HibernateJPA中的继承映射实体extends的关系(每个子类独立一个表)

技术2022-06-25 42

Hibernate/JPA中的继承映射

Hibernate中的继承映射

Hibernate的继承映射包含了三种不同的策略：

每簇类使用一个表；每个子类一个表；每个具体内一个表（有限制）。

假设我们有四个类Animal，Dog，Cat，其代码如下：文件名：Animal.java

class Animal { private String identifier; private String name; private String category; // setter and getter}

文件名：Dog.java

class Dog extends Animal { private String // setter and getter}

文件名：Cat.java

class Cat extends Animal { private String // setter and getter}

每簇类使用一个表

使用每簇类使用一个表的策略时，有一个限制就时子类不能有NOT NULL，映射文件为：文件名：Animal.hbm.xml

< class name ="Animal" table ="TB_ANIMAL" > < id name ="identifier" type ="string" column ="IDENTIFIER" > < generator class ="uuid.hex" /> </ id > < discriminator column ="ANIMAL_TYPE" type ="string" /> < property name ="name" column ="NAME" type ="string" /> < subclass name ="Dog" discriminator-value ="DOG" > </ subclass > < subclass name ="Cat" discriminator-value ="CAT" > </ subclass > </ class >

每个子类一个表

使用每个子类一个表的策略时，可以使用一个映射文件实现，也可以分成多个映射文件来实现。每个子类一个映射文件的情况：文件名：Animal.hbm.xml

< class name ="Animal" table ="ANIMAL" > < id name ="identifier" column ="IDENTIFIER" type ="string" > < generator class ="uuid.hex" /> </ id > < property > </ class > 文件名：Dog.hbm.xml < joined-subclass name ="Dog" table ="DOG" extends ="Animal" > < key column ="DOG_ID" /> </ joined-subclass > 文件名：Cat.hbm.xml < joined-subclass name ="Cat" table ="CAT" extends ="Cat" > < key column ="CAT_ID" /> </ joined-subclass >

每个子类一个表的策略实际上一种one-to-one的映射。

每个具体内一个表（有限制）

使用每个具体内一个表（有限制）策略时，每一个子类的映射文件将要包含所有父类中的属性，映射文件：文件名：Dog.hbm.xml

< class name ="Dog" table ="DOG" > < id name ="identifier" column ="IDENTIFIER" type ="string" > < generator class ="uuid.hex" /> </ id > < property name ="name" column ="NAME" type ="string" /> </ class > 文件名：Cat.hbm.xml < class name ="Cat" table ="CAT" > < id name ="identifier" column ="IDENTIFIER" type ="string" > < generator class ="uuid.hex" /> </ id > < property name ="name" column ="NAME" type ="string" /> </ class >

JPA中的实体层次设计

这部分的内容基本与Hibernate一致.JPA同样支持3种类型的继承形式:

1.Single Table Strategy ,单表策略,一张表包含基类与子类的所有数据,很多情况下都是采用这样的冗余设计,通过一个discriminator来区分

2.Table Per Class Strategy ,每个子类对应一张表,每张表都拥有基类的属性

3.Join Strategy ,仍然是每个子类对应一张表，但此表中不包含基类的属性,仅仅是此子类的扩展属性,共享基类的属性

以一个例子来说明3种情况:

一.单表策略

比如Pet作为基类,Cat和Dog继承此类并拥有自己的扩展属性,如:

package com.denny_blue.ejb3.inheritance;

import java.io.Serializable;

import javax.persistence.DiscriminatorColumn;import javax.persistence.DiscriminatorType;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.Inheritance;import javax.persistence.InheritanceType;

@Entity@Inheritance(strategy = InheritanceType.SINGLE_TABLE)@DiscriminatorColumn(name = "animal_type", discriminatorType = DiscriminatorType.STRING)public class Pet implements Serializable {private int id;

private String name;

private double weight;

public Pet() {}

@Id@GeneratedValue(strategy = GenerationType.AUTO)public int getId() { return id;}

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

public String getName() { return name;}

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

public double getWeight() { return weight;}

public void setWeight(double weight) { this.weight = weight;}

}

Pet类值的注意的就是通过@Inheritance(strategy = InheritanceType.SINGLE_TABLE)确定采用单表策略,通过@DiscriminatorColumn确定了标志值的字段和类型，我想熟悉hibernate的朋友对这些都应该很熟悉.然后是两个子类:

//Cat.java

package com.denny_blue.ejb3.inheritance;

import javax.persistence.DiscriminatorColumn;import javax.persistence.DiscriminatorType;import javax.persistence.DiscriminatorValue;import javax.persistence.Entity;import javax.persistence.Inheritance;import javax.persistence.InheritanceType;

@Entity@Inheritance(strategy = InheritanceType.SINGLE_TABLE)@DiscriminatorColumn(discriminatorType = DiscriminatorType.STRING)@DiscriminatorValue("cat")public class Cat extends Pet {private String HairBall;

public String getHairBall() { return HairBall;}

public void setHairBall(String hairBall) { HairBall = hairBall;}

}

//Dog.java

package com.denny_blue.ejb3.inheritance;

@Entity@Inheritance(strategy=InheritanceType.SINGLE_TABLE)@DiscriminatorColumn(discriminatorType=DiscriminatorType.STRING)@DiscriminatorValue("dog")public class Dog extends Pet {private String trick;

public String getTrick() { return trick;}

public void setTrick(String trick) { this.trick = trick;}

}

两个子类最值的关注的就是@DiscriminatorValue注释,比如Cat的此值为cat,意味着当Cat类型的Entity存入数据库时,JPA将自动把cat的值赋给animal_type字段,Dog的值则为dog,由此就可以在同一张表中区分开两个不同的子类.

二.Table per Class

采用Table Per Class策略的话，每个子类都将单独建表，并且都独立拥有基类中的所有属性,互相之间不共享,在我们的例子中所要进行的修改很小,像这样:

//基类

@Entity@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)public class Pet implements Serializable {private int id;

private String name;

private double weight;

........

//子类:不需要任何设置

@Entitypublic class Dog extends Pet {private String trick;

.......

}

例：

@Entity@Table(name = "TEST_A")@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)public class TestA implements java.io.Serializable {

// Fields

/*** */private static final long serialVersionUID = 5931010626625178698L;private Long id;private String name;private String code;

// Constructorspublic TestA() {

}

/** minimal constructor */public TestA(Long id) { this.id = id;}

/** full constructor */public TestA(Long id, String name, String code) { this.id = id; this.name = name; this.code = code;}

// Property accessors@Id@Column(name = "ID")@GeneratedValue(strategy = GenerationType.AUTO, generator = "SEQ")@SequenceGenerator(name="SEQ", sequenceName = "SEQ_TEMP_VOICE_FILE")public Long getId() { return this.id;}

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

@Column(name = "NAME", unique = false, nullable = true, insertable = true, updatable = true, length = 200)public String getName() { return this.name;}

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

@Column(name = "CODE", unique = false, nullable = true, insertable = true, updatable = true, length = 200)public String getCode() { return this.code;}

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

}

@Entity@Table(name = "TEST_B")public class TestB extends TestA {

// Fields

/*** */private static final long serialVersionUID = 4942564072409132522L;

/** minimal constructor */public TestB() {

}public TestB(Long id) { this.setId(id);}

@Column(name = "FLAG", unique = false, nullable = true, insertable = true, updatable = true, precision = 1, scale = 0)public Long getFlag() { return this.flag;}

public void setFlag(Long flag) { this.flag = flag;}

}

三.Join策略

每个子类同样独立建表,基类也独立建表，只不过所有的子类的表中只有扩展属性,他们共享基类的表,在我们的例子中修改下即可:

//基类

@Entity@Inheritance(strategy = InheritanceType.JOINED)public class Pet implements Serializable {private int id;

private String name;

private double weight;

........

//子类

@Entity

@Inheritance(strategy = InheritanceType.JOINED)public class Dog extends Pet {private String trick;

专利

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HibernateJPA中的继承映射 实体extends的关系(每个子类独立一个表)

Hibernate/JPA中的继承映射

JPA中的实体层次设计

专利

HibernateJPA中的继承映射实体extends的关系(每个子类独立一个表)