.setName("牛顿");
            student.setTearcher(tearcher);
    
            Student student1 = new Student();
            student1.setName("爱因斯坦");
            student1.setTearcher(tearcher);
    
            Student student2 = new Student();
            student2.setName("霍金");
            student2.setTearcher(tearcher);
    
            Student student3 = new Student();
            student3.setName("杨振宁");
            student3.setTearcher(tearcher);
    
            EntityManager jpa = JPAUtil.getJpa();
            EntityTransaction transaction = jpa.getTransaction();
            transaction.begin();
            jpa.persist(tearcher);
            jpa.persist(student);
            jpa.persist(student1);
            jpa.persist(student2);
            jpa.persist(student3);
            transaction.commit();
            jpa.close();
        }
    
        @Test
        public void test3(){
        
    
            EntityManager jpa = JPAUtil.getJpa();
            Student student = jpa.find(Student.class, 3L);
            System.out.println(student.hashCode());
            //懒加载之后
            Tearcher tearcher = student.getTearcher();
            System.out.println(tearcher);
            jpa.close();
        }
    
    }

#### fetch抓取策略 ####

**FetchType.EAGER：fetch的默认值，立即加载**

默认情况下，JPA持续性提供程序使用获取类型EAGER：这将要求持续性提供程序运行时必须迫切左外连接获取数据。

**FetchType.LAZY：延迟加载**

FetchType.LAZY：这将提示持续性提供程序在首次访问数据时并不急于获取数据。

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