在上一篇文章中,使用單表策略將一個表從邏輯上分成了多個表.但這樣可能會造成空巢欄位,也就是說,一個邏輯表只由部分欄位組成,而物理的表的很多欄位的值就會為null.為了解決這個問題,可以將t_accounts表物理地分成多個表.為了與t_accounts表進行對比,新建一個t_myaccounts表,結構如圖1所示.
從t_myaccounts的結構可以看出,在該表中只包含了t_accounts表的前三個欄位,而後兩個在邏輯上分到了不同的表,因此,
要建立兩個物理表：t_checkingaccount和t_savingsaccount.這兩個表的結構如下：
圖2 t_checkingaccount表
在t_checkingaccount和t_savingsaccount表中都有一個account_id,這個account_id的值依賴於t_myaccounts表中的account_id.
下面先來編寫與t_myaccounts對應的實體Bean,代碼如下：
| package entity; import javax.persistence.Column; import javax.persistence.DiscriminatorColumn; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.GenerationType; import javax.persistence.Id; import javax.persistence.Inheritance; import javax.persistence.Table; @Entity @Table(name="t_myaccounts") @Inheritance(strategy=InheritanceType.JOINED) public class Account { protected String id; protected float balance; protected String type; @Id @GeneratedValue(strategy=GenerationType.IDENTITY) @Column(name="account_id") public String getId() { return id; } public void setId(String id) { this.id = id; } public float getBalance() { return balance; } public void setBalance(float balance) { this.balance = balance; } @Column(name="account_type") public String getType() { return type; } public void setType(String type) { this.type = type; } } |
從上面的代碼可以看出,只使用了@Inheritance對實體Bean進行註釋.
下面編寫MyCheckingAccount和MySavingsAccount類的代碼：
MyCheckingAccount類的代碼：
| package entity; import javax.persistence.Column; import javax.persistence.DiscriminatorValue; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.PrimaryKeyJoinColumn; import javax.persistence.Table; @Entity @Table(name="t_checkingaccount") // 指定與Account類共享的主鍵名 @PrimaryKeyJoinColumn(name="account_id") public class MyCheckingAccount extends Account { private double overdraftLimit; public MyCheckingAccount() { // 為account_type欄位賦默認值 } @Column(name="overdraft_limit") public double getOverdraftLimit() { return overdraftLimit; } public void setOverdraftLimit(double overdraftLimit) { this.overdraftLimit = overdraftLimit; } } |
MySavingsAccount類的代碼：
| package entity; import javax.persistence.Column; import javax.persistence.DiscriminatorValue; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.PrimaryKeyJoinColumn; import javax.persistence.Table; @Entity @Table(name="t_savingsaccount") @PrimaryKeyJoinColumn(name="account_id") public class MySavingsAccount extends Account { private double interestRate; public MySavingsAccount() { // 為account_type欄位賦默認值 setType("S"); } @Column(name="interest_rate") public double getInterestRate() { return interestRate; } public void setInterestRate(double interestRate) { this.interestRate = interestRate; } } |
在上面的代碼中使用構造方法來初始化了t_myaccounts表的account_type欄位的值.
可以使用下面的代碼進行測試：
| System.out.println(((MyCheckingAccount)em.createQuery("from MyCheckingAccount where id=12") .getSingleResult()).getBalance()); MyCheckingAccount ca = new MyCheckingAccount(); ca.setBalance(342); ca.setOverdraftLimit(120); em.persist(ca); MySavingsAccount sa = new MySavingsAccount(); sa.setBalance(200); sa.setInterestRate(321); em.persist(sa); |
[火星人 ] Eclipse JBoss 5 EJB3開發指南（11）：實體Bean的連接策略（JOINED Strategy）已經有557次圍觀
