JPA规范：一对多、一对一、多对多的双向关联与级联操作以及JPA联合主键

通常在企业开发中，开发Dao层有两种做法： 
（1）先建表，后再根据表来编写配置文件和实体bean。使用这种方案的开发人员受到了传统数据库建模的影响。 
（2）先编写配置文件和实体bean，然后再生成表，使用这种方案的开发人员采用的是领域建模思想，这种思想相对前一种思想更加OOP。

建议使用第二种（领域建模思想），从软件开发来想，这种思想比第一种思想更加面向对象。 领域建模思想也是目前比较新的一门建模思想，第一种是传统的建模思想，已经有10来年的发展历程了，而领域建模思想是近几年才兴起的，这种思想更加的面向对象。

 

 

一、一对多双向关联与级联操作：

以订单类和订单商品类为例：

多的一方为关系维护端，关系维护端负责外键记录的更新，关系被维护端是没有权利更新外键记录。

1、订单类：

@Entity
public class Orders {
	private String orderid;
	private Float amount = 0f;
	private Set items=new HashSet();
	
	@Id @Column(length=12)
	public String getOrderid() {
		return orderid;
	}
	public void setOrderid(String orderid) {
		this.orderid = orderid;
	}
	
	@Column(nullable=false)
	public Float getAmount() {
		return amount;
	}
	public void setAmount(Float amount) {
		this.amount = amount;
	}
	
	//REFRESH，级联刷新(调用refresh方法才会起作用);PERSIST，级联保存(persist);
	//MERGE,级联更新（merge方法）;REMOVE,级联删除(remove方法);
	//级联:cascade={CascadeType.ALL})如果要使用上面四项的使用，可以使用ALL来代替
	//@OneToMany默认行为是延迟加载
	//mappedBy:指定关系被维护端，指定OrderItem里面的order，相当于hibernate的inverse放弃维护
	@OneToMany(cascade={CascadeType.REFRESH,CascadeType.PERSIST,CascadeType.MERGE,CascadeType.REMOVE}
	,fetch=FetchType.LAZY,mappedBy="orders")
	public Set getItems() {
		return items;
	}
	public void setItems(Set items) {
		this.items = items;
	}
	
	public void addOrderItem(OrderItem orderItem){
		orderItem.setOrders(this);
		this.items.add(orderItem);
	}
}

2、订单列表类：

@Entity
public class OrderItem {
	private Integer id;
	private String productName;
	private Float sellPrice = 0f;
	
	private Orders orders;//对应Order类里面指定的关系被维护端
	
	@Id @GeneratedValue
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	
	@Column(length=40,nullable=false)
	public String getProductName() {
		return productName;
	}
	public void setProductName(String productName) {
		this.productName = productName;
	}
	
	@Column(nullable=false)
	public Float getSellPrice() {
		return sellPrice;
	}
	public void setSellPrice(Float sellPrice) {
		this.sellPrice = sellPrice;
	}
	
	//默认立即加载
	//optional=true,选项允许为null，false时，不能为null
	@ManyToOne(cascade={CascadeType.MERGE,CascadeType.REFRESH},optional=false)
	@JoinColumn(name="order_id")//设置外键名称
	public Orders getOrders() {
		return orders;
	}
	public void setOrders(Orders orders) {
		this.orders = orders;
	}
}

3、一对多的测试类：

//JPA的Dao层
@Transactional
public class JpaDaoImpl implements JpaDao {
	
	//事务管理
	@PersistenceContext
	private EntityManager em;
	
	//JPA一对多测试类
	@Override
	public void jpaTest() {
		Orders orders=new Orders();
		orders.setAmount(34f);
		orders.setOrderid("999");
		
		OrderItem orderItem1=new OrderItem();
		orderItem1.setProductName("篮球");
		orderItem1.setSellPrice(150f);
		OrderItem orderItem2=new OrderItem();
		orderItem2.setProductName("足球");
		orderItem2.setSellPrice(90f);
		
		orders.addOrderItem(orderItem1);
		orders.addOrderItem(orderItem2);
		
		em.merge(orders);
	}
}

由于配置了事务管理，这里就不需要手动开启、提交事务和关闭资源等重复的代码，直接交由事务进行管理。

具体配置步骤可以参看这篇博客：https://blog.csdn.net/a745233700/article/details/81415550

 

 

二、一对一双向关联与级联操作：

以身份证类和人为例：

1、Persion类：

@Entity
public class Person {
	public Person() {
	}
	public Person(String name) {
		this.name=name;
	}
	
	private Integer id;
	private String name;
	private IDcard idcard;
	 
	@Id @GeneratedValue
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	
	@Column(length=10,nullable=false)
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
        //一对一配置
	@OneToOne(optional=false,cascade={CascadeType.ALL})
	@JoinColumn(name="idcard_id")
	public IDcard getIdcard() {
		return idcard;
	}
	public void setIdcard(IDcard idcard) {
		this.idcard = idcard;
	}
}

2、IDcard类：

@Entity
public class IDcard {
	public IDcard() {}
	public IDcard(String cardno) {
		this.cardno = cardno;
	}

	private Integer id;
	private String cardno;
	private Person person;
	
	@Id @GeneratedValue
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	
	@Column(length=18,nullable=false)
	public String getCardno() {
		return cardno;
	}
	public void setCardno(String cardno) {
		this.cardno = cardno;
	}
	
	//一对一配置：
	@OneToOne(mappedBy="idcard",cascade={CascadeType.PERSIST,CascadeType.MERGE,CascadeType.REFRESH})
	public Person getPerson() {
		return person;
	}
	public void setPerson(Person person) {
		this.person = person;
	}
}

3、一对一测试类：

	//JPA一对多测试类
	@Override
	public void jpaTest() {
		
		Person person=new Person("小张");		
		person.setIdcard(new IDcard("448xxx1990xxxx1234"));
		
		em.persist(person);
	}

 

 

三、多对多双向关联与级联操作：

以教师类和学生类为例：

1、教师类：

//老师为关系被维护端
@Entity
public class Teacher {
	public Teacher(){}
	public Teacher(String name) {
		this.name = name;
	}
	private Integer id;
	private String name;
	private Set students=new HashSet();
	
	@Id @GeneratedValue
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	
	@Column(length=10,nullable=false)
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	@ManyToMany(cascade=CascadeType.REFRESH,mappedBy="teachers")
	public Set getStudents() {
		return students;
	}
	public void setStudents(Set students) {
		this.students = students;
	}
	
	@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;
		Teacher other = (Teacher) obj;
		if (id == null) {
			if (other.id != null)
				return false;
		} else if (!id.equals(other.id))
			return false;
		return true;
	}
}

2、学生类：

//学生为关系维护端
@Entity
public class Student {
	public Student(){}
	public Student(String name) {
		this.name = name;
	}
	
	private Integer id;
	private String name;
	private Set teachers=new HashSet();
	
	@Id @GeneratedValue
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	
	@Column(length=10,nullable=false)
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	@ManyToMany(cascade=CascadeType.REFRESH)
	@JoinTable(name="student_teacher",//设置第三张表的表名
	inverseJoinColumns=@JoinColumn(name="teacher_id"),//设置被维护端在第三张表中的外键名称
	joinColumns=@JoinColumn(name="student_id"))//设置维护端在第三张表中的外键名称
	public Set getTeachers() {
		return teachers;
	}
	public void setTeachers(Set teachers) {
		this.teachers = teachers;
	}
	
	public void addTeacher(Teacher teacher){
		this.teachers.add(teacher);
	}
	
	public void removeTeacher(Teacher teacher){
		if(this.teachers.contains(teacher)){
			this.teachers.remove(teacher);
		}
	}
}

3、多对多测试类：

	//JPA多对多测试类：没有建立关系联系的添加
	@Override
	public void jpaTest() {
		
		//没有建立关系联系的添加
		em.persist(new Student("小张"));
		em.persist(new Teacher("李老师"));
	}

	//JPA多对多测试类：建立学生跟老师的联系
	@Override
	public void jpaTest() {
		
		//建立学生和老师的关系
		Student student=em.find(Student.class, 15);
		student.addTeacher(em.getReference(Teacher.class, 16));
	}

	//JPA多对多测试类：删除学生跟老师的联系
	@Override
	public void jpaTest() {
		
		//删除学生跟老师的联系
		Student student=em.find(Student.class, 15);
		student.removeTeacher(em.getReference(Teacher.class, 16));
	}

	//JPA多对多测试类：删除对象:只删除教师
	//直接不接触外键，直接删除老师，这种方式删除不了，被维护端没有权限删除外键，抛异常
	@Override
	public void jpaTest() {
		
		em.remove(em.getReference(Teacher.class, 16));
	}

	//JPA多对多测试类：删除对象:只删除教师
	//先解除学生与老师的关系，再删除教师对象
	@Override
	public void jpaTest() {
		
		Student student=em.find(Student.class, 15);
		Teacher teacher=em.getReference(Teacher.class, 16);
		student.removeTeacher(teacher);
		em.remove(teacher);
	}

	//JPA多对多测试类：删除对象:学生,并删除第三表中的记录，不删除老师
	//关系维护端有权限删除外键
	@Override
	public void jpaTest() {
		
		em.remove(em.getReference(Student.class, 15));
	}

 

 

四、联合主键：

以飞机航线为例：两个城市决定一条航线。

1、联合主键的三个要求：

（1）必须定义无参构造函数；

（2）必须实现序列化接口Serializable；

（3）必须重写hashCode()和equals()方法。

2、AirLinkPK联合主键类：

/*联合主键的三个要求：
1.必须定义无参构造函数
2.必须实现序列化接口Serializable
3.必须重写hashCode()和equals()方法
*/
@Embeddable
public class AirLinePK implements Serializable {		
	public AirLinePK(){}
	public AirLinePK(String startCity, String endCity) {
		this.startCity = startCity;
		this.endCity = endCity;
	}
	
	private String startCity;//开始城市
	private String endCity;//结束城市
	
	@Column(length=3)
	public String getStartCity() {
		return startCity;
	}
	public void setStartCity(String startCity) {
		this.startCity = startCity;
	}
	
	@Column(length=3)
	public String getEndCity() {
		return endCity;
	}
	public void setEndCity(String endCity) {
		this.endCity = endCity;
	}
	
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((endCity == null) ? 0 : endCity.hashCode());
		result = prime * result + ((startCity == null) ? 0 : startCity.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;
		AirLinePK other = (AirLinePK) obj;
		if (endCity == null) {
			if (other.endCity != null)
				return false;
		} else if (!endCity.equals(other.endCity))
			return false;
		if (startCity == null) {
			if (other.startCity != null)
				return false;
		} else if (!startCity.equals(other.startCity))
			return false;
		return true;
	}
}

3、AirLine类：

@Entity
public class AirLine {
	
	public AirLine(){}
	public AirLine(AirLinePK id) {
		this.id = id;
	}
	public AirLine(String startCity,String endCity,String name){
		this.id=new AirLinePK(startCity,endCity);
		this.name=name;
	}
	
	private AirLinePK id;
	private String name;
	
	//联合主键的实体标识符
	@EmbeddedId
	public AirLinePK getId() {
		return id;
	}
	public void setId(AirLinePK id) {
		this.id = id;
	}
	
	@Column(length=20)
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

3、联合主键测试类：

	//联合主键测试类
	@Override
	public void jpaTest() {
		
		em.persist(new AirLine("PEK","SHA","北京飞上海"));
	}