Hibernate的关联映射(4)-单向N-N关联

骑猪看日落 2022-07-21 00:17 176阅读 0赞

单向N-N关联 (必须使用连接表)  
Address实体类

package com.anlw.entity;
    
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;
    import javax.persistence.Table;
    
    @Entity
    @Table(name="address_inf")
    public class Address {
    	//标识属性
    	@Id
    	@Column(name="address_id11d")
    	@GeneratedValue(strategy=GenerationType.IDENTITY)
    	private Integer addressId;
    	//定义地址详细的成员变量
    	private String addressDetail;
    	//无参构造器
    	public Address(){}
    	//初始化全部成员变量的构造器
    	public Address(String addressDetail) {
    		this.addressDetail = addressDetail;
    	}
    	public Integer getAddressId() {
    		return addressId;
    	}
    	public void setAddressId(Integer addressId) {
    		this.addressId = addressId;
    	}
    	public String getAddressDetail() {
    		return addressDetail;
    	}
    	public void setAddressDetail(String addressDetail) {
    		this.addressDetail = addressDetail;
    	}
    	
    }

Person实体类

package com.anlw.entity;
    
    import java.util.HashSet;
    import java.util.Set;
    
    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.JoinTable;
    import javax.persistence.ManyToMany;
    import javax.persistence.Table;
    
    
    
    @Entity
    @Table(name="person_inf")
    public class Person{
    	//标识属性
    	@Id
    	@Column(name="person_id12")
    	@GeneratedValue(strategy=GenerationType.IDENTITY)
    	private Integer id;
    	private String name;
    	private int age;
    	//定义该Person实体所有关联的Address实体
    	@ManyToMany(targetEntity=Address.class)
    	//映射连接表为p_a
    	@JoinTable(name="p_a",
    	//定义连接表中名为person_id的外键列，该外键列参照当前实体对应表的主键列
    	joinColumns=@JoinColumn(name="p",referencedColumnName="person_id12"),
    	//定义连接表中名为address_id的外键列
    	//该外键列参照当前实体的关联实体对应表的主键列,没有指定unique=true
    	inverseJoinColumns=@JoinColumn(name="a",referencedColumnName="address_id11d")
    			)
    	private Set<Address> addresses = new HashSet<Address>();
    	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;
    	}
    	public int getAge() {
    		return age;
    	}
    	public void setAge(int age) {
    		this.age = age;
    	}
    	public Set<Address> getAddresses() {
    		return addresses;
    	}
    	public void setAddresses(Set<Address> addresses) {
    		this.addresses = addresses;
    	}
    
    	
    }

核心测试

package com.anlw.test;
    
    import org.hibernate.Session;
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.service.ServiceRegistry;
    
    import com.anlw.entity.Address;
    import com.anlw.entity.Person;
    
    public class Test {
    	public static void main(String[] args) {
    		Configuration conf = new Configuration().configure();
    		ServiceRegistry st = new StandardServiceRegistryBuilder().applySettings(conf.getProperties()).build();
    		SessionFactory sf = conf.buildSessionFactory(st);
    		Session sess = sf.openSession();
    		Transaction tx = sess.beginTransaction();
    		// 创建一个Person对象
    		Person p = new Person();
    		// 创建一个瞬态的Address对象
    		Address a = new Address("guangzhou");
    		// 必须先持久化Address对象
    		sess.persist(a);
    		p.setName("kobe");
    		p.setAge(36);
    		// 设置Person和Address之间的关联关系
    		p.getAddresses().add(a);
    		// 持久化Person对象
    		sess.save(p);
    		// 创建一个瞬态的Address对象
    		Address a2 = new Address("shanghai");
    		// 必须先持久化Address对象
    		sess.persist(a2);
    		// 修改持久化状态的Person对象
    		p.getAddresses().add(a2);
    		tx.commit();
    		sess.close();
    		sf.close();
    	}
    }

单向的N-N关联和1-N关联的持久化类代码完全相同，控制着关系的一端需要增加一个Set类型的属性，被关联的持久化实例以集合的形式存在；

N-N关联需要使用@ManyToMany注解来修饰代表关联实体的集合属性；

@ManyToMany支持下面属性：

cascade:(非必须属性)指定Hibernate对关联实体采用怎样的级联策略，该级联策略支持如下五个属性：

1.CascadeType.ALL：指定Hibernate将多有的持久化操作都级联到关联实体；

2.CascadeType.MERGE：指定Hibernate将merge操作级联到关联实体；

3.CascadeType.PERSIST：指定Hibernate将persist操作级联到关联实体；

4.CascadeType.REFRESH：指定Hibernate将refresh操作级联到关联实体；

5.CascadeType.REMOVE：指定Hibernate将remove操作级联到关联实体；

fetch:(非必须属性)指定抓取关联实体时的抓取策略，该属性支持如下两个属性值：

1.FetchType.EAGER：抓取实体时，立即抓取关联实体，这是默认值；

2.FetchType.LAZY：抓取实体时，延迟抓取关联实体，等到正真用到关联实体时采取抓取；