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 + "]";
}
}
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