Springdata jpa 入门

1.使用开始

配置maven

1 <dependency>
2   <groupId>org.springframework.boot</groupId>
3   <artifactId>spring-boot-starter-data-jpa</artifactId>
4 </dependency>

配置properties

# database
spring.datasource.url=jdbc:mysql://127.0.0.1:3306/stuadmin?useUnicode=true&characterEncoding=UTF-8&useSSL=false&serverTimezone=GMT%2B8
spring.datasource.username=root
spring.datasource.password=root
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.type = com.alibaba.druid.pool.DruidDataSource

spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=none
spring.jpa.database-platform=org.hibernate.dialect.MySQL55Dialect

创建实体

2.一对一关系

• Teacher 教师

@Entity //表示实体
@Table(name = "t_teacher") // 表示数据库表名
// 数据库json序列化异常
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class Teacher {

    @Id //表示id主键
    @GeneratedValue(strategy = GenerationType.IDENTITY) //自增长
    @Column(name = "id", precision = 8) // 列名 总长度
    private Long id;

    @Column(name = "name", length = 20) // length只作用于varchar类型
    private String name;

    @Column(name = "subject", columnDefinition = "varchar(25) not null COMMENT '教授课程'") // columnDefinition 相当于添加一些细微属性操作 会拼接到数据
    private String subject;
}

• WorkCard 工作证

@Entity
@Table(name = "t_work_card")
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class WorkCard {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "number", unique = true)
    private Long number;

    @Column(name = "overtime", columnDefinition = "datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT '过期时间'")
    private Date overtime;

    // 级联 懒加载
    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    //当前表id参照teacher表teacher_id字段 无外键(逻辑外键) 
    @JoinColumn(name = "teacher_id", referencedColumnName = "id",foreignKey = @ForeignKey(name = "none",value = ConstraintMode.NO_CONSTRAINT))
    private Teacher teacher;
}

• 定义接口repository

public interface WorkCardRepository extends JpaRepository<WorkCard, Long> {
}

• 使用

@Autowired
WorkCardRepository workCardRepository;

@GetMapping("/getteacherandworkcard")
@ResponseBody
public WorkCard queryWorkcardAndTeacher() {

    // 级联查询
    WorkCard one = workCardRepository.getOne(1L);
    return one;
}

多对一

• Classz 教室

@Entity
@Table(name = "t_class")
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class Classz {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "name", columnDefinition = "varchar(25) not null COMMENT '班级名称'")
    private String name;

    //一对多 当前教室为1 学生为多
    //当前属性映射到classz中
    //懒加载
    @OneToMany(mappedBy = "classz", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Student> studentList;
}

• Student 学生

@Entity
@Table(name = "t_student")
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "name", columnDefinition = "varchar(25) not null COMMENT '姓名'")
    private String name;

    @Column(name = "age")
    private Integer age;

    @Column(name = "gender", columnDefinition = "varchar(2) not null default '男' COMMENT '性别'")
    private String gender;

    @Column(name = "distance",  columnDefinition = "decimal(5,2) COMMENT '离校距离'")
    private BigDecimal distance;

    @ManyToOne(fetch = FetchType.LAZY) // 多对一
    // class_id字段映射 Class
    @JoinColumn(name = "class_id", foreignKey = @ForeignKey(name = "none",value = ConstraintMode.NO_CONSTRAINT))
    @JsonIgnore
    private Classz classz;
}

• 定义repository

public interface ClasszRepository extends JpaRepository<Classz, Long> {
}

• 使用

@GetMapping("/getclassz")
@ResponseBody
public Classz queryClassz() {
    //查修班级
    Classz one = classzRepository.getOne(3L);
    return one;
}

多对多

• 教室

@Entity @Table(name = "t_class")
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class Classz {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "name", columnDefinition = "varchar(25) not null COMMENT '班级名称'")
    private String name;

    @ManyToMany(mappedBy = "classzList", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JsonIgnore
    private List<Teacher> teachers;
}

• 老师

@Entity @Table(name = "t_teacher")
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class Teacher {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", precision = 8)
    private Long id;

    @Column(name = "name", length = 20)
    private String name;

    @Column(name = "subject", columnDefinition = "varchar(25) not null COMMENT '教授课程'")
    private String subject;

    @ManyToMany(cascade=CascadeType.ALL, fetch = FetchType.LAZY) //Teacher是关系的维护端
    //将会生成中间关联表
    @JoinTable(name = "t_teacher_class_ref",
            joinColumns = @JoinColumn(name="teacher_id", foreignKey = @ForeignKey(name = "none",value = ConstraintMode.NO_CONSTRAINT)),
            inverseJoinColumns = @JoinColumn(name = "class_id", foreignKey = @ForeignKey(name = "none",value = ConstraintMode.NO_CONSTRAINT)),
            uniqueConstraints = {@UniqueConstraint(columnNames={"class_id", "teacher_id"})}) // class_id teacher_id 联合索引
    private List<Classz> classzList;//教室
}

• 定义repository

• 使用

@GetMapping("/getteachers")
@ResponseBody
public Teacher queryTeachers() {

    // 级联查询
    Teacher one = teacherRepository.getOne(1L);

    return one;
}

自定义方法名

public interface UserRepository extends JpaRepository<User, Long> {

    List<User> findByAge(Integer age);

    List<User> findAllByAgeBetween(Integer start, Integer end);

    List<User> findUsersByAgeAndGenderLessThan(Integer age, Integer gender);

    @Query(nativeQuery = true, value = "select * from t_user where username = :username")
    List<User> findUsersByName(@Param("username") String username);
}

native sql

    @Query(nativeQuery = true, value = "select * from t_user where username = :username")
    List<User> findUsersByName(@Param("username") String username);

jpql

    @Query("select s from Student s join s.classz c where c.name = :classname and s.name like :sname%")
    List<Student> getStudentByName(@Param("classname") String classname, @Param("sname") String sname);

分页

public interface StudentPagingAndSortRepository extends PagingAndSortingRepository<Student, Long> {
}

@GetMapping("/get/studensbypage")
@ResponseBody
public List<Student> getStudentsByPage(@Param("page") Integer page, @Param("size") Integer size) {

    Pageable pageRequest = PageRequest.of(page, size);
    Page<Student> findStudentsByPage = studentPagingAndSortRepository.findAll(pageRequest);
    return findStudentsByPage.getContent();
}
或者使用JpaSpecificationExecutor 动态查询 和 pagerequest联合使用

Pageable pageable = PageRequest.of(page,size);
Specification<Student> specification = new Specification<Student>() {
    @Override
    public Predicate toPredicate(Root<Student> root, CriteriaQuery<?> criteriaQuery, CriteriaBuilder criteriaBuilder) {

        List<Predicate> predicates = new ArrayList<Predicate>();
        //age
        Predicate age = criteriaBuilder.between(root.<Integer>get("age"), agegt, agelt);
        predicates.add(age);
        //dis
        Predicate distance = criteriaBuilder.equal(root.<Double>get("distance"), BigDecimal.valueOf(dis));
        predicates.add(distance);
        //gender
        Predicate gender = criteriaBuilder.equal(root.<String>get("gender"), gend);
        predicates.add(gender);

        Predicate endset = criteriaBuilder.and(predicates.toArray(new Predicate[predicates.size()]));
        return endset;
    }
};
// 查询
Page<Student> students = studentSpecificationRepository.findAll(specification, pageable);
return students.getContent();
}

级联关系 - CascadeType

https://www.jianshu.com/p/e8caafce5445

• CascadeType.REMOVE

• CascadeType.MERGE

• CascadeType.DETACH

• CascadeType.REFRESH

• CascadeType.ALL

@EntityGhpra解决n+1问题

• 1.用EntityGhpra解决n+1

　　

@Entity
@Table(name = "t_class")
@NamedEntityGraph(name = "Classz.Graph", attributeNodes = {@NamedAttributeNode("studentList")}) //注意这句，先用@NamedEntityGraph指名要加载的studentList属性
@JsonIgnoreProperties(value={"hibernateLazyInitializer","handler","fieldHandler"})
public class Classz {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "name", columnDefinition = "varchar(25) not null COMMENT '班级名称'")
    private String name;

    //一对多 当前教室为1 学生为多
    //当前属性映射到classz中
    //懒加载
    @OneToMany(mappedBy = "classz", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Student> studentList;
}

• 2.调用方法时，指名要进行加载优化的对象

public interface ClasszRepository extends JpaRepository<Classz, Long> {
　　// 解决懒加载n+1问题
　　@EntityGraph(value = "Classz.Graph", type = EntityGraph.EntityGraphType.FETCH)
　　public List<Classz> findAll();
}

自定义repository

 https://blog.csdn.net/chengqiuming/article/details/82531227

jpa与hibernate的关系

1. JPA

 JPA全称： Java Persistence API，JPA通过JDK 5.0注解或XML描述对象－关系表的映射关系，并将运行期的实体对象持久化到数据库中。  JPA的出现有两个原因：
  　　1. 其一，简化现有Java EE和Java SE应用的对象持久化的开发工作；
 　　 2. 其二，Sun希望整合对ORM技术，实现持久化领域的统一。

 Sun之所以提出JPA规范，其目的是以官方身份来统一各种ORM框架的规范，包括著名的Hibernate、TopLink等，不过JPA规范给开发者带来了福音：

开发者面向JPA规范的接口，但底层的JPA实现可以任意切换：

　　觉得Hibernate好的，可以选择Hibernate JPA实现；觉得TopLink好的，可以选择TopLink JPA实现……

这样开发者可以避免为使用Hibernate学习一套ORM框架，为使用TopLink又要再学习一套ORM框架

  JPA提供的技术：

•   (1)ORM映射元数据

  　　JPA支持XML和JDK 5.0注解两种元数据的形式，元数据描述对象和表之间的映射关系，框架据此将实体对象持
  久化到数据库表中；

•   (2)JPA 的API

  　　用来操作实体对象，执行CRUD操作，框架在后台替我们完成所有的事情，开发者从繁琐的JDBC和SQL代码中解
  脱出来。

•   (3)查询语言

 　　 通过面向对象而非面向数据库的查询语言查询数据，避免程序的SQL语句紧密耦合

1. Hibernate

　　JPA是需要Provider来实现其功能的，Hibernate就是JPA Provider中很强的一个。

Java 8日期和时间支持

SQL片段映射

不可变的实体类型

实体过滤器

SQL片段匹配

手动冲洗模式

二级缓存查询

软删除

 这些高级功能都是hibernate所拥有的。