Java持久性API（JPA）第5讲——关系建立

目标：建立实体类之间的关系来体现数据库中实体之间的关联关系。

主要内容：

n 创建数据库表

n 添加测试数据

n 生成实体类

n 关系分析

n 主键类

1、实例说明

本实例模拟简单学生成绩管理系统，为了使用说明一对一的关系，每个学生对一个一台电脑。涉及的实体有：学生、电脑、课程、成绩。具体信息参考《创建数据库表》部分。

涉及的关系如下：

n 每个电脑属于一个学生；

n 学生和课程之间是多对多的关系；

n 学生和成绩之间是一对多的关系；

n 课程和成绩之间是一对多的关系。

2、创建数据库表

数据库表的定义语句如下：

create table student

(

s_id char(10) not null,

s_name char(10) not null,

s_birthday date,

primary key(s_id)

);

create table computer

(

c_id char(10) not null,

s_id char(10),

c_price float,

primary key(s_id),

foreign key(s_id) references student(s_id)

);

create table course

(

c_id char(8) not null,

c_name varchar(50) not null,

primary key(c_id)

);

create table score

(

s_id char(10) not null,

c_id char(8) not null,

grade float,

primary key(s_id,C_id),

foreign key(s_id) references student(s_id),

foreign key(c_id) references course(c_id)

);

3、添加测试数据

添加测试数据的SQL语句如下：

insert into student values

(

'0511370101',

'张小菲',

'1987-03-04'

);

insert into student values

(

'0511370104',

'刘慧',

'1988-11-04'

);

insert into student values

(

'0511370102',

'付方',

'1988-9-9'

);

insert into student values

(

'0511370103',

'郭好',

'1987-6-5'

);

insert into computer values

(

'cp001',

'0511370101',

8500

);

insert into computer values

(

'cp002',

'0511370102',

8600

);

insert into computer values

(

'cp003',

'0511370103',

8400

);

insert into computer values

(

'cp004',

'0511370104',

8700

);

insert into course values

(

'JK0301',

'Java Web开发'

);

insert into course values

(

'JK0302',

'Java EE'

);

insert into course values

(

'JK0401',

'.NET'

);

insert into course values

(

'JK0402',

'C#语言'

);

insert into score VALUES(

'0511370101','JK0301',70

);

insert into score VALUES(

'0511370101','JK0302',71

);

insert into score VALUES(

'0511370101','JK0401',72

);

insert into score VALUES(

'0511370101','JK0402',73

);

insert into score VALUES(

'0511370103','JK0301',90

);

insert into score VALUES(

'0511370103','JK0302',91

);

insert into score VALUES(

'0511370103','JK0401',92

);

insert into score VALUES(

'0511370103','JK0402',93

);

insert into score VALUES(

'0511370104','JK0301',80

);

insert into score VALUES(

'0511370104','JK0302',81

);

insert into score VALUES(

'0511370104','JK0401',82

);

insert into score VALUES(

'0511370104','JK0402',83

);

4、使用向导生成持久单元

和第1讲中的过程相同。

5、使用向导生成实体类

和第1讲中的过程相同。

6、关系分析

1）一对一的关系

学生和电脑之间的关系是一对一的关系

在Student类中有如下表示关系的代码：

@OneToOne(cascade = CascadeType.ALL, mappedBy = "student")

private Computer computer;

在Computer类中有如下表示关系的代码：

@JoinColumn(name = "s_id", referencedColumnName = "s_id", insertable = false, updatable = false)

@OneToOne

private Student student;

2）一对多的关系

学生和成绩之间是一对多的关系。

在Student类中有如下表示关系的代码：

@OneToMany(cascade = CascadeType.ALL, mappedBy = "student")

private Collection<Score> scoreCollection;

3）多对一的关系

成绩和学生之间的关系是多对一的关系。

在成绩类中有如下表示关系的代码：

@JoinColumn(name = "s_id", referencedColumnName = "s_id", insertable = false, updatable = false)

@ManyToOne

private Student student;

7、主键类

在成绩表中，学生学号和课程号组成联合主键。在创建实体类的时候需要一个主键类来标识这个实体。

主键类的代码如下：

/*

* ScorePK.java

*

* Created on 2007年5月30日, 上午1:11

*

* To change this template, choose Tools | Template Manager

* and open the template in the editor.

*/

package jpa;

import java.io.Serializable;

import javax.persistence.Column;

import javax.persistence.Embeddable;

/**

* Primary Key class ScorePK for entity class Score

*

* @author Administrator

*/

@Embeddable

public class ScorePK implements Serializable {

@Column(name = "s_id", nullable = false)

private String sId;

@Column(name = "c_id", nullable = false)

private String cId;

/** Creates a new instance of ScorePK */

public ScorePK() {

}

/**

* Creates a new instance of ScorePK with the specified values.

* @param cId the cId of the ScorePK

* @param sId the sId of the ScorePK

*/

public ScorePK(String cId, String sId) {

this.cId = cId;

this.sId = sId;

}

/**

* Gets the sId of this ScorePK.

* @return the sId

*/

public String getSId() {

return this.sId;

}

/**

* Sets the sId of this ScorePK to the specified value.

* @param sId the new sId

*/

public void setSId(String sId) {

this.sId = sId;

}

/**

* Gets the cId of this ScorePK.

* @return the cId

*/

public String getCId() {

return this.cId;

}

/**

* Sets the cId of this ScorePK to the specified value.

* @param cId the new cId

*/

public void setCId(String cId) {

this.cId = cId;

}

/**

* Returns a hash code value for the object.This implementation computes

* a hash code value based on the id fields in this object.

* @return a hash code value for this object.

*/

@Override

public int hashCode() {

int hash = 0;

hash += (this.cId != null ? this.cId.hashCode() : 0);

hash += (this.sId != null ? this.sId.hashCode() : 0);

return hash;

}

/**

* Determines whether another object is equal to this ScorePK.The result is

* <code>true</code> if and only if the argument is not null and is a ScorePK object that

* has the same id field values as this object.

* @param object the reference object with which to compare

* @return <code>true</code> if this object is the same as the argument;

* <code>false</code> otherwise.

*/

@Override

public boolean equals(Object object) {

// TODO: Warning - this method won't work in the case the id fields are not set

if (!(object instanceof ScorePK)) {

return false;

}

ScorePK other = (ScorePK)object;

if (this.cId != other.cId && (this.cId == null || !this.cId.equals(other.cId)))

return false;

if (this.sId != other.sId && (this.sId == null || !this.sId.equals(other.sId)))

return false;

return true;

}

/**

* Returns a string representation of the object.This implementation constructs

* that representation based on the id fields.

* @return a string representation of the object.

*/

@Override

public String toString() {

return "jpa.ScorePK[cId=" + cId + ", sId=" + sId + "]";

}

}