JPA实体继承的映射

标签：jpa继承映射   jpa继承关系   jpa实体继承   

注：文章中据说的实体指的是被@Entity注解的类。

JPA中对象关系映射通常情况下是一个实体对应一个表，两个实体之间没有任何关系。如果两个实体之间是继承关系，那么该如何映射呢？

映射超类能够像实体类一样被映射，只是它的映射将作用于继承自它的实体类，因为它本身不存在单独的表。当作用于子类时，继承的映射信息将作用于子类对应的表上。子类可以通过@AttributeOverride和AssociationOverride注解或对应的XML描述符元素来覆盖映射超类的映射信息。下面来看一个实例：

package com.mikan;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;

@MappedSuperclass
public class Employee implements Serializable {

	private static final long serialVersionUID = -7674269980281525370L;
	
	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	protected Integer empId;
	
	@Column
	protected String name;

	// getter/setter方法
	
}

package com.mikan;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name = "FT_EMP")
public class FullTimeEmployee extends Employee {

	private static final long serialVersionUID = 9115429216382631425L;

	// 继承映射超类的empId和name属性

	@Column
	private Double salary;

	// getter/setter方法
	
}

package com.mikan;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name = "PT_EMP")
public class PartTimeEmployee extends Employee {

	private static final long serialVersionUID = -6122347374515830424L;

	// 继承映射超类的empId和name属性

	@Column(name = "hourly_wage")
	private Float hourlyWage;

	// getter/setter方法
	
}

public class Cart {
	protected Integer operationCount; // transient state

	public Cart() {
		operationCount = 0;
	}

	public Integer getOperationCount() {
		return operationCount;
	}

	public void incrementOperationCount() {
		operationCount++;
	}
}

@Entity
public class ShoppingCart extends Cart {
	Collection<Item> items = new Vector<Item>();

	public ShoppingCart() {
		super();
	}

	@OneToMany
	public Collection<Item> getItems() {
		return items;
	}

	public void addItem(Item item) {
		items.add(item);
		incrementOperationCount();
	}
}

ShoppingCart对应的表中不会包含operationCount字段。

JPA实体继承的映射

标签：jpa继承映射   jpa继承关系   jpa实体继承   

原文地址：http://blog.csdn.net/mhmyqn/article/details/37994707