SpringData JPA复合主键

　　上一篇博客简单介绍了SpringData JPA实现简单的CRUD，分页与多条件的排序，那里的主键类型是Long,有时我们会遇到主键不是一个的，复合主键，经过调研如下。确定一个人，不能只根据他的姓名来确定，因为会有重名，现在我们假设姓名、身份证号确定唯一一个人。

复合主键：一张表存在多个字段共同组成一个主键，这多个字段的组合不能重复，但是单独一个可以重复。

例子：姓名和省份证号共同组成了主键

 一、Spring Data Jpa 复合主键  

1.1、编写一个复合主键类：PeopleKey　

@Embeddable
public class PeopleKey implements Serializable  {
	
	@Column(name = "name")
	private String name;
	
	@Column(name = "idcardno")
	private String idcardno;
	// 省略setter,getter方法

	@Override
	public String toString() {
		return "PeopleKey [name=" + name + ", idcardno=" + idcardno + "]";
	}
}　

　　注意：　　

　　1) 实现Serializable接口(否则会报错，错误会直接显示); 

　　2)在复合主键的类上，使用注解@Embeddable

　　3) 有默认的public无参数的构造方法（在我这个实例中，我没有添加有参构造方法，所以采用默认的构造方法）

　　如果你在实体类里有有参构造方法，那么一定要有一个无参构造方法，否则运行的时候会报错

org.hibernate.InstantiationException: No default constructor for entity:  : com.my.model.People

这个就是没有默认的构造方法造成的，所以要在实体类中加入默认的无参构造方法。

　　4) 重写equals和hashCode方法。equals方法用于判断两个对象是否相同，EntityManger通过find方法来查找Entity时，是根据equals的返回值来判断的。hashCode方法返回当前对象的哈希码（我验证EntityManger，不重写也没事。）；

1.2、编写实体类：People 

package com.my.model;

import javax.persistence.*;
@Entity
@Table(name = "people")
//@IdClass(PeopleKey.class)
public class People extends PeopleKey{
　　　　// 复合主键要用这个注解
	@EmbeddedId
	private PeopleKey id;

	@Column(name = "age")
	private int age;
	
	@Column(name = "address")
	private String address;

	// 省略setter,getter方法
	@Override
	public String toString() {
		return "People [id=" + id + ", age=" + age + ", address=" + address
				+ "]";
	}	
}

1.3 测试：

@Service
public class PeopleService {
	
	@Resource
	private PeopleRepository peopleRepository;

	public People findOne() {
		PeopleKey peopleKey = new PeopleKey();
		peopleKey.setName("张三");
		peopleKey.setIdcardno("340123");
		People people = peopleRepository.findOne(peopleKey);
		return people;
	}

}

控制台上的输出结果：

People [id=PeopleKey [name=张三, idcardno=340123], age=3, address=分解分]　

二、采用@IdClass来注解复合主键


过程和@Embeddable差不多，这里直接贴例子。

@Entity
@Table(name = "people")
@IdClass(PeopleKey.class)
public class People  implements Serializable {

//	@EmbeddedId
//	private PeopleKey id;
	
	@Id
	@Column(name = "name")
	private String name;
	@Id
	@Column(name = "idcardno")
	private String idcardno;
	
	@Column(name = "age")
	private int age;
	
	@Column(name = "address")
	private String address;

}

public class PeopleKey implements Serializable  {
//	@Id
//	@Column(name = "name")
	private String name;
//	@Id
//	@Column(name = "idcardno")
	private String idcardno;

}

采用这个方法的我参考博客里有一篇，写的比较详细，但是感觉这个方法不好，本身就已经在PeopleKey中把主键给封装了，但是在实体类People中还要把复合主键给加入进去，不够简介，采用第一种方法，就很简单，而且也体现了Java类封装的思想。

 三、EntityManager的验证，直接上代码

package com.my.model;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Embeddable;
import javax.persistence.Entity;
import javax.persistence.Id;
//@Embeddable
public class PeopleKey implements Serializable  {
//	@Id
//	@Column(name = "name")
	private String name;
//	@Id
//	@Column(name = "idcardno")
	private String idcardno;
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getIdcardno() {
		return idcardno;
	}
	public void setIdcardno(String idcardno) {
		this.idcardno = idcardno;
	}

	@Override
	public String toString() {
		return "PeopleKey [name=" + name + ", idcardno=" + idcardno + "]";
	}	

}

　　

package com.my.model;

import java.io.Serializable;

import javax.persistence.*;
@Entity
@Table(name = "people")
@IdClass(PeopleKey.class)
public class People  {

//	@EmbeddedId
//	private PeopleKey id;
	

	@Column(name = "age")
	private int age;
	
	@Column(name = "address")
	private String address;

//	public PeopleKey getId() {
//		return id;
//	}
//
//	public void setId(PeopleKey id) {
//		this.id = id;
//	}
//	
	@Id
	@Column(name = "name")
	private String name;
	@Id
	@Column(name = "idcardno")
	private String idcardno;
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getIdcardno() {
		return idcardno;
	}
	public void setIdcardno(String idcardno) {
		this.idcardno = idcardno;
	}
	public int getAge() {
		return age;
	}

	public People() {
		super();
	}

	public void setAge(int age) {
		this.age = age;
	}


	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}

	@Override
	public String toString() {
		return "People [age=" + age + ", address=" + address + ", name=" + name
				+ ", idcardno=" + idcardno + "]";
	}
	
	
}

测试：

 
	 @RequestMapping(value = "/useEntityManager")
	 public void findUseEntityManager() throws Exception
	    {
		 	PeopleKey peopleKey = new PeopleKey();
			peopleKey.setName("张三");
			peopleKey.setIdcardno("340123");
	         People people = entityManager.find(People.class,peopleKey);
	         System.out.println(people.toString());
	    }　

 结果：

People [age=3, address=分解分, name=张三, idcardno=340123]

　　

参考博客：

1、https://www.cnblogs.com/linjiqin/archive/2011/03/09/1978680.html

2、http://blog.csdn.net/qq_35056292/article/details/77892012