JPA中一对多和多对一双向关系中,多的一方为关系维护端 。有mappedBy的一方为关系被维护端。
关系维护端负责外键纪录的更新 ,关系被维护端是没有权力更新外键纪录的。只能通过关系维护端设置与关系被维护端的关系。
一对多:
@OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
CascadeType.MERGE, CascadeType.REMOVE, CascadeType.ALL }, fetch = FetchType.EAGER, mappedBy = "order")
cascade:设置级联操作,对JPQL语句进行的操作没有任何影响
CascadeType.PERSIST对应实体管理器的persist()方法
CascadeType.REFRESH对应实体管理器的refresh()方法
CascadeType.MERGE对应实体管理器的merge()方法
CascadeType.REMOVE对应实体管理器的remove()方法
CascadeType.ALL对应实体管理器的以上方法:persist()、refresh()、merge()、remove()方法
fetch:设置是否延迟加载,FetchType.LZAY为延迟加载, FetchType.EAGER时立即加载。如果加载的是多的一方的话,默认的是延迟加载,否则是加载。
多对一:
@ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
@JoinColumn(name = "order_id")
optional:代表外键字段是否可以为空。true可以为空,false不可以为空。
@JoinColumn(name = "order_id"):设置关联外键字段的名称。
例:
1.两个实体的一对多和多对一
使用现实生活中的订单和订单项的关系,代码为:
- package com.taoistwar.jpa.entity.onetomany;
- import java.util.HashSet;
- import java.util.Set;
- import javax.persistence.CascadeType;
- import javax.persistence.Entity;
- import javax.persistence.FetchType;
- import javax.persistence.GeneratedValue;
- import javax.persistence.GenerationType;
- import javax.persistence.Id;
- import javax.persistence.OneToMany;
- import javax.persistence.Table;
- @Entity
- @Table(name = "order_info")
- public class OrderInfo {
- private Integer id;
- private String name;
- private Set<OrderItem> items = new HashSet<OrderItem>();
- @Id
- @GeneratedValue(strategy = GenerationType.AUTO)
- public Integer getId() {
- return id;
- }
- public void setId(Integer id) {
- this.id = id;
- }
- public String getName() {
- return name;
- }
- public void setName(String name) {
- this.name = name;
- }
- @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
- CascadeType.MERGE, CascadeType.REMOVE }, fetch = FetchType.EAGER, mappedBy = "order")
- //fetch是否延迟加载,mappedBy有它的一方为关系被维护端。
- public Set<OrderItem> getItems() {
- return items;
- }
- public void setItems(Set<OrderItem> items) {
- this.items = items;
- }
- public void addOrderItem(OrderItem orderItem) {
- orderItem.setOrder(this);
- this.items.add(orderItem);
- }
- }
- package com.taoistwar.jpa.entity.onetomany;
- import javax.persistence.CascadeType;
- import javax.persistence.Column;
- import javax.persistence.Entity;
- import javax.persistence.GeneratedValue;
- import javax.persistence.GenerationType;
- import javax.persistence.Id;
- import javax.persistence.JoinColumn;
- import javax.persistence.ManyToOne;
- import javax.persistence.Table;
- @Entity
- @Table(name = "order_item")
- public class OrderItem {
- private Integer Id;
- private String name;
- private OrderInfo order;
- @Id
- @GeneratedValue(strategy = GenerationType.AUTO)
- public Integer getId() {
- return Id;
- }
- public void setId(Integer id) {
- Id = id;
- }
- @Column(length = 20, nullable = true)
- public String getName() {
- return name;
- }
- public void setName(String name) {
- this.name = name;
- }
- @ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
- @JoinColumn(name = "order_id")
- // optional是否可以为空
- public OrderInfo getOrder() {
- return order;
- }
- public void setOrder(OrderInfo order) {
- this.order = order;
- }
- @Override
- public int hashCode() {
- final int prime = 31;
- int result = 1;
- result = prime * result + ((Id == null) ? 0 : Id.hashCode());
- return result;
- }
- @Override
- public boolean equals(Object obj) {
- if (this == obj)
- return true;
- if (obj == null)
- return false;
- if (getClass() != obj.getClass())
- return false;
- OrderItem other = (OrderItem) obj;
- if (Id == null) {
- if (other.Id != null)
- return false;
- } else if (!Id.equals(other.Id))
- return false;
- return true;
- }
- }
2.单个实体的一对多和多对一
使用软件开发中的模块为例子,代码如下:
- package com.taoistwar.ejb.sys.entity;
- import java.io.Serializable;
- import java.util.Set;
- import javax.persistence.CascadeType;
- import javax.persistence.Column;
- import javax.persistence.Entity;
- import javax.persistence.FetchType;
- import javax.persistence.GeneratedValue;
- import javax.persistence.GenerationType;
- import javax.persistence.Id;