Hibernate多对一ManytoOne

 ------------------------Hibernate多对一ManytoOne
 要点：
 ManytoOne配置在多端
 可以配置级联操作 @ManyToOne(cascade=CascadeType.ALL)

 实体：

 @Entity
public class Person {//一端

	@Id
	@GeneratedValue
	private long id;

	@Column(name="name")
	private String name;

	public long getId() {
		return id;
	}
	public void setId(long id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

@Entity
public class Phone {//多端

	@Id
	@GeneratedValue
	private Long id;
	@Column(name="number")
	private String number;

	@ManyToOne
	private Person p;

	public Person getP() {
		return p;
	}
	public void setP(Person p) {
		this.p = p;
	}
	public Long getId() {
		return id;
	}
	public void setId(Long id) {
		this.id = id;
	}
	public String getNumber() {
		return number;
	}
	public void setNumber(String number) {
		this.number = number;
	}
}

实例1：只在多端配置映射，只保存一端
		Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

        session.save(p);

结果：
Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)
只保存了Person为张三的对象。因为Person端无映射，且为瞬时态对象，直接持久化Person。

实例2.只在多端配置映射，只保存多端
		Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

		session.save(ph1);
        session.save(ph2);

结果：[org.hibernate.TransientObjectException: object references an unsaved transient instance
Hibernate抛出异常：Phone对象引用了未持久化瞬时态的Person对象。
因为没有做级联。

实例3.只在多端配置映射，先保存一端，再保存多端
        Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

        session.save(p);
        session.save(ph1);
        session.save(ph2); 

结果：
Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
先持久化person，再持久化Phone，外键关联正确，执行结果完美。

实例4.只在多端配置映射，先保存多端，再保存一端
        Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

        session.save(ph1);
        session.save(ph2);
        session.save(p); 

结果：
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)
Hibernate:
    update
        Phone
    set
        number=?,
        p_id=?
    where
        id=?
Hibernate:
    update
        Phone
    set
        number=?,
        p_id=?
    where
        id=?
先保存ph1,ph2,然后保存了person，最后将外键更新至ph1，ph2。执行了5条sql，有点浪费资源。
因为保存ph1,ph2时person还未保存，所以ph1和ph2的外键为null。最后保存了person后再更新外键。
当设置关联为@ManyToOne(optional=false)外键不许为空时，则抛出异常，因为保存ph1，ph2时外键为null。		

---------------在多端设置级联
要点：设置级联后，可以只保存多端，而不保存一端。持久化操作进行了传递即级联

@Entity
public class Phone {

	@Id
	@GeneratedValue
	private Long id;
	@Column(name="number")
	private String number;

	@ManyToOne(cascade=CascadeType.ALL)//多对一，设置级联
	private Person p;

	public Person getP() {
		return p;
	}
	public void setP(Person p) {
		this.p = p;
	}
	public Long getId() {
		return id;
	}
	public void setId(Long id) {
		this.id = id;
	}
	public String getNumber() {
		return number;
	}
	public void setNumber(String number) {
		this.number = number;
	}

}

实例5：只在多端配置映射并且设置级联，只保存一端
        Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

        session.save(p);
结果：
Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)

实例6.只在多端配置映射并且设置级联，只保存多端
        Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

        session.save(ph1);
        session.save(ph2);

结果：Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)

先保存person，再保存phone，执行3条sql语句，运行结果完美

实例7.只在多端配置映射并且设置级联，先保存一端，再保存多端
        Person p  = new Person();
        p.setName("zhangsan");

        Phone ph1 = new Phone();
        Phone ph2 = new Phone();
        ph1.setNumber("13109361111");
        ph2.setNumber("13109362222");

        ph1.setP(p);
        ph2.setP(p);

        session.save(p);
        session.save(ph1);
        session.save(ph2);
结果：
Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
运行同上，结果完美

实例7.只在多端配置映射并且设置级联，先保存多端，再保存一端
        session.save(ph1);
        session.save(ph2);
        session.save(p);
结果：
Hibernate:
    insert
    into
        Person
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
Hibernate:
    insert
    into
        Phone
        (number, p_id, id)
    values
        (?, ?, ?)
运行同上，结果完美