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");

today lazy . tomorrow die .