目標：建立實體類之間的關係來體現資料庫中實體之間的關聯關係。主要內容：n 建立資料庫表n 添加測試資料n 產生實體類n 關係分析n 主鍵類1、執行個體說明本執行個體類比簡單學產生績管理系統，為了使用說明一對一的關係，每個學生對一個一台電腦。涉及的實體有：學生、電腦、課程、成績。具體資訊參考《建立資料庫表》部分。涉及的關係如下：n 每個電腦屬於一個學生；n 學生和課程之間是多對多的關係；n 學生和成績之間是一對多的關係；n 課程和成績之間是一對多的關係。2、建立資料庫表資料庫表的定義語句如下：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、添加測試資料 添加測試資料的SQL語句如下：insert into student values( '0511370101', '張小菲', '1987-03-04');insert into student values( '0511370104', '劉慧', '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開發'); insert into course values( 'JK0302', 'Java EE'); insert into course values( 'JK0401', '.NET'); insert into course values( 'JK0402', 'C#語言'); 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、使用嚮導產生持久單元 和第1講中的過程相同。5、使用嚮導產生實體類 和第1講中的過程相同。6、關係分析 1）一對一的關係學生和電腦之間的關係是一對一的關係在Student類中有如下表示關係的代碼： @OneToOne(cascade = CascadeType.ALL, mappedBy = "student") private Computer computer;在Computer類中有如下表示關係的代碼： @JoinColumn(name = "s_id", referencedColumnName = "s_id", insertable = false, updatable = false) @OneToOne private Student student; 2）一對多的關係學生和成績之間是一對多的關係。在Student類中有如下表示關係的代碼： @OneToMany(cascade = CascadeType.ALL, mappedBy = "student") private Collection<Score> scoreCollection;3）多對一的關係成績和學生之間的關係是多對一的關係。在成績類中有如下表示關係的代碼： @JoinColumn(name = "s_id", referencedColumnName = "s_id", insertable = false, updatable = false) @ManyToOne private Student student;7、主鍵類在成績表中，學生學號和課程號組成聯合主鍵。在建立實體類的時候需要一個主鍵類來標識這個實體。主鍵類的代碼如下：/* * 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 */@Embeddablepublic 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 + "]"; } }
