本文为原创,如需转载,请注明作者和出处,谢谢!
上一篇: eclipse + JBoss 5 + EJB3开发指南(10):通过继承实体Bean,将单个表映射成多个表(单表策略,SINGLE_TABLE)
在上一篇文章中,使用单表策略将一个表从逻辑上分成了多个表。但这样可能会造成空巢字段,也就是说,一个逻辑表只由部分字段组成,而物理的表的很多字段的值就会为null 。为了解决这个问题,可以将t_accounts 表物理地分成多个表。为了与t_accounts 表进行对比,新建一个t_myaccounts 表,结构如图1 所示。
下一篇: eclipse + JBoss 5 + EJB3开发指南(12):使用命名查询执行JPQL
国内最棒的Google Android技术社区(eoeandroid),欢迎访问!
《银河系列原创教程》发布
《Java Web开发速学宝典》出版,欢迎定购
上一篇: eclipse + JBoss 5 + EJB3开发指南(10):通过继承实体Bean,将单个表映射成多个表(单表策略,SINGLE_TABLE)
在上一篇文章中,使用单表策略将一个表从逻辑上分成了多个表。但这样可能会造成空巢字段,也就是说,一个逻辑表只由部分字段组成,而物理的表的很多字段的值就会为null 。为了解决这个问题,可以将t_accounts 表物理地分成多个表。为了与t_accounts 表进行对比,新建一个t_myaccounts 表,结构如图1 所示。
图1 t_myaccounts表
从t_myaccounts的结构可以看出,在该表中只包含了t_accounts表的前三个字段,而后两个在逻辑上分到了不同的表,因此,首先要建立两个物理表:t_checkingaccount和t_savingsaccount。这两个表的结构如下:
图2 t_checkingaccount表
图3 t_savingsaccount表
图2 t_checkingaccount表
图3 t_savingsaccount表
在t_checkingaccount和t_savingsaccount表中都有一个account_id,这个account_id的值依赖于t_myaccounts表中的account_id。
下面先来编写与t_myaccounts对应的实体Bean,代码如下:
下面先来编写与t_myaccounts对应的实体Bean,代码如下:
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.InheritanceType;
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;
}
}
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.InheritanceType;
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;
}
}
从上面的代码可以看出,只使用了@Inheritance对实体Bean进行注释。
下面编写MyCheckingAccount和MySavingsAccount类的代码:
MyCheckingAccount类的代码:
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 " )
// 指定与Account类共享的主键名
@PrimaryKeyJoinColumn(name = " account_id " )
public class MyCheckingAccount extends Account
{
private double overdraftLimit;
public MyCheckingAccount()
{
// 为account_type字段赋默认值
setType( " C " );
}
@Column(name = " overdraft_limit " )
public double getOverdraftLimit()
{
return overdraftLimit;
}
public void setOverdraftLimit( double overdraftLimit)
{
this .overdraftLimit = overdraftLimit;
}
}
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 " )
// 指定与Account类共享的主键名
@PrimaryKeyJoinColumn(name = " account_id " )
public class MyCheckingAccount extends Account
{
private double overdraftLimit;
public MyCheckingAccount()
{
// 为account_type字段赋默认值
setType( " C " );
}
@Column(name = " overdraft_limit " )
public double getOverdraftLimit()
{
return overdraftLimit;
}
public void setOverdraftLimit( double overdraftLimit)
{
this .overdraftLimit = overdraftLimit;
}
}
MySavingsAccount类的代码:
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()
{
// 为account_type字段赋默认值
setType( " S " );
}
@Column(name = " interest_rate " )
public double getInterestRate()
{
return interestRate;
}
public void setInterestRate( double interestRate)
{
this .interestRate = interestRate;
}
}
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()
{
// 为account_type字段赋默认值
setType( " S " );
}
@Column(name = " interest_rate " )
public double getInterestRate()
{
return interestRate;
}
public void setInterestRate( double interestRate)
{
this .interestRate = interestRate;
}
}
在上面的代码中使用构造方法来初始化了t_myaccounts表的account_type字段的值。
可以使用下面的代码进行测试:
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);
.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);
下一篇: eclipse + JBoss 5 + EJB3开发指南(12):使用命名查询执行JPQL
国内最棒的Google Android技术社区(eoeandroid),欢迎访问!
《银河系列原创教程》发布
《Java Web开发速学宝典》出版,欢迎定购