欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页

hibernate关联关系映射

程序员文章站 2022-04-22 18:04:25
...

1.单向关联

a.多对一(many-to-one)

@Entity(name = "COLOR")
public class Color implements Serializable {

	private static final long serialVersionUID = 6402753847018667163L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private Long id;

	@Column
	private String color;

	@ManyToOne(fetch = FetchType.EAGER)
	@JoinColumns({
		@JoinColumn(name = "DOG_ID",nullable = false,referencedColumnName="id"),
		@JoinColumn(name = "DOG_NAME",nullable = false,referencedColumnName="DOG_NAME")
	})
	private Dog dogs;

	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	public String getColor() {
		return color;
	}

	public void setColor(String color) {
		this.color = color;
	}

	public Dog getDogs() {
		return dogs;
	}

	public void setDogs(Dog dogs) {
		this.dogs = dogs;
	}

	@Override
	public String toString() {
		return "Color [id=" + id + ", color=" + color + "]";
	}
}

 

@Entity(name = "DOG")
@IdClass(DogPrimarykey.class)
public class Dog {
	private static final long serialVersionUID = 7219216851705947642L;

	@Id
	private String id;
	@Id
	private String name;
	@Column(name = "DOG_AGE", length = 2)
	private int age;

	public String getId() {
		return id;
	}

	public void setId(String 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) {
		if (age > 100 || age < 1) {
			this.age = 1;
		} else {
			this.age = age;
		}
	}

	public void say() {
		// TODO Auto-generated method stub
		System.out.println("汪  汪  汪");
	}

	@Override
	public String toString() {
		return "Dog [id=" + id + ", name=" + name + ", age=" + age + "]";
	}
}

 因为一的那一段采用的联合主键,因此采用@JoinColumns的方式注解

  @ManyToOne(fetch = FetchType.EAGER) 和 lazy 区别

如果是EAGER,那么表示取出这条数据时,它关联的数据也同时取出放入内存中。

如果是LAZY,那么取出这条数据时,它关联的数据并不取出来,在同一个session中,什么时候要用,就什么时候取(再次访问数据库)。

但是,在session外,就不能再取了。用EAGER时,因为在内存里,所以在session外也可以取。

CascadeType.PERSIST--级联保存,CascadeType.MERGE--级联更新,CascadeType.REFRESH--级联刷新,CascadeType.ALL--包含所有级联属性,所以可以多个。他们代表的操作是不一样的,如果你想包括所有级联关系就只需要用一个CascadeType.ALL就可以

b.一对一(One-to-one)

把上面的ManyToOne改为了OneToOne

 

2.双向关联(Bidirectional associations)

a.一对多(one to many)/多对一(many to one)

 

@Entity(name = "DOG")
@IdClass(DogPrimarykey.class)
public class Dog {
	private static final long serialVersionUID = 7219216851705947642L;

	@Id
	private String id;
	@Id
	private String name;
	@Column(name = "DOG_AGE", length = 2)
	private int age;

	@OneToMany(cascade=CascadeType.ALL,mappedBy="dogs")
	private Set<Color> colors;
	
	public Set<Color> getColors() {
		return colors;
	}

	public void setColors(Set<Color> colors) {
		this.colors = colors;
	}

	public String getId() {
		return id;
	}

	public void setId(String 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) {
		if (age > 100 || age < 1) {
			this.age = 1;
		} else {
			this.age = age;
		}
	}

	public void say() {
		// TODO Auto-generated method stub
		System.out.println("汪  汪  汪");
	}

	@Override
	public String toString() {
		return "Dog [id=" + id + ", name=" + name + ", age=" + age + "]";
	}
}

 

@Entity(name = "COLOR")
public class Color implements Serializable {

	private static final long serialVersionUID = 6402753847018667163L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private Long id;

	@Column
	private String color;

	@OneToOne
	@JoinColumns({
		@JoinColumn(name = "DOG_ID",nullable = false,referencedColumnName="id"),
		@JoinColumn(name = "DOG_NAME",nullable = false,referencedColumnName="DOG_NAME")
	})
	private Dog dogs;

	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	public String getColor() {
		return color;
	}

	public void setColor(String color) {
		this.color = color;
	}

	public Dog getDogs() {
		return dogs;
	}

	public void setDogs(Dog dogs) {
		this.dogs = dogs;
	}

	@Override
	public String toString() {
		return "Color [id=" + id + ", color=" + color + "]";
	}
}