hibernate--联合主键(了解+，掌握-)

如果一个表有多个主键（= =一般比较少）

8.4. 组件作为联合标识符(Components as composite identifiers)

先定义一个类OrderLineId
(实现接口，implems java.io.Serializable（把当前对象序列化的接口）)，

重写

为什么要序列化？内存满和集群化的时候用，具体百度(= =！其实我就是不懂)

里面有多个成员变量(字段(这些都是主键))

package com.zjx.hibernate;

public class StudentPK implements java.io.Serializable{
	private int id;
	private String name;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

	@Override
	public boolean equals(Object o) {         //判断是否相等，相等就找它
		if(o instanceof StudentPK) {  //如果o是一个StudentPK对象
			StudentPK pk = (StudentPK)o;
			if(this.id == pk.getId() && this.name.equals(pk.getName())) {
				return true;
			}
		}
		return false;
	}

	@Override
	public int hashCode() {   //计算主键的hash码  if有多个对象 就直接找hash码
		return this.name.hashCode();
	}
}

然后在 类Student中 创建OrderLineId成员变量，set,get一下，

package com.zjx.hibernate;

public class Student {

	private StudentPK pk;

	private int age;
	private String sex;
	private boolean good;
	public boolean isGood() {
		return good;
	}
	public void setGood(boolean good) {
		this.good = good;
	}
	/*public int getId() {
		return id;
	}
	public void setId(int 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 String getSex() {
		return sex;
	}
	public void setSex(String sex) {
		this.sex = sex;
	}
	public StudentPK getPk() {
		return pk;
	}
	public void setPk(StudentPK pk) {
		this.pk = pk;
	}

}

 其他都一样 就是hbm.xml配置文件改成如下

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
	<class name="com.bjsxt.hibernate.Student">

		<composite-id name="pk" class="com.zjx.hibernate.StudentPK">
			<key-property name="id"></key-property>
			<key-property name="name"></key-property>
		</composite-id>

		<property name="age" />
		<property name="sex" />
		<property name="good" type="yes_no"></property>
    </class>

</hibernate-mapping>

如果是使用注释？

2.2.. 映射主键属性

下面是定义组合主键的几种语法:

• 将组件类注解为@Embeddable,并将组件的属性注解为@Id(比较少用)
• 将组件的属性注解为@EmbeddedId       
• 将类注解为@IdClass,并将该实体中所有属于主键的属性都注解为@Id

对于EJB2的开发人员来说 @IdClass是很常见的, 但是对于Hibernate的用户来说就是一个崭新的用法.

组合主键类对应了一个实体类中的多个字段或属性,

而且主键类中用于定义主键的字段或属性和 实体类中对应的字段或属性在类型上必须一致.下面我们看一个例子:

@Entity
@IdClass(FootballerPk.class)
public class Footballer {
    //part of the id key
    @Id public String getFirstname() {
        return firstname;
    }

    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }

    //part of the id key
    @Id public String getLastname() {
        return lastname;
    }

    public void setLastname(String lastname) {
        this.lastname = lastname;
    }

    public String getClub() {
        return club;
    }

    public void setClub(String club) {
        this.club = club;
    }

    //appropriate equals() and hashCode() implementation
}

@Embeddable
public class FootballerPk implements Serializable {
    //same name and type as in Footballer
    public String getFirstname() {
        return firstname;
    }

    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }

    //same name and type as in Footballer
    public String getLastname() {
        return lastname;
    }

    public void setLastname(String lastname) {
        this.lastname = lastname;
    }

    //appropriate equals() and hashCode() implementation
}

如上, @IdClass指向对应的主键类.

Hibernate支持在组合标识符中定义关联(就像使用普通的注解一样),而EJB3规范并不支持此类用法.

@Entity
@AssociationOverride( name="id.channel", joinColumns = @JoinColumn(name="chan_id") )
public class TvMagazin {
    @EmbeddedId public TvMagazinPk id;
    @Temporal(TemporalType.TIME) Date time;
}

@Embeddable
public class TvMagazinPk implements Serializable {
    @ManyToOne
    public Channel channel;
    public String name;
    @ManyToOne
    public Presenter presenter;
}

@IdClass(TeacherPK.class)；

那么就能使用TeacherPK的东西；

t.setId(1);

t.setName("t1");

版权声明：本文为博主原创文章，未经博主允许不得转载。