@OneToMany 注解用来定义具有一对多的多值关联。例如：汽车和轮胎，一辆汽车可以拥有多个轮胎，但是一个轮胎只能属于某辆车。
如果集合使用泛型来定义元素类型，则不需要指定关联的目标实体类型；否则必须指定目标实体类。例如：
@OneToMany private List<CarTyre> CarTyre;
如果关系是双向的，则必须使用 mappedBy 元素来指定作为关系所有者的实体的关系字段或属性。
@OneToMany 注释可以在包含在实体类中的嵌入类中使用，以指定与实体集合的关系。如果关系是双向的，则必须使用 mappedBy 元素来指定关系的所有者实体的关系字段或属性。当集合是 java.util.Map 时，cascade 元素和 orphanRemoval 元素应用于映射值。
实例1: 使用泛型的一对多关联
// 在 Customer 中:
@OneToMany(cascade=ALL, mappedBy="customer")
public Set<Order> getOrders() {
return orders;
}
// 在 Order 类中:
@ManyToOne
@JoinColumn(name="CUST_ID", nullable=false)
public Customer getCustomer() {
return customer;
}实例2: 不使用泛型的一对多关联
// 在 Customer 中:
@OneToMany(targetEntity=com.acme.Order.class, cascade=ALL, mappedBy="customer")
public Set getOrders() {
return orders;
}
// 在 Order 类中:
@ManyToOne
@JoinColumn(name="CUST_ID", nullable=false)
public Customer getCustomer() {
return customer;
}实例3: 使用外键映射的单向一对多关联
// 在 Customer 中:
@OneToMany(orphanRemoval=true)
@JoinColumn(name="CUST_ID") // join 列在 Order 表中
public Set<Order> getOrders() {
return orders;
}定义一方（One）和多方（Many）对象的级联关系，当对一方（One）对象进行了某个操作后，怎样影响到多方（Many）对象的操作。级联可取值如下：
（1）不定义，则对关系表不会产生任何影响
（2）CascadeType.PERSIST 级联新建
（3）CascadeType.REMOVE 级联删除
（4）CascadeType.REFRESH 级联刷新
（5）CascadeType.MERGE 级联更新
（6）CascadeType.ALL 表示同时选择 CascadeType.PERSIST、CascadeType.REMOVE、CascadeType.REFRESH 和 CascadeType.MERGE
定义关联属性何时进行加载，可选择项包括：FetchType.EAGER 和 FetchType.LAZY。如下：
FetchType.EAGER：表示关系类在主类加载的时候同时加载
FetchType.LAZY：表示关系类在被访问时才加载，默认值是 FetchType.LAZY。
拥有关联关系的字段，如果关系是单向的就不需要；如果是双向关系表，那么拥有关系的这一方有建立、解除和更新与另一方关系的能力；而另一方没有这种能力，只能被动管理；这个属性被定义在关系的被拥有方。支持双向 @OneToOne，双向 @OneToMany，双向 @ManyToMany。
指定在删除一方（One）数据的同时是否删除掉多方（Many）的数据。默认为 false，不删除多方（Many）数据；仅仅将一方（One）和连接表的数据进行删除。例如：一个主记录包含三条副记录
@OneToMany(cascade={
CascadeType.MERGE, CascadeType.PERSIST, CascadeType.REFRESH
}, orphanRemoval = false)
private Collection<UserOneToManyExt> userOneToManyExt;客户端代码
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
// 插入数据
String uuid = UUID.randomUUID().toString();
List<UserOneToManyExt> userExts = new ArrayList<UserOneToManyExt>();
userExts.add(new UserOneToManyExt(uuid + "@qq.com", "http://www.hxstrive1.com"));
userExts.add(new UserOneToManyExt(uuid + "@163.com", "http://www.hxstrive2.com"));
userExts.add(new UserOneToManyExt(uuid + "@outlook.com", "http://www.hxstrive3.com"));
UserOneToMany user = new UserOneToMany();
user.setName("张三-" + uuid);
user.setAge(28);
user.setUserOneToManyExt(userExts);
em.persist(user);
// 查询数据
String qlString = "select t from UserOneToMany t";
Query query = em.createQuery(qlString);
List<UserOneToMany> userList = (List<UserOneToMany>) query.getResultList();
for (UserOneToMany item : userList) {
// 删除数据
em.remove(item);
}
em.getTransaction().commit();执行 SQL 脚本如下：
SELECT SEQUENCE_VALUE FROM OPENJPA_SEQUENCE_TABLE WHERE ID = ? FOR UPDATE [params=?] UPDATE OPENJPA_SEQUENCE_TABLE SET SEQUENCE_VALUE = ? WHERE ID = ? AND SEQUENCE_VALUE = ? [params=?, ?, ?] SELECT SEQUENCE_VALUE FROM OPENJPA_SEQUENCE_TABLE WHERE ID = ? FOR UPDATE [params=?] UPDATE OPENJPA_SEQUENCE_TABLE SET SEQUENCE_VALUE = ? WHERE ID = ? AND SEQUENCE_VALUE = ? [params=?, ?, ?] INSERT INTO UserOneToMany (id, age, name) VALUES (?, ?, ?) [params=?, ?, ?] INSERT INTO UserOneToManyExt (id, email, homeUrl, user16_id) VALUES (?, ?, ?, ?) [params=?, ?, ?, ?] INSERT INTO UserOneToManyExt (id, email, homeUrl, user16_id) VALUES (?, ?, ?, ?) [params=?, ?, ?, ?] INSERT INTO UserOneToManyExt (id, email, homeUrl, user16_id) VALUES (?, ?, ?, ?) [params=?, ?, ?, ?] INSERT INTO UserOneToMany_UserOneToManyExt (USERONETOMANY_ID, USERONETOMANYEXT_ID) VALUES (?, ?) [params=?, ?] INSERT INTO UserOneToMany_UserOneToManyExt (USERONETOMANY_ID, USERONETOMANYEXT_ID) VALUES (?, ?) [params=?, ?] INSERT INTO UserOneToMany_UserOneToManyExt (USERONETOMANY_ID, USERONETOMANYEXT_ID) VALUES (?, ?) [params=?, ?] SELECT t0.id, t0.age, t0.name FROM UserOneToMany t0 DELETE FROM UserOneToMany_UserOneToManyExt WHERE USERONETOMANY_ID = ? [params=?] DELETE FROM UserOneToMany WHERE id = ? [params=?]
如果 orphanRemoval = true，删除数据执行的 SQL 如下：
SELECT SEQUENCE_VALUE FROM OPENJPA_SEQUENCE_TABLE WHERE ID = ? FOR UPDATE [params=?] UPDATE OPENJPA_SEQUENCE_TABLE SET SEQUENCE_VALUE = ? WHERE ID = ? AND SEQUENCE_VALUE = ? [params=?, ?, ?] SELECT SEQUENCE_VALUE FROM OPENJPA_SEQUENCE_TABLE WHERE ID = ? FOR UPDATE [params=?] UPDATE OPENJPA_SEQUENCE_TABLE SET SEQUENCE_VALUE = ? WHERE ID = ? AND SEQUENCE_VALUE = ? [params=?, ?, ?] INSERT INTO UserOneToMany (id, age, name) VALUES (?, ?, ?) [params=?, ?, ?] INSERT INTO UserOneToManyExt (id, email, homeUrl, user16_id) VALUES (?, ?, ?, ?) [params=?, ?, ?, ?] INSERT INTO UserOneToManyExt (id, email, homeUrl, user16_id) VALUES (?, ?, ?, ?) [params=?, ?, ?, ?] INSERT INTO UserOneToManyExt (id, email, homeUrl, user16_id) VALUES (?, ?, ?, ?) [params=?, ?, ?, ?] INSERT INTO UserOneToMany_UserOneToManyExt (USERONETOMANY_ID, USERONETOMANYEXT_ID) VALUES (?, ?) [params=?, ?] INSERT INTO UserOneToMany_UserOneToManyExt (USERONETOMANY_ID, USERONETOMANYEXT_ID) VALUES (?, ?) [params=?, ?] INSERT INTO UserOneToMany_UserOneToManyExt (USERONETOMANY_ID, USERONETOMANYEXT_ID) VALUES (?, ?) [params=?, ?] SELECT t0.id, t0.age, t0.name FROM UserOneToMany t0 DELETE FROM UserOneToMany_UserOneToManyExt WHERE USERONETOMANY_ID = ? [params=?] DELETE FROM UserOneToMany WHERE id = ? [params=?] DELETE FROM UserOneToManyExt WHERE id = ? [params=?] DELETE FROM UserOneToManyExt WHERE id = ? [params=?] DELETE FROM UserOneToManyExt WHERE id = ? [params=?]
关联目标的实体类。一般不用指定，JPA 厂商会自动发现。例如：
@OneToMany(cascade=ALL, mappedBy="customer")
public Set<Order> getOrders() {
return orders;
}上面实例中，targetEntity 默认就是 Order.class；如果没有使用泛型，则需要手动指定，如下：
@OneToMany(cascade=ALL, targetEntity=Order.class)
public Set getOrders() {
return orders;
}
