【SpringData&JPA从入门到精通】03-映射关联关系
目录
映射关联关系
1、单向多对一
Order 实体类（多）
@Table(name = "JPA_ORDER")
@Entity
public class Order {
private Integer id;
private String orderName;
private Customer customer;
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "ORDER_NAME")
public String getOrderName() {
return orderName;
}
public void setOrderName(String orderName) {
this.orderName = orderName;
}
@JoinColumn(name = "CUSTOMERS_ID")
@ManyToOne
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
}
Customer 实体类（一）
@Entity
@Table(name = "JPA_CUSTOMERS")
public class Customer {
private Integer id;
private String lastName;
private String email;
private int age;
private Date birthDay;
private Date createTime;
@GeneratedValue(strategy = GenerationType.AUTO)
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "LAST_NAME", length = 50, nullable = false)
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Temporal(TemporalType.DATE)
public Date getBirthDay() {
return birthDay;
}
public void setBirthDay(Date birthDay) {
this.birthDay = birthDay;
}
@Temporal(TemporalType.TIMESTAMP)
public Date getCreateTime() {
return createTime;
}
public void setCreateTime(Date createTime) {
this.createTime = createTime;
}
}
1.1、保存
测试方法 1：先保存 Customer 对象（1 的一方），后保存 Order 对象（n 的一方）
@Test
public void testPersistence() {
Customer customer1 = new Customer();
customer1.setLastName("customer1");
customer1.setAge(21);
customer1.setEmail("customer1@qq.com");
customer1.setCreateTime(new Date());
customer1.setBirthDay(new Date());
Order order1 = new Order();
order1.setOrderName("Customer1-Order1");
Order order2 = new Order();
order2.setOrderName("Customer1-Order2");
// 设置关联关系
order1.setCustomer(customer1);
order2.setCustomer(customer1);
entityManager.persist(customer1);
entityManager.persist(order1);
entityManager.persist(order2);
}
日志信息
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
测试方法 2：先保存 Order 对象（n 的一方），后保存 Customer 对象（1 的一方）
@Test
public void testPersist2() {
Customer customer2 = new Customer();
customer2.setLastName("customer2");
customer2.setAge(21);
customer2.setEmail("customer2@qq.com");
customer2.setCreateTime(new Date());
customer2.setBirthDay(new Date());
Order order1 = new Order();
order1.setOrderName("Customer2-Order1");
Order order2 = new Order();
order2.setOrderName("Customer2-Order2");
// 设置关联关系
order1.setCustomer(customer2);
order2.setCustomer(customer2);
entityManager.persist(order1);
entityManager.persist(order2);
entityManager.persist(customer2);
}
日志信息
Hibernate:
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
update
JPA_ORDER
set
CUSTOMERS_ID=?,
ORDER_NAME=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMERS_ID=?,
ORDER_NAME=?
where
id=?
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
测试方法 1 和测试方法 2 的区别：
- insert 语句顺序不一致
- 方法 2 多了两条 update
建议：在保存多对一的关联关系时，建议先保存 1 的一端，后保存 n 的一端，这样不会多出额外的 update 语句
1.2、获取
@Test
public void testFind() {
Order order = entityManager.find(Order.class, 1);
System.out.println(order.getOrderName());
System.out.println(order.getCustomer().getLastName());
}
日志信息
Hibernate:
select
order0_.id as id1_1_1_,
order0_.CUSTOMERS_ID as CUSTOMER3_1_1_,
order0_.ORDER_NAME as ORDER_NA2_1_1_,
customer1_.id as id1_0_0_,
customer1_.age as age2_0_0_,
customer1_.birthDay as birthDay3_0_0_,
customer1_.createTime as createTi4_0_0_,
customer1_.email as email5_0_0_,
customer1_.LAST_NAME as LAST_NAM6_0_0_
from
JPA_ORDER order0_
left outer join
JPA_CUSTOMERS customer1_
on order0_.CUSTOMERS_ID=customer1_.id
where
order0_.id=?
Customer1-Order1
customer1
注意：默认情况下，使用左外连接获取 n 的一端的对象和其关联的 1 的一端的对象
想要改变其默认的加载策略，可以修改 Order 实体类中 @ManyToOne 的 fetch 属性值为 LAZY
@JoinColumn(name = "CUSTOMERS_ID")
// fetch 设置关联属性的默认加载策略
@ManyToOne(fetch = FetchType.LAZY)
public Customer getCustomer() {
return customer;
}
再次测试，查看日志信息
Hibernate:
select
order0_.id as id1_1_0_,
order0_.CUSTOMERS_ID as CUSTOMER3_1_0_,
order0_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_ORDER order0_
where
order0_.id=?
Customer1-Order1
Hibernate:
select
customer0_.id as id1_0_0_,
customer0_.age as age2_0_0_,
customer0_.birthDay as birthDay3_0_0_,
customer0_.createTime as createTi4_0_0_,
customer0_.email as email5_0_0_,
customer0_.LAST_NAME as LAST_NAM6_0_0_
from
JPA_CUSTOMERS customer0_
where
customer0_.id=?
customer1
1.3、删除
删除 n 的一端的记录
@Test
public void testRemoveN() {
Order order = entityManager.find(Order.class, 2);
entityManager.remove(order);
}
日志信息
Hibernate:
select
order0_.id as id1_1_0_,
order0_.CUSTOMERS_ID as CUSTOMER3_1_0_,
order0_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_ORDER order0_
where
order0_.id=?
Hibernate:
delete
from
JPA_ORDER
where
id=?
删除 1 的一端的记录
@Test
public void testRemove1() {
Customer customer = entityManager.find(Customer.class, 9);
entityManager.remove(customer);
}
日志信息
Hibernate:
select
customer0_.id as id1_0_0_,
customer0_.age as age2_0_0_,
customer0_.birthDay as birthDay3_0_0_,
customer0_.createTime as createTi4_0_0_,
customer0_.email as email5_0_0_,
customer0_.LAST_NAME as LAST_NAM6_0_0_
from
JPA_CUSTOMERS customer0_
where
customer0_.id=?
Hibernate:
delete
from
JPA_CUSTOMERS
where
id=?
ERROR: Cannot delete or update a parent row: a foreign key constraint fails (`jpa`.`jpa_order`, CONSTRAINT `FK_ly166u7lew35w5ap1s4ocb5l6` FOREIGN KEY (`CUSTOMERS_ID`) REFERENCES `jpa_customers` (`id`))
javax.persistence.RollbackException: Error while committing the transaction
<1 internal line>
at helloworld.Many2OneTest.destory(Many2OneTest.java:30) <26 internal line>
Caused by: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1387)
at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1310)
at org.hibernate.ejb.TransactionImpl.commit(TransactionImpl.java:80)
... 27 more
Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement <19 internal line>
... 27 more
Caused by: java.sql.SQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails (`jpa`.`jpa_order`, CONSTRAINT `FK_ly166u7lew35w5ap1s4ocb5l6` FOREIGN KEY (`CUSTOMERS_ID`) REFERENCES `jpa_customers` (`id`))
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:117)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:953)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdateInternal(ClientPreparedStatement.java:1098)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdateInternal(ClientPreparedStatement.java:1046)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeLargeUpdate(ClientPreparedStatement.java:1371)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdate(ClientPreparedStatement.java:1031) <1 internal line>
... 41 more
注意：不能直接删除 1 的一端，因为有外键约束
1.4、更新
@Test
public void testUpdate() {
Order order = entityManager.find(Order.class, 1);
order.getCustomer().setLastName("Manny2One-Update");
}
日志信息
Hibernate:
select
order0_.id as id1_1_0_,
order0_.CUSTOMERS_ID as CUSTOMER3_1_0_,
order0_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_ORDER order0_
where
order0_.id=?
Hibernate:
select
customer0_.id as id1_0_0_,
customer0_.age as age2_0_0_,
customer0_.birthDay as birthDay3_0_0_,
customer0_.createTime as createTi4_0_0_,
customer0_.email as email5_0_0_,
customer0_.LAST_NAME as LAST_NAM6_0_0_
from
JPA_CUSTOMERS customer0_
where
customer0_.id=?
Hibernate:
update
JPA_CUSTOMERS
set
age=?,
birthDay=?,
createTime=?,
email=?,
LAST_NAME=?
where
id=?
JPA_CUSTOMERS 表数据
2、单向一对多
Customer 实体类（一）
@Entity
@Table(name = "JPA_CUSTOMERS")
public class Customer {
private Integer id;
private String lastName;
private String email;
private int age;
private Date birthDay;
private Date createTime;
private Set<Order> orders = new HashSet<>();
@GeneratedValue(strategy = GenerationType.AUTO)
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "LAST_NAME", length = 50, nullable = false)
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
@Basic
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Temporal(TemporalType.DATE)
public Date getBirthDay() {
return birthDay;
}
public void setBirthDay(Date birthDay) {
this.birthDay = birthDay;
}
@Temporal(TemporalType.TIMESTAMP)
public Date getCreateTime() {
return createTime;
}
public void setCreateTime(Date createTime) {
this.createTime = createTime;
}
// @JoinColumn 映射外键列名称
@JoinColumn(name = "CUSTOMER_ID")
// @OneToMany 映射 1-n 的关联关系
@OneToMany
public Set<Order> getOrders() {
return orders;
}
public void setOrders(Set<Order> orders) {
this.orders = orders;
}
}
Order 实体类（多）
@Table(name = "JPA_ORDER")
@Entity
public class Order {
private Integer id;
private String orderName;
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "ORDER_NAME")
public String getOrderName() {
return orderName;
}
public void setOrderName(String orderName) {
this.orderName = orderName;
}
}
2.1、保存
测试方法 1：先保存 Customer 对象，后保存 Order 对象
@Test
public void testPersist() {
Customer customer1 = new Customer();
customer1.setLastName("customer1");
customer1.setAge(11);
customer1.setEmail("customer1@qq.com");
customer1.setCreateTime(new Date());
customer1.setBirthDay(new Date());
Order order1 = new Order();
order1.setOrderName("Customer1-Order1");
Order order2 = new Order();
order2.setOrderName("Customer1-Order2");
// 设置关联关系
Set<Order> orders = customer1.getOrders();
orders.add(order1);
orders.add(order2);
entityManager.persist(customer1);
entityManager.persist(order1);
entityManager.persist(order2);
}
日志信息
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
insert
into
JPA_ORDER
(ORDER_NAME)
values
(?)
Hibernate:
insert
into
JPA_ORDER
(ORDER_NAME)
values
(?)
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
测试方法 2：先保存 Order 对象，后保存 Customer 对象
@Test
public void testPersist2() {
Customer customer2 = new Customer();
customer2.setLastName("customer2");
customer2.setAge(22);
customer2.setEmail("customer2@qq.com");
customer2.setCreateTime(new Date());
customer2.setBirthDay(new Date());
Order order1 = new Order();
order1.setOrderName("Customer2-Order1");
Order order2 = new Order();
order2.setOrderName("Customer2-Order2");
// 设置关联关系
Set<Order> orders = customer2.getOrders();
orders.add(order1);
orders.add(order2);
entityManager.persist(order1);
entityManager.persist(order2);
entityManager.persist(customer2);
}
日志信息
Hibernate:
insert
into
JPA_ORDER
(ORDER_NAME)
values
(?)
Hibernate:
insert
into
JPA_ORDER
(ORDER_NAME)
values
(?)
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
注意：尽管我们对 Order 和 Customer 保存操作进行了位置的调换，但从其执行过程可以发现：
单向 1-n 关联关系执行保存时，一定会多出 UPDATE 语句 ==> 因为 n 的一端在插入时不会同时插入外键列
2.2、获取
@Test
public void testFind() {
Customer customer = entityManager.find(Customer.class, 1);
System.out.println(customer.getLastName());
System.out.println(customer.getOrders().size());
}
日志信息
Hibernate:
select
customer0_.id as id1_0_0_,
customer0_.age as age2_0_0_,
customer0_.birthDay as birthDay3_0_0_,
customer0_.createTime as createTi4_0_0_,
customer0_.email as email5_0_0_,
customer0_.LAST_NAME as LAST_NAM6_0_0_
from
JPA_CUSTOMERS customer0_
where
customer0_.id=?
customer1
Hibernate:
select
orders0_.CUSTOMER_ID as CUSTOMER3_0_1_,
orders0_.id as id1_1_1_,
orders0_.id as id1_1_0_,
orders0_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_ORDER orders0_
where
orders0_.CUSTOMER_ID=?
2
注意：默认情况下，对关联的多的一方使用懒加载的加载策略
这里注意与单向多对一的区别：单向多对一——默认情况下，使用左外连接获取 n 的一端的对象和其关联的 1 的一端的对象
同理，可以通过修改 Customer 实体类中 @OneToMany 的 fetch 属性值，来改变其默认的加载策略
@JoinColumn(name = "CUSTOMER_ID")
// - fetch 设置默认的加载策略
@OneToMany(fetch = FetchType.EAGER)
public Set<Order> getOrders() {
return orders;
}
再次测试，查看日志信息
Hibernate:
select
customer0_.id as id1_0_1_,
customer0_.age as age2_0_1_,
customer0_.birthDay as birthDay3_0_1_,
customer0_.createTime as createTi4_0_1_,
customer0_.email as email5_0_1_,
customer0_.LAST_NAME as LAST_NAM6_0_1_,
orders1_.CUSTOMER_ID as CUSTOMER3_0_3_,
orders1_.id as id1_1_3_,
orders1_.id as id1_1_0_,
orders1_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_CUSTOMERS customer0_
left outer join
JPA_ORDER orders1_
on customer0_.id=orders1_.CUSTOMER_ID
where
customer0_.id=?
customer1
2
可以看到，fetch 属性值修改为 EAGER 时，JPA 会使用左外连接获取 1 的一端的对象和其关联的 n 的一端的对象（注意这里与单向多对一的区别！！！）
2.3、删除
删除 n 的一端
@Test
public void testRemoveN() {
Order order = entityManager.find(Order.class, 5);
entityManager.remove(order);
}
日志信息
Hibernate:
select
order0_.id as id1_1_0_,
order0_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_ORDER order0_
where
order0_.id=?
Hibernate:
delete
from
JPA_ORDER
where
id=?
删除 1 的一端
@Test
public void testRemove1() {
Customer customer = entityManager.find(Customer.class, 1);
entityManager.remove(customer);
}
日志信息
Hibernate:
select
customer0_.id as id1_0_1_,
customer0_.age as age2_0_1_,
customer0_.birthDay as birthDay3_0_1_,
customer0_.createTime as createTi4_0_1_,
customer0_.email as email5_0_1_,
customer0_.LAST_NAME as LAST_NAM6_0_1_,
orders1_.CUSTOMER_ID as CUSTOMER3_0_3_,
orders1_.id as id1_1_3_,
orders1_.id as id1_1_0_,
orders1_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_CUSTOMERS customer0_
left outer join
JPA_ORDER orders1_
on customer0_.id=orders1_.CUSTOMER_ID
where
customer0_.id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=null
where
CUSTOMER_ID=?
Hibernate:
delete
from
JPA_CUSTOMERS
where
id=?
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
可以发现，默认情况下，若删除 1 的一端，则会先将有关联的数据外键值置为空后，再去执行 delete 操作
注意：这里注意单向一对多与单向多对一的区别。单向多对一不能直接删除 1 的一端，因为有外键约束；而单向一对多可以删除 1 的一端
Q：那可以对单向一对多删除 1 的一端时，不选择将有关联的数据外键值置为空，而选择将有关联的数据直接删除，允许这样操作吗？
A：答案是肯定的，当然可以！只需要为 @OneToMany 指定 cascade 属性值即可
@JoinColumn(name = "CUSTOMER_ID")
// - cascade 设置级联策略为级联删除
@OneToMany(cascade = {CascadeType.REMOVE})
public Set<Order> getOrders() {
return orders;
}
再次测试，查看日志信息进行验证
Hibernate:
select
customer0_.id as id1_0_1_,
customer0_.age as age2_0_1_,
customer0_.birthDay as birthDay3_0_1_,
customer0_.createTime as createTi4_0_1_,
customer0_.email as email5_0_1_,
customer0_.LAST_NAME as LAST_NAM6_0_1_,
orders1_.CUSTOMER_ID as CUSTOMER3_0_3_,
orders1_.id as id1_1_3_,
orders1_.id as id1_1_0_,
orders1_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_CUSTOMERS customer0_
left outer join
JPA_ORDER orders1_
on customer0_.id=orders1_.CUSTOMER_ID
where
customer0_.id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=null
where
CUSTOMER_ID=?
Hibernate:
delete
from
JPA_ORDER
where
id=?
Hibernate:
delete
from
JPA_ORDER
where
id=?
Hibernate:
delete
from
JPA_CUSTOMERS
where
id=?
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
可以发现，当 @OneToMany 的 cascade 属性值设置为 Remove 时，首先会将多的一端的外键列置空。然后会对 JPA_ORDER 表进行数据的删除操作
2.4、更新
@Test
public void testUpdate() {
Customer customer = entityManager.find(Customer.class, 2);
customer.getOrders().iterator().next().setOrderName("Customer2-Order1-Update");
}
日志信息
Hibernate:
select
customer0_.id as id1_0_1_,
customer0_.age as age2_0_1_,
customer0_.birthDay as birthDay3_0_1_,
customer0_.createTime as createTi4_0_1_,
customer0_.email as email5_0_1_,
customer0_.LAST_NAME as LAST_NAM6_0_1_,
orders1_.CUSTOMER_ID as CUSTOMER3_0_3_,
orders1_.id as id1_1_3_,
orders1_.id as id1_1_0_,
orders1_.ORDER_NAME as ORDER_NA2_1_0_
from
JPA_CUSTOMERS customer0_
left outer join
JPA_ORDER orders1_
on customer0_.id=orders1_.CUSTOMER_ID
where
customer0_.id=?
Hibernate:
update
JPA_ORDER
set
ORDER_NAME=?
where
id=?
JPA_ORDERS 表数据
3、双向一对多（双向多对一）
双向一对多 和 双向多对一 本质是一样的。从左侧来看就是 一对多，从右侧来看就是 多对一
Customer 实体类（一）
@Entity
@Table(name = "JPA_CUSTOMERS")
public class Customer {
private Integer id;
private String lastName;
private String email;
private int age;
private Date birthDay;
private Date createTime;
private Set<Order> orders = new HashSet<>();
@GeneratedValue(strategy = GenerationType.AUTO)
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "LAST_NAME", length = 50, nullable = false)
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
@Basic
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Temporal(TemporalType.DATE)
public Date getBirthDay() {
return birthDay;
}
public void setBirthDay(Date birthDay) {
this.birthDay = birthDay;
}
@Temporal(TemporalType.TIMESTAMP)
public Date getCreateTime() {
return createTime;
}
public void setCreateTime(Date createTime) {
this.createTime = createTime;
}
@JoinColumn(name = "CUSTOMER_ID")
@OneToMany
public Set<Order> getOrders() {
return orders;
}
public void setOrders(Set<Order> orders) {
this.orders = orders;
}
}
Order 实体类（多）
@Table(name = "JPA_ORDER")
@Entity
public class Order {
private Integer id;
private String orderName;
private Customer customer;
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "ORDER_NAME")
public String getOrderName() {
return orderName;
}
public void setOrderName(String orderName) {
this.orderName = orderName;
}
@JoinColumn(name = "CUSTOMER_ID")
@ManyToOne
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
}
3.1、保存
测试方法 1：先保存 Customer 对象，后保存 Order 对象
@Test
public void testPersist() {
Customer customer1 = new Customer();
customer1.setLastName("customer1");
customer1.setAge(11);
customer1.setEmail("customer1@qq.com");
customer1.setCreateTime(new Date());
customer1.setBirthDay(new Date());
Order order1 = new Order();
order1.setOrderName("Customer1-Order1");
Order order2 = new Order();
order2.setOrderName("Customer1-Order2");
// 设置关联关系——一对多
Set<Order> orders = customer1.getOrders();
orders.add(order1);
orders.add(order2);
// 设置关联关系——多对一
order1.setCustomer(customer1);
order2.setCustomer(customer1);
entityManager.persist(customer1);
entityManager.persist(order1);
entityManager.persist(order2);
}
日志信息
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
分析：由于是先保存 Customer 对象（即一的一方），所以在保存 Order 对象（即多的一方）时，CUSTOMER_ID 已经有了，因此在执行
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)时，CUSTOMER_ID 就已经被插入了；而多出来的
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?是由 Customer 来维护的
测试方法 2：先保存 Order 对象，后保存 Customer 对象
@Test
public void testPersist2() {
Customer customer2 = new Customer();
customer2.setLastName("customer2");
customer2.setAge(22);
customer2.setEmail("customer2@qq.com");
customer2.setCreateTime(new Date());
customer2.setBirthDay(new Date());
Order order1 = new Order();
order1.setOrderName("Customer2-Order1");
Order order2 = new Order();
order2.setOrderName("Customer2-Order2");
// 设置关联关系——一对多
Set<Order> orders = customer2.getOrders();
orders.add(order1);
orders.add(order2);
// 设置关联关系——多对一
order1.setCustomer(customer2);
order2.setCustomer(customer2);
entityManager.persist(order1);
entityManager.persist(order2);
entityManager.persist(customer2);
}
日志信息
Hibernate:
insert
into
JPA_ORDER
(CUSTOMER_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMER_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?,
ORDER_NAME=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?,
ORDER_NAME=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
Hibernate:
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
分析：由于是先保存 Order 对象（即多的一方），此时 CUSTOMER_ID 还没有值，因此在执行
insert
into
JPA_ORDER
(CUSTOMERS_ID, ORDER_NAME)
values
(?, ?)时，CUSTOMER_ID 为空；而多出来的 update 语句，即
update
JPA_ORDER
set
CUSTOMER_ID=?,
ORDER_NAME=?
where
id=?是由 Order 来维护的，
update
JPA_ORDER
set
CUSTOMER_ID=?
where
id=?是有 Customer 来维护的
由上可以得到一个小总结
小结：双向一对多（双向多对一）执行保存时
1）先保存 1 的一方，默认会多出 m 条 update 语句（m 为 n 的一方个数）
2）先保存 n 的一方，默认会多出 2m 条 update 语句（m 为 n 的一方个数）
建议：当关联关系为双向 1-n 时，建议使用 n 的一方来维护关联关系，1 的一方不维护关联关系，这样可以有效减少 SQL 语句
Q：那么如何让 1 的一方不维护关联关系呢？
A：可以通过在 1 的一方的 @OneToMany 注解中使用 mappedBy 属性来实现
@Entity
@Table(name = "JPA_CUSTOMERS")
public class Customer {
// ...
@JoinColumn(name = "CUSTOMER_ID")
@OneToMany(mappedBy = "customer")
public Set<Order> getOrders() {
return orders;
}
// ...
}
属性值指定为 n 的一方中 1 的一方属性名，什么意思呢？
@Table(name = "JPA_ORDER")
@Entity
public class Order {
// ...
private Customer customer; // 该属性名就是 mappedBy 属性值
// ...
}
现在直接来执行测试方法 1，观察日志信息有何问题
javax.persistence.PersistenceException: [PersistenceUnit: jpa-1] Unable to build EntityManagerFactory
<3 internal lines>
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:63)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:47)
at helloworld.bilateral.One2ManyTest.init(One2ManyTest.java:23) <27 internal lines>
Caused by: org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn: com.vectorx.jpa.helloworld.bilateral.one2many.Customer.orders <10 internal lines>
... 32 more
这里，报错信息告诉我们，使用 mappedBy 时，不能在 Customer 中使用 @JoinColumn 注解了。既然如此，那就注释掉呗
@Entity
@Table(name = "JPA_CUSTOMERS")
public class Customer {
// ...
//@JoinColumn(name = "CUSTOMER_ID")
@OneToMany(mappedBy = "customer")
public Set<Order> getOrders() {
return orders;
}
// ...
}
再来执行测试方法 1，观察日志信息是否能正常执行
Hibernate:
insert
into
JPA_CUSTOMERS
(age, birthDay, createTime, email, LAST_NAME)
values
(?, ?, ?, ?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMER_ID, ORDER_NAME)
values
(?, ?)
Hibernate:
insert
into
JPA_ORDER
(CUSTOMER_ID, ORDER_NAME)
values
(?, ?)
可以发现，执行没有报错，而且也不会多出 update 语句
JPA_CUSTOMERS 表数据
JPA_ORDERS 表数据
3.2、获取、删除、更新
这里的操作都能在 单向多对一 和 单向一对多 中找到对应的实现，这里就不一一细说了
4、双向一对一
基于外键的 1-1 关联关系：在双向的一对一关联中，需要在关系被维护端（inverse side）中的 @OneToOne 注释中指定 mappedBy，以指定是这一关联中的被维护端。同时需要在关系维护端（owner side）建立外键列指向关系被维护端的主键列
Manager 实体类
@Table(name = "JPA_MANAGERS")
@Entity
public class Manager {
private Integer id;
private String mgrName;
private Department dept;
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "MGR_NAME")
public String getMgrName() {
return mgrName;
}
public void setMgrName(String mgrName) {
this.mgrName = mgrName;
}
/**
* 使用 @OneToOne 注解来映射 1-1 关联关系
* 使用 mappedBy 属性来不维护关联关系（没有外键的一方）
*/
@OneToOne(mappedBy = "mgr")
public Department getDept() {
return dept;
}
public void setDept(Department dept) {
this.dept = dept;
}
}
Department 实体类
@Table(name = "JPA_DEPARTMENTS")
@Entity
public class Department {
private Integer id;
private String deptName;
private Manager mgr;
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "DEPT_NAME")
public String getDeptName() {
return deptName;
}
public void setDeptName(String deptName) {
this.deptName = deptName;
}
/**
* 使用 @OneToOne 注解来映射 1-1 关联关系
* 使用 @JoinColumn 注解来映射当前数据表的外键
* 注意：1-1 关联关系需要添加 unique = true
*
* @return
*/
@JoinColumn(name = "MGR_ID", unique = true)
@OneToOne
public Manager getMgr() {
return mgr;
}
public void setMgr(Manager mgr) {
this.mgr = mgr;
}
}
4.1、保存
测试方法 1：先保存 Manager 即不维护关联关系的一方（没有外键的一方），再保存 Department 即维护关联关系的一方（有外键的一方）
@Test
public void testPersist() {
Manager mgr = new Manager();
mgr.setMgrName("Mgr1");
Department dept = new Department();
dept.setDeptName("Dept1");
// 设置 1-1 关联关系
mgr.setDept(dept);
dept.setMgr(mgr);
entityManager.persist(mgr);
entityManager.persist(dept);
}
日志信息
Hibernate:
insert
into
JPA_MANAGERS
(MGR_NAME)
values
(?)
Hibernate:
insert
into
JPA_DEPARTMENTS
(DEPT_NAME, MGR_ID)
values
(?, ?)
JPA_MANAGERS 表数据
JPA_DEPARTMENT 表数据
测试方法 2：先保存 Department 即维护关联关系的一方（有外键的一方），再保存 Manager 即不维护关联关系的一方（没有外键的一方）
@Test
public void testPersist2() {
Manager mgr = new Manager();
mgr.setMgrName("Mgr2");
Department dept = new Department();
dept.setDeptName("Dept2");
// 设置 1-1 关联关系
mgr.setDept(dept);
dept.setMgr(mgr);
entityManager.persist(dept);
entityManager.persist(mgr);
}
日志信息
Hibernate:
insert
into
JPA_DEPARTMENTS
(DEPT_NAME, MGR_ID)
values
(?, ?)
Hibernate:
insert
into
JPA_MANAGERS
(MGR_NAME)
values
(?)
Hibernate:
update
JPA_DEPARTMENTS
set
DEPT_NAME=?,
MGR_ID=?
where
id=?
JPA_MANAGERS 表数据
JPA_DEPARTMENT 表数据
小结：双向一对一关联关系中，建议先保存不维护关联关系的一方（即没有外键的一方），这样不会多出 update 语句
4.2、获取
测试方法 1：查询 Department 即维护关联关系的一方（有外键的一方）
@Test
public void testFind1() {
Department dept = entityManager.find(Department.class, 1);
System.out.println(dept.getDeptName());
System.out.println(dept.getMgr().getClass().getName());
}
- 情况 1：默认情况
日志信息
Hibernate:
select
department0_.id as id1_0_1_,
department0_.DEPT_NAME as DEPT_NAM2_0_1_,
department0_.MGR_ID as MGR_ID3_0_1_,
manager1_.id as id1_1_0_,
manager1_.MGR_NAME as MGR_NAME2_1_0_
from
JPA_DEPARTMENTS department0_
left outer join
JPA_MANAGERS manager1_
on department0_.MGR_ID=manager1_.id
where
department0_.id=?
Hibernate:
select
department0_.id as id1_0_1_,
department0_.DEPT_NAME as DEPT_NAM2_0_1_,
department0_.MGR_ID as MGR_ID3_0_1_,
manager1_.id as id1_1_0_,
manager1_.MGR_NAME as MGR_NAME2_1_0_
from
JPA_DEPARTMENTS department0_
left outer join
JPA_MANAGERS manager1_
on department0_.MGR_ID=manager1_.id
where
department0_.MGR_ID=?
Dept1
com.vectorx.jpa.helloworld.bilateral.one2one.Manager
小结：双向一对一关联关系中，获取维护关联关系的一方（即有外键的一方）时，默认会通过左外连接获取其关联对象
对此，我们尝试进行优化
- 情况 2：使用
fetch属性来修改默认的加载策略为懒加载
@Table(name = "JPA_DEPARTMENTS")
@Entity
public class Department {
// ...
@JoinColumn(name = "MGR_ID", unique = true)
@OneToOne(fetch = FetchType.LAZY)
public Manager getMgr() {
return mgr;
}
// ...
}
日志信息
Hibernate:
select
department0_.id as id1_0_0_,
department0_.DEPT_NAME as DEPT_NAM2_0_0_,
department0_.MGR_ID as MGR_ID3_0_0_
from
JPA_DEPARTMENTS department0_
where
department0_.id=?
Dept1
com.vectorx.jpa.helloworld.bilateral.one2one.Manager_$$_javassist_1
可以看到，使用懒加载的加载策略后，只发送了一条 select 语句，而且 Manager 对象为代理对象
小结：双向一对一关联关系中，获取维护关联关系的一方（即有外键的一方）时，若加载策略修改为懒加载，则会通过代理获取其关联对象
测试方法 2：查询 Manager 即不维护关联关系的一方（没有外键的一方）
@Test
public void testFind2() {
Manager mgr = entityManager.find(Manager.class, 1);
System.out.println(mgr.getMgrName());
System.out.println(mgr.getDept().getClass().getName());
}
- 情况 1：默认情况
日志信息
Hibernate:
select
manager0_.id as id1_1_1_,
manager0_.MGR_NAME as MGR_NAME2_1_1_,
department1_.id as id1_0_0_,
department1_.DEPT_NAME as DEPT_NAM2_0_0_,
department1_.MGR_ID as MGR_ID3_0_0_
from
JPA_MANAGERS manager0_
left outer join
JPA_DEPARTMENTS department1_
on manager0_.id=department1_.MGR_ID
where
manager0_.id=?
Mgr1
com.vectorx.jpa.helloworld.bilateral.one2one.Department
小结：双向一对一关联关系中，获取不维护关联关系的一方（即没有外键的一方）时，默认也会通过左外连接获取其关联对象
同理，我们尝试进行优化
- 情况 2：使用
fetch属性来修改默认的加载策略为懒加载
@Table(name = "JPA_MANAGERS")
@Entity
public class Manager {
//...
@OneToOne(mappedBy = "mgr", fetch = FetchType.LAZY)
public Department getDept() {
return dept;
}
//...
}
日志信息
Hibernate:
select
manager0_.id as id1_1_0_,
manager0_.MGR_NAME as MGR_NAME2_1_0_
from
JPA_MANAGERS manager0_
where
manager0_.id=?
Hibernate:
select
department0_.id as id1_0_0_,
department0_.DEPT_NAME as DEPT_NAM2_0_0_,
department0_.MGR_ID as MGR_ID3_0_0_
from
JPA_DEPARTMENTS department0_
where
department0_.MGR_ID=?
Mgr1
com.vectorx.jpa.helloworld.bilateral.one2one.Department
小结：双向一对一关联关系中，获取不维护关联关系的一方（即没有外键的一方）时，即使指定懒加载的加载策略，也会额外发送一条 select 语句获取其关联对象，而不是使用代理。
因此，在不维护关联关系的一方，不建议修改
fetch属性
双向 1-1 不延迟加载的问题
- 如果延迟加载要起作用，就必须设置一个代理对象
- Manager 其实可以不关联一个 Department
- 如果有 Department 关联就设置为代理对象而延迟加载，如果不存在关联的 Department 就设置 null。因为外键字段是定义在 Department 表中的，Hibernate 在不读取 Department 表的情况是无法判断是否有关联 Deparmtment，因此无法判断设置 null 还是代理对象。而统一设置为代理对象，也无法满足不关联的情况，所以无法使用延迟加载，只有显式读取 Department
4.3、删除、更新
双向一对一关联关系中删除和更新操作，与单向一对多和单向多对一的情况类似，这里不再赘述
5、双向多对多
在双向多对多关联关系中，我们必须指定一个关系维护端（owner side），可以通过在 @ManyToMany 注解中指定 mappedBy 属性来标识其为关系维护端
Item 实体类
@Table(name = "JPA_ITEMS")
@Entity
public class Item {
private Integer id;
private String itemName;
private Set<Category> categories = new HashSet<>();
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "ITEM_NAME")
public String getItemName() {
return itemName;
}
public void setItemName(String itemName) {
this.itemName = itemName;
}
@JoinTable(
name = "ITEM_CATEGORY",
joinColumns = {@JoinColumn(name = "ITEM_ID", referencedColumnName = "ID")},
inverseJoinColumns = {@JoinColumn(name = "CATEGORY_ID", referencedColumnName = "ID")}
)
@ManyToMany
public Set<Category> getCategories() {
return categories;
}
public void setCategories(Set<Category> categories) {
this.categories = categories;
}
}
Category 实体类
@Table(name = "JPA_CATEGORIES")
@Entity
public class Category {
private Integer id;
private String categoryName;
private Set<Item> items = new HashSet<>();
@GeneratedValue
@Id
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name = "CATEGORY_NAME")
public String getCategoryName() {
return categoryName;
}
public void setCategoryName(String categoryName) {
this.categoryName = categoryName;
}
@ManyToMany(mappedBy = "categories")
public Set<Item> getItems() {
return items;
}
public void setItems(Set<Item> items) {
this.items = items;
}
}
执行初始化生成数据表，可以发现会生成一个名为 ITEM_CATEGORY 的中间数据表，查看其 DDL 语句
create table if not exists jpa.item_category
(
ITEM_ID int not null,
CATEGORY_ID int not null,
primary key (ITEM_ID, CATEGORY_ID),
constraint FK_6ui7ejm313518nu71evuhstxy
foreign key (ITEM_ID) references jpa.jpa_items (id),
constraint FK_gtonlc7pn4w3o4yljbkj9n7wj
foreign key (CATEGORY_ID) references jpa.jpa_categories (id)
);
通过 @JoinTable 注解，我们发现 JPA 帮我们生成了连接两个表的中间表，其中
ITEM_ID外键指向 JPA_ITEMS 的主键 idCATEGORY_ID外键指向 JPA_CATEGORIES 的主键 id
@JoinTable 注解详解：
name：中间表表名joinColumns：name——外键列列名，referencedColumnName——外键列指向关联当前表的字段inverseJoinColumns：name——外键列列名，referencedColumnName——外键列指向另一关联表的字段
5.1、保存
@Test
public void testPersist() {
// 商品
Item i1 = new Item();
i1.setItemName("i-1");
Item i2 = new Item();
i2.setItemName("i-2");
// 类别
Category c1 = new Category();
c1.setCategoryName("c-1");
Category c2 = new Category();
c2.setCategoryName("c-2");
// 设置双向 n-n 关联关系——Category
i1.getCategories().add(c1);
i1.getCategories().add(c2);
i2.getCategories().add(c1);
i2.getCategories().add(c2);
// 设置双向 n-n 关联关系——Item
c1.getItems().add(i1);
c1.getItems().add(i2);
c2.getItems().add(i1);
c2.getItems().add(i2);
entityManager.persist(i1);
entityManager.persist(i2);
entityManager.persist(c1);
entityManager.persist(c2);
}
日志信息
Hibernate:
insert
into
JPA_ITEMS
(ITEM_NAME)
values
(?)
Hibernate:
insert
into
JPA_ITEMS
(ITEM_NAME)
values
(?)
Hibernate:
insert
into
JPA_CATEGORIES
(CATEGORY_NAME)
values
(?)
Hibernate:
insert
into
JPA_CATEGORIES
(CATEGORY_NAME)
values
(?)
Hibernate:
insert
into
ITEM_CATEGORY
(ITEM_ID, CATEGORY_ID)
values
(?, ?)
Hibernate:
insert
into
ITEM_CATEGORY
(ITEM_ID, CATEGORY_ID)
values
(?, ?)
Hibernate:
insert
into
ITEM_CATEGORY
(ITEM_ID, CATEGORY_ID)
values
(?, ?)
Hibernate:
insert
into
ITEM_CATEGORY
(ITEM_ID, CATEGORY_ID)
values
(?, ?)
JPA_ITEMS 表数据
JPA_CATEGORY 表数据
ITEM_CATEGORY 表数据
5.2、获取
测试方法 1：获取 Item 即维护关联关系的一方
@Test
public void testFind1() {
Item item = entityManager.find(Item.class, 1);
System.out.println(item.getItemName());
System.out.println(item.getCategories().size());
}
日志信息
Hibernate:
select
item0_.id as id1_3_0_,
item0_.ITEM_NAME as ITEM_NAM2_3_0_
from
JPA_ITEMS item0_
where
item0_.id=?
i-1
Hibernate:
select
categories0_.ITEM_ID as ITEM_ID1_3_1_,
categories0_.CATEGORY_ID as CATEGORY2_0_1_,
category1_.id as id1_1_0_,
category1_.CATEGORY_NAME as CATEGORY2_1_0_
from
ITEM_CATEGORY categories0_
inner join
JPA_CATEGORIES category1_
on categories0_.CATEGORY_ID=category1_.id
where
categories0_.ITEM_ID=?
2
测试方法 2：获取 Category 即不维护关联关系的一方
@Test
public void testFind2() {
Category category = entityManager.find(Category.class, 1);
System.out.println(category.getCategoryName());
System.out.println(category.getItems().iterator().size());
}
日志信息
Hibernate:
select
category0_.id as id1_1_0_,
category0_.CATEGORY_NAME as CATEGORY2_1_0_
from
JPA_CATEGORIES category0_
where
category0_.id=?
c-1
Hibernate:
select
items0_.CATEGORY_ID as CATEGORY2_1_1_,
items0_.ITEM_ID as ITEM_ID1_0_1_,
item1_.id as id1_3_0_,
item1_.ITEM_NAME as ITEM_NAM2_3_0_
from
ITEM_CATEGORY items0_
inner join
JPA_ITEMS item1_
on items0_.ITEM_ID=item1_.id
where
items0_.CATEGORY_ID=?
2
小结：双向多对多关联关系中，不论是获取维护关联关系的一方，还是获取不维护关联关系的一方，都是默认使用懒加载的加载策略
总结
本节重点关注集中映射关联关系，其中包括
- 单向映射关联关系：单向多对一（n-1）、单向一对多（1-n）
- 双向映射关联关系：双向一对多/双向多对一、双向一对一（1-1）、双向多对多（n-n）
4 个关联关系注解：@OneToMany、@ManyToOne、@OneToOne、@ManyToMany，其中属性
mappedBy：不维护关联关系一方指定维护关联关系一方中当前实体属性名fetch：加载策略，LAZY为懒加载，EAGER为饿汉式加载cascade：级联策略，REMOVE为级联删除（注意该属性只有OneToOne、OneToMany中有）
另外还有 2 个注解
@JoinColumn：映射外键列列名，和上述四中注解配合使用@JoinTable：映射中间表
附上导图，仅供参考
