hibernate关联关系映射

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 + "]";
	}
}