Hibernate支持三种类型的继承关系：每个具体类一张表(Table per concrete class)：表与子类之间的独立一对一关系。
每个子类一张表(Table per subclass)：每个子类对应一张子表，并与主类共享主表。
每个类分层结构一张表(Table per class hierarchy)：表与类的一对多关系。
实例场景：产品(Product)、DVD和书籍(Book)
1、每个具体类一张表。注解使用@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)表结构如下：
@Entity @Table(name="t_product") @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS) public class Product { private Integer id; private String manufacturer;//生产商 private String name;//商品名 @Id @GeneratedValue(strategy=GenerationType.AUTO) public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getManufacturer() { return manufacturer; } public void setManufacturer(String manufacturer) { this.manufacturer = manufacturer; } public String getName() { return name; } public void setName(String name) { this.name = name; } } @Entity @Table(name="t_dvd") public class DVD extends Product { private String regionCode; @Column(name="region_code") public String getRegionCode() { return regionCode; } public void setRegionCode(String regionCode) { this.regionCode = regionCode; } } @Entity @Table(name="t_book") public class Book extends Product { private Integer pageCount; @Column(name="page_count") public Integer getPageCount() { return pageCount; } public void setPageCount(Integer pageCount) { this.pageCount = pageCount; } }
测试方法：
public void inheritanceTestInsert() { Product product1=new Product(); product1.setName("书籍"); product1.setManufacturer("出版商"); Product product2=new Product(); product2.setName("食品"); product2.setManufacturer("农场"); DVD dvd1=new DVD(); dvd1.setName("泰山"); dvd1.setManufacturer("北影"); dvd1.setRegionCode("美国"); DVD dvd2=new DVD(); dvd2.setName("让子弹飞"); dvd2.setManufacturer("大幅度"); dvd2.setRegionCode("中国"); Book book1=new Book(); book1.setName("大话"); book1.setManufacturer("备有"); book1.setPageCount(10000); Book book2=new Book(); book2.setName("托福"); book2.setManufacturer("科技"); book2.setPageCount(23434); Transaction tx=session.beginTransaction(); session.save(product1); session.save(product2); session.save(dvd1); session.save(dvd2); session.save(book1); session.save(book2); tx.commit(); }
Hibernate生成的Sql语句：Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_dvd (manufacturer, name, region_code, id) values (?, ?, ?, ?)Hibernate: insert into t_dvd (manufacturer, name, region_code, id) values (?, ?, ?, ?)Hibernate: insert into t_book (manufacturer, name, page_count, id) values (?, ?, ?, ?)Hibernate: insert into t_book (manufacturer, name, page_count, id) values (?, ?, ?, ?)
测试方法：
public void inheritanceTestQuery() { Query query=session.createQuery("from Product"); List<Product> products=query.list(); }
Hibernate生成的Sql语句：select product0_.id as id6_, product0_.manufacturer as manufact2_6_, product0_.name as name6_, product0_.page_count as page1_7_, product0_.region_code as region1_8_, product0_.clazz_ as clazz_ from (select manufacturer, null as page_count, null as region_code, name, id, 0 as clazz_ from t_product union all select manufacturer, page_count, null as region_code, name, id, 1 as clazz_ from t_book union all select manufacturer, null as page_count, region_code, name, id, 2 as clazz_ from t_dvd) product0_Hibernate会自动把t_product,t_book和t_dvd表里的内容全部查询出来，这是因为Product类也是实体，在数据库中也有对应的表。如果不希望Product类成为映射实体，可以将其定义成抽象类（即public abstract class Product {}）或者使用 @MappedSuperclass 注解，这样测试方法inheritanceTestQuery()对应Hibernate生成的sql语句为：select product0_.id as id6_, product0_.manufacturer as manufact2_6_, product0_.name as name6_, product0_.page_count as page1_7_, product0_.region_code as region1_8_, product0_.clazz_ as clazz_ from (select manufacturer, page_count, null as region_code, name, id, 1 as clazz_ from t_book union all select manufacturer, null as page_count, region_code, name, id, 2 as clazz_ from t_dvd) product0_可见Hibernate只从t_dvd和t_book里取数据。
2、每个子类一张表。注解使用@Inheritance(strategy=InheritanceType.JOINED)表结构
@Entity @Table(name="t_product") @Inheritance(strategy=InheritanceType.JOINED) public class Product { private Integer id; private String manufacturer;//生产商 private String name;//商品名 @Id @GeneratedValue(strategy=GenerationType.AUTO) public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getManufacturer() { return manufacturer; } public void setManufacturer(String manufacturer) { this.manufacturer = manufacturer; } public String getName() { return name; } public void setName(String name) { this.name = name; } } @Entity @Table(name="t_dvd") public class DVD extends Product { private String regionCode; @Column(name="region_code") public String getRegionCode() { return regionCode; } public void setRegionCode(String regionCode) { this.regionCode = regionCode; } } @Entity @Table(name="t_book") public class Book extends Product { private Integer pageCount; @Column(name="page_count") public Integer getPageCount() { return pageCount; } public void setPageCount(Integer pageCount) { this.pageCount = pageCount; } }
执行测试方法inheritanceTestInsert();Hibernate生成的Sql语句：Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_dvd (region_code, id) values (?, ?)Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_dvd (region_code, id) values (?, ?)Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_book (page_count, id) values (?, ?)Hibernate: insert into t_product (manufacturer, name, id) values (?, ?, ?)Hibernate: insert into t_book (page_count, id) values (?, ?)
执行测试方法inheritanceTestQuery() ;Hibernate生成的Sql语句：select product0_.id as id6_, product0_.manufacturer as manufact2_6_, product0_.name as name6_, product0_1_.page_count as page2_7_, product0_2_.region_code as region2_8_, case when product0_1_.id is not null then 1 when product0_2_.id is not null then 2 when product0_.id is not null then 0 end as clazz_ from t_product product0_ left outer join t_book product0_1_ on product0_.id = product0_1_.id left outer join t_dvd product0_2_ on product0_.id = product0_2_.idHibernate会自动把t_product、t_book与t_dvd表里的内容全部连接查询出来。
3、每个类分层结构一张表。注解使用@Inheritance(strategy=InheritanceType.SINGLE_TABLE)表结构
@Entity @Table(name="t_product") @Inheritance(strategy=InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name="category",discriminatorType=DiscriminatorType.STRING)//定义用于区分的列名为category，列类型为String @DiscriminatorValue("product")//可选，指定Product类对应的记录的category字段的值为“product”，默认值为类名，即"Product" public class Product { private Integer id; private String manufacturer;//生产商 private String name;//商品名 @Id @GeneratedValue(strategy=GenerationType.AUTO) public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getManufacturer() { return manufacturer; } public void setManufacturer(String manufacturer) { this.manufacturer = manufacturer; } public String getName() { return name; } public void setName(String name) { this.name = name; } } @Entity @DiscriminatorValue("dvd")//可选，指定DVD类对应的记录的category字段的值为“dvd”，默认值为类名，即"DVD" public class DVD extends Product { private String regionCode; @Column(name="region_code") public String getRegionCode() { return regionCode; } public void setRegionCode(String regionCode) { this.regionCode = regionCode; } } @Entity @DiscriminatorValue("book")//可选，指定Book类对应的记录的category字段的值为“book”，默认值为类名，即"Book" public class Book extends Product { private Integer pageCount; @Column(name="page_count") public Integer getPageCount() { return pageCount; } public void setPageCount(Integer pageCount) { this.pageCount = pageCount; } }
执行测试方法inheritanceTestInsert();Hibernate生成的sql语句：Hibernate: insert into t_product (manufacturer, name, category, id) values (?, ?, 'product', ?)Hibernate: insert into t_product (manufacturer, name, category, id) values (?, ?, 'product', ?)Hibernate: insert into t_product (manufacturer, name, region_code, category, id) values (?, ?, ?, 'dvd', ?)Hibernate: insert into t_product (manufacturer, name, region_code, category, id) values (?, ?, ?, 'dvd', ?)Hibernate: insert into t_product (manufacturer, name, page_count, category, id) values (?, ?, ?, 'book', ?)Hibernate: insert into t_product (manufacturer, name, page_count, category, id) values (?, ?, ?, 'book', ?)可见，Product、DVD和Book对应的记录都插入了t_product表中，并通过category字段来区分。
执行测试方法
public void inheritanceTestQuery() { Query query=session.createQuery("from Book"); List<Product> products=query.list(); } Hibernate生成的Sql语句：select book0_.id as id6_, book0_.manufacturer as manufact3_6_, book0_.name as name6_, book0_.page_count as page5_6_ from t_product book0_ where book0_.category = 'book'Hibernate会根据配置自动进行类型识别，把t_product表中category=book的数据(Book) 全部查询出来。
