2)映射关系
JPA定义了one-to-one、one-to-many、many-to-one、many-to-many 4种关系。
对于数据库来说，通常在一个表中记录对另一个表的外键关联；对应到实体对象，持有关联数据的一方称为owning-side，另一方称为inverse-side。
为了编程的方便，我们经常会希望在inverse-side也能引用到owning-side的对象，此时就构建了双向关联关系。 在双向关联中，需要在inverse-side定义mappedBy属性，以指明在owning-side是哪一个属性持有的关联数据。
对关联关系映射的要点如下：
关系类型 Owning-Side Inverse-Side
one-to-one @OneToOne @OneToOne(mappedBy="othersideName")
one-to-many / many-to-one @ManyToOne @OneToMany(mappedBy="xxx")
many-to-many @ManyToMany @ManyToMany(mappedBy ="xxx")
其中 many-to-many关系的owning-side可以使用@JoinTable声明自定义关联表，比如Book和Author之间的关联表：
@JoinTable(name = "BOOKAUTHOR", joinColumns = { @JoinColumn(name = "BOOKID", referencedColumnName = "id") }, inverseJoinColumns = { @JoinColumn(name = "AUTHORID", referencedColumnName = "id") })
关联关系还可以定制延迟加载和级联操作的行为（owning-side和inverse-side可以分别设置）：
通过设置fetch=FetchType.LAZY 或 fetch=FetchType.EAGER来决定关联对象是延迟加载或立即加载。
通过设置cascade={options}可以设置级联操作的行为，其中options可以是以下组合：
CascadeType.MERGE 级联更新CascadeType.PERSIST 级联保存CascadeType.REFRESH 级联刷新CascadeType.REMOVE 级联删除
CascadeType.ALL 级联上述4种操作
4、事件及监听
通过在实体的方法上标注@PrePersist，@PostPersist等声明即可在事件发生时触发这些方法。
三、JPA应用
1、Dependencies
<dependencies>
<dependency>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa</artifactId>
</dependency>
<dependencies>
2、JPA提供的接口
主要来看看Spring Data JPA提供的接口，也是Spring Data JPA的核心概念：
1)：Repository：最顶层的接口，是一个空的接口，目的是为了统一所有Repository的类型，且能让组件扫描的时候自动识别。
2)：CrudRepository ：是Repository的子接口，提供CRUD的功能
public interface CrudRepository<T, ID extends Serializable>extends Repository<T, ID> {
<S extends T> S save(S entity);
T findOne(ID primaryKey);
Iterable<T> findAll();
Long count();
void delete(T entity);
boolean exists(ID primaryKey);
// … more functionality omitted
}
3)：PagingAndSortingRepository：是CrudRepository的子接口，添加分页和排序的功能
public interface PagingAndSortingRepository<T, ID extends Serializable> extends CrudRepository<T, ID> {
Iterable<T> findAll(Sort sort);
Page<T> findAll(Pageable pageable);
}
4)：JpaRepository：是PagingAndSortingRepository的子接口，增加了一些实用的功能，比如：批量操作等。
5)：JpaSpecificationExecutor：用来做负责查询的接口
public interface JpaSpecificationExecutor<T>{
T findOne(Specification<T> spec);
List<T> findAll(Specification<T> spec);
Page<T> findAll(Specification<T> spec, Pageable pageable);
List<T> findAll(Specification<T> spec, Sort sort);
long count(Specification<T> spec);
}
6)：Specification：是Spring Data JPA提供的一个查询规范，要做复杂的查询，只需围绕这个规范来设置查询条件即可
3、查询语言
3.1 根据名称判别
Keyword Sample JPQL snippet
And findByLastnameAndFirstname … where x.lastname = ?1 and x.firstname = ?2
Or findByLastnameOrFirstname … where x.lastname = ?1 or x.firstname = ?2
Is,Equals findByFirstname,findByFirstnameIs,findByFirstnameEquals … where x.firstname = ?1
Between findByStartDateBetween … where x.startDate between ?1 and ?2
LessThan findByAgeLessThan … where x.age < ?1
LessThanEqual findByAgeLessThanEqual … where x.age ⇐ ?1
GreaterThan findByAgeGreaterThan … where x.age > ?1
GreaterThanEqual findByAgeGreaterThanEqual … where x.age >= ?1
After findByStartDateAfter … where x.startDate > ?1
Before findByStartDateBefore … where x.startDate < ?1
IsNull findByAgeIsNull … where x.age is null
IsNotNull,NotNull findByAge(Is)NotNull … where x.age not null
Like findByFirstnameLike … where x.firstname like ?1
NotLike findByFirstnameNotLike … where x.firstname not like ?1
StartingWith findByFirstnameStartingWith … where x.firstname like ?1(parameter bound with appended %)
EndingWith findByFirstnameEndingWith … where x.firstname like ?1(parameter bound with prepended %)
Containing findByFirstnameContaining … where x.firstname like ?1(parameter bound wrapped in%)
OrderBy findByAgeOrderByLastnameDesc … where x.age = ?1 order by x.lastname desc
Not findByLastnameNot … where x.lastname <> ?1
In findByAgeIn(Collection<Age> ages) … where x.age in ?1
NotIn findByAgeNotIn(Collection<Age> age) … where x.age not in ?1
True findByActiveTrue() … where x.active = true
False findByActiveFalse() … where x.active = false
IgnoreCase findByFirstnameIgnoreCase … where UPPER(x.firstame) = UPPER(?1)
3.2 @Query
public interface UserRepository extends JpaRepository<User, Long> {
//Declare query at the query method using @Query
@Query("select u from User u where u.emailAddress = ?1")
User findByEmailAddress(String emailAddress);
//Advanced like-expressions in @Query
@Query("select u from User u where u.firstname like %?1")
List<User> findByFirstnameEndsWith(String firstname);
//Declare a native query at the query method using @Query
@Query(value = "SELECT * FROM USERS WHERE EMAIL_ADDRESS = ?1", nativeQuery = true)
User findByEmailAddress(String emailAddress);
//Declare native count queries for pagination at the query method using @Query
@Query(value = "SELECT * FROM USERS WHERE LASTNAME = ?1",countQuery = "SELECT count(*) FROM USERS WHERE LASTNAME = ?1",nativeQuery = true)
Page<User> findByLastname(String lastname, Pageable pageable);
//Declaring manipulating queries
@Modifying
@Query("update User u set u.firstname = ?1 where u.lastname = ?2")
int setFixedFirstnameFor(String firstname, String lastname);
}
3.3 复杂查询 JpaSpecificationExecutor
Criteria 查询：是一种类型安全和更面向对象的查询
这个接口基本是围绕着Specification接口来定义的， Specification接口中只定义了如下一个方法：
Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb);
Criteria查询
基本对象的构建
1：通过EntityManager的getCriteriaBuilder或EntityManagerFactory的getCriteriaBuilder方法可以得到CriteriaBuilder对象
2：通过调用CriteriaBuilder的createQuery或createTupleQuery方法可以获得CriteriaQuery的实例
3：通过调用CriteriaQuery的from方法可以获得Root实例
过滤条件
1：过滤条件会被应用到SQL语句的FROM子句中。在criteria 查询中，查询条件通过Predicate或Expression实例应用到CriteriaQuery对象上。
2：这些条件使用 CriteriaQuery .where 方法应用到CriteriaQuery 对象上
3：CriteriaBuilder也作为Predicate实例的工厂，通过调用CriteriaBuilder 的条件方法（ equal，notEqual， gt， ge，lt， le，between，like等）创建Predicate对象。
4：复合的Predicate 语句可以使用CriteriaBuilder的and, or andnot 方法构建
实例：
ImTeacher.java
@Entity
@Table(name = "im_teacher")
public class ImTeacher implements Serializable{
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue
@Column(name = "id")
private int id;
@Column(name = "teacher_id")
private int teacherId;
@Column(name = "name")
private String name = "";
@Column(name = "age")
private int age;
@Column(name = "sex")
private String sex = "";
...
}
ImTeacherDao.java
public interface ImTeacherDao extends PagingAndSortingRepository<ImTeacher, Integer>,JpaSpecificationExecutor{
...
}
@Service
public class ImTeacherDaoService {
@Autowired
ImTeacherDao imTeacherDao;
/**
* 复杂查询测试
* @param page
*/
public Page<ImTeacher> findBySepc(int page, int size){
PageRequest pageReq = this.buildPageRequest(page, size);
Page<ImTeacher> imTeachers = this.imTeacherDao.findAll(new MySpec(), pageReq);
return imTeachers;
}
/**
* 建立分页排序请求
*/
private PageRequest buildPageRequest(int page, int size) {
Sort sort = new Sort(Direction.DESC,"age");
return new PageRequest(page,size, sort);
}
private class MySpec implements Specification<ImTeacher>{
@Override
public Predicate toPredicate(Root<ImTeacher> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
//1.混合条件查询
Path<String> exp1 = root.get("name");
Path<String> exp2 = root.get("age");
query.where(cb.like(exp1, "%王%"),cb.equal(exp2, "45"));
//2.多表查询
/*Join<ImTeacher,ImStudent> join = root.join("imStudent", JoinType.INNER);
Path<String> exp3 = join.get("name");
return cb.like(exp3, "%jy%");*/
return null;
}
}
}
3.4 分页
上个实例的发杂查询已经带有分页，若实例的DAO接口有继承PagingAndSortingRepository接口，则可以直接调用
Page<ImTeacher> impeacher = imTeacherDao.findAll(new PageRequest(1,20));
3.5 联表查询
方法：
法一：直接用Query语句或者上节复杂的连接查询，查出两张或多张表的数据。
法二：映射，接下来将详细介绍。
1）ImStudent.java
@Entity
@Table(name = "im_student")
public class ImStudent {
@Id
@GeneratedValue
@Column(name = "id")
private int id;
@Column(name = "student_id")
private int studentId;
@Column(name = "name")
private String name = "";
@Column(name = "age")
private int age;
@Column(name = "sex")
private String sex = "";
@Column(name = "teacher_id")
private int teacherId;