繼承映射（一）整個類繼承樹只一張表SINGLE_TABLE

根類一張表，將所有子類的屬性都合并到根類中，此外還需要一個欄位用於區分子類的類型。

company：

package com.persia.singletable;import java.io.Serializable;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToMany;import javax.persistence.OrderBy;import javax.persistence.Table;@Entity@Table(name="company")public class Company implements Serializable {   private Integer id;private String name;private Set<Employee> employees=new HashSet<Employee>();@Id@GeneratedValuepublic Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@OneToMany(mappedBy="company",cascade=CascadeType.ALL,fetch=FetchType.EAGER)@OrderBy(value="id asc")public Set<Employee> getEmployees() {return employees;}public void setEmployees(Set<Employee> employees) {this.employees = employees;}public void addEmployee(Employee em){if(!this.employees.contains(em)){this.employees.add(em);em.setCompany(this);}}public void removeEmployee(Employee em){em.setCompany(null);this.employees.remove(em);}}

 

Employee:

package com.persia.singletable;import java.io.Serializable;import javax.persistence.CascadeType;import javax.persistence.DiscriminatorColumn;import javax.persistence.DiscriminatorType;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Inheritance;import javax.persistence.InheritanceType;import javax.persistence.ManyToOne;import javax.persistence.Table;import javax.persistence.JoinColumn;@Entity@Table(name="employee")@Inheritance(strategy=InheritanceType.SINGLE_TABLE)@DiscriminatorColumn(name="Discriminator",discriminatorType=DiscriminatorType.STRING,length=30)public class Employee implements Serializable {private Integer id;private String name;private Company company;@Id@GeneratedValuepublic Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@ManyToOne(cascade=CascadeType.PERSIST,optional=false)@JoinColumn(name="company_id")public Company getCompany() {return company;}public void setCompany(Company company) {this.company = company;}}

 

繼承類：HourEmployee:

package com.persia.singletable;import java.io.Serializable;import javax.persistence.Column;import javax.persistence.DiscriminatorValue;import javax.persistence.Entity;@Entity@DiscriminatorValue("rate")public class HourEmployee extends Employee {   private Float rate;   @Column(nullable=true)public Float getRate() {return rate;}public void setRate(Float rate) {this.rate = rate;}      }

 

SalaryEmployee:

package com.persia.singletable;import java.io.Serializable;import javax.persistence.Column;import javax.persistence.DiscriminatorValue;import javax.persistence.Entity;@Entity@DiscriminatorValue("salary")public class SalaryEmployee extends Employee {private Float salary;@Column(nullable=true)public Float getSalary() {return salary;}public void setSalary(Float salary) {this.salary = salary;}}

 

dao:

package com.persia.singletable;import java.util.HashSet;import java.util.Iterator;import java.util.List;import java.util.Set;import javax.ejb.Remote;import javax.ejb.Stateless;import javax.persistence.EntityManager;import javax.persistence.PersistenceContext;import javax.persistence.Query;@Stateless@Remote({EmployeeDAO.class})public class EmployeeDAOBean implements EmployeeDAO {@PersistenceContextprivate EntityManager em;@Overridepublic void deleteCompany(Integer companyId) {Company c=em.find(Company.class, companyId);em.remove(c);}@Overridepublic void deleteEmployee(Integer employeeId) {Employee e=em.find(Employee.class, employeeId);e.getCompany().getEmployees().remove(e);e.setCompany(null);em.remove(e);}@Overridepublic List getAllCompany() {Query q=em.createQuery("from Company");return q.getResultList();}@Overridepublic void insertCompany() {Company c=new Company();c.setName("waltmart");SalaryEmployee se=new SalaryEmployee();se.setCompany(c);se.setName("salary-a");se.setSalary(9999.0f);HourEmployee he=new HourEmployee();he.setCompany(c);he.setName("hour-b");he.setRate(888.0f);Set set=new HashSet();set.add(se);set.add(he);c.setEmployees(set);em.persist(c);}@Overridepublic void updateHourEmployee(Integer companyId, String name, float rate) {Company c=em.find(Company.class, companyId);Set set=c.getEmployees();Iterator it=set.iterator();while(it.hasNext()){Employee e=(Employee)it.next();if(e instanceof HourEmployee){if(e.getName().equals(name))((HourEmployee)e).setRate(rate);}}}}

 

用戶端：

package com.persia.singletabletest;import java.util.Iterator;import java.util.List;import java.util.Properties;import javax.naming.InitialContext;import javax.naming.NamingException;import com.persia.singletable.Company;import com.persia.singletable.Employee;import com.persia.singletable.EmployeeDAO;import com.persia.singletable.HourEmployee;import com.persia.singletable.SalaryEmployee;public class TestSingleTable {/** * @param args * @throws NamingException  */public static void main(String[] args) throws NamingException {Properties props=new Properties();      props.setProperty("java.naming.factory.initial","org.jnp.interfaces.NamingContextFactory");      props.setProperty("java.naming.provider.url","localhost:1099");      props.setProperty("java.naming.factory.url.pkgs","org.jboss.naming");InitialContext context=new InitialContext(props);try{EmployeeDAO edao=(EmployeeDAO)context.lookup("EmployeeDAOBean/remote");edao.insertCompany();//edao.deleteCompany(new Integer(1));//edao.deleteEmployee(new Integer(1));//edao.updateHourEmployee(new Integer(1), "hour-b", 95.0f);List ls=edao.getAllCompany();if(ls!=null){for(int i=0;i<ls.size();i++){Company c=(Company)ls.get(i);if(c!=null){System.out.println("=====公司:"+c.getName()+"=====");Iterator it=c.getEmployees().iterator();while(it.hasNext()){Employee e=(Employee)it.next();if(e instanceof HourEmployee)System.out.println("臨時工："+((HourEmployee)e).getName()+" 薪水："+((HourEmployee)e).getRate());elseSystem.out.println("正式員工："+((SalaryEmployee)e).getName()+" 薪水："+((SalaryEmployee)e).getSalary());}}}}}catch(Exception e){e.printStackTrace();}}}

 

運行結果：

=====公司:waltmart=====正式員工：salary-a 薪水：9999.0臨時工：hour-b 薪水：888.0

資料庫：

mysql> desc company;+-------+--------------+------+-----+---------+----------------+| Field | Type         | Null | Key | Default | Extra          |+-------+--------------+------+-----+---------+----------------+| id    | int(11)      | NO   | PRI | NULL    | auto_increment || name  | varchar(255) | YES  |     | NULL    |                |+-------+--------------+------+-----+---------+----------------+2 rows in set (0.03 sec)mysql> desc employee;+---------------+--------------+------+-----+---------+----------------+| Field         | Type         | Null | Key | Default | Extra          |+---------------+--------------+------+-----+---------+----------------+| Discriminator | varchar(30)  | NO   |     | NULL    |                || id            | int(11)      | NO   | PRI | NULL    | auto_increment || name          | varchar(255) | YES  |     | NULL    |                || rate          | float        | YES  |     | NULL    |                || salary        | float        | YES  |     | NULL    |                || company_id    | int(11)      | NO   | MUL | NULL    |                |+---------------+--------------+------+-----+---------+----------------+6 rows in set (0.02 sec)mysql> select * from company;+----+----------+| id | name     |+----+----------+|  1 | waltmart |+----+----------+1 row in set (0.00 sec)mysql> select * from employee;+---------------+----+----------+------+--------+------------+| Discriminator | id | name     | rate | salary | company_id |+---------------+----+----------+------+--------+------------+| rate          |  1 | hour-b   |  888 |   NULL |          1 || salary        |  2 | salary-a | NULL |   9999 |          1 |+---------------+----+----------+------+--------+------------+2 rows in set (0.00 sec)

 

進行刪除和修改：

修改

=====公司:waltmart=====臨時工：hour-b 薪水：95.0正式員工：salary-a 薪水：9999.0

 

刪除company後：

mysql> select * from employee;Empty set (0.00 sec)mysql> select * from company;Empty set (0.00 sec)

 

若刪除SalaryEmployee：

=====公司:waltmart=====臨時工：hour-b 薪水：888.0

 

mysql> select * from employee;+---------------+----+--------+------+--------+------------+| Discriminator | id | name   | rate | salary | company_id |+---------------+----+--------+------+--------+------------+| rate          |  2 | hour-b |  888 |   NULL |          1 |+---------------+----+--------+------+--------+------------+1 row in set (0.00 sec)mysql> select * from company;+----+----------+| id | name     |+----+----------+|  1 | waltmart |+----+----------+1 row in set (0.00 sec)

 

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SINGLE_TABLE的映射策略在所有繼承策略中是最簡單的，同時也是執行效率最高的，它僅僅需要對一個表進行管理和操作。在載入entity和多態串連時不需要進行任何關聯，聯合或子查詢，因為所有的資料都儲存在一個表中。

缺點是：

1.不能為所有子類的屬性對應的欄位定義not null 約束

2.在子類屬性不是非常多的時候優先選擇。（查詢效能高）