Hibernate @OneToOne懒加载实现解决方案

浏览数：27 / 时间：2015年06月09日

在hibernate注解（三）中，我提高过一对一（@OneToOne）懒加载失效的问题。虽然给出了解决方法，但并没有给出完整的解决方案。今天我专门针对该问题进行讨论。至于懒加载失效的原因，在之前的文章中已经我已经叙述过了，就不再重复了，不明白的可以去看看。

一、测试环境

数据库：myqsl

代码：主：Student，从：Card

表：

DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `ID` int(11) NOT NULL,
  `NAME` varchar(50) NOT NULL,
  `CARD_ID` int(11) DEFAULT NULL,
  PRIMARY KEY (`ID`),
  KEY `PK_CARD_ID` (`CARD_ID`),
  CONSTRAINT `PK_CARD_ID` FOREIGN KEY (`CARD_ID`) REFERENCES `card` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

DROP TABLE IF EXISTS `card`;
CREATE TABLE `card` (
  `ID` int(11) NOT NULL,
  `CODE` varchar(32) NOT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

代码：

package com.po;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Entity
@Table(name = "Student")
public class Student {

	private int id;
	private String name;
	private Card card;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	@Column(name = "NAME", nullable = false, length = 50)
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
	@JoinColumn(name = "CARD_ID")
	public Card getCard() {
		return card;
	}

	public void setCard(Card card) {
		this.card = card;
	}
}

package com.po;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Entity
@Table(name = "card")
public class Card {

	private int id;
	private String code;
	private Student student;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	@Column(name = "CODE", length = 32, nullable = false)
	public String getCode() {
		return code;
	}

	public void setCode(String code) {
		this.code = code;
	}

	@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "card")
	public Student getStudent() {
		return student;
	}

	public void setStudent(Student student) {
		this.student = student;
	}
}

方案一

在card表增加一个student表的外键字段STUDENT_ID，并在Card类的@OneToOne下增加@JoinColumn(name = "STUDENT_ID")，去掉mappedBy = "card"，即

DROP TABLE IF EXISTS `card`;
CREATE TABLE `card` (
  `ID` int(11) NOT NULL,
  `CODE` varchar(32) NOT NULL,
  `STUDENT_ID` int(11) DEFAULT NULL,
  PRIMARY KEY (`ID`),
  KEY `PK_STUDENT_ID` (`STUDENT_ID`),
  CONSTRAINT `PK_STUDENT_ID` FOREIGN KEY (`STUDENT_ID`) REFERENCES `student` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

public class Card {

	// ... 略

	@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
	@JoinColumn(name = "STUDENT_ID")
	public Student getStudent() {
		return student;
	}

	// ... 略
}

优点：不改变Student与Card在代码中的对应关系（一对一）

缺点：需要同时维护Student和Card的两个外键。

方案二

改为主键关联。

DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `ID` int(11) NOT NULL,
  `NAME` varchar(50) NOT NULL,
  PRIMARY KEY (`ID`),
  CONSTRAINT `PK_CARD_ID` FOREIGN KEY (`ID`) REFERENCES `card` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

public class Student {

	// ... 略

	@Id
	@GenericGenerator(name = "PK_Card", strategy = "foreign", parameters = @Parameter(name = "property", value = "card"))
	@GeneratedValue(generator = "PK_Card")
	@Column(name = "ID", unique = true, nullable = false)
	public int getId() {
		return id;
	}

	// ... 略

	@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL, optional = false)
	@PrimaryKeyJoinColumn
	public Card getCard() {
		return card;
	}

	// ... 略
}

public class Card {

	// ... 略

	@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "card", optional = false)
	public Student getStudent() {
		return student;
	}

	// ... 略
}

除了改变student表的主键、外键结构外，Student类和Card类也要做相应修改，尤其注意“optional”，要设置为false，否则无法实现懒加载。

优点：不改变Student与Card在代码中的对应关系（一对一）

缺点：改动较大，且使用主键关联具有局限性。

PS：主键关联的局限性

使用主键关联会影响数据存储结构，主键关联是一种强耦合，以上述为例：Card存在时，Student才能存在，Card消亡时，Student也随之消失。这是因为Student的主键依赖于Card主键，Student无法独立存在（就是说必须先有学生卡，才能有学生）。

方案三

将Card类中的OneToOne改为OneToMany（一对多）。

public class Card {

	private Set students;

	// ... 略

	@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "card")
	public Set<Student> getStudents() {
		return students;
	}

	public void setStudents(Set students) {
		this.students = students;
	}

	// ... 略
}

优点：数据库不用修改

缺点：需要修改Student与Card在代码中的对应关系