How does Hibernate implement polymorphic mapping?

Hibernate polymorphic mapping can map inherited classes to the database, providing the following mapping types: joined-subclass: Create a separate table for the subclass, including all columns of the parent class. table-per-class: Create a separate table for subclasses, containing only subclass-specific columns. union-subclass: Similar to joined-subclass, but the parent class table joins all subclass columns.

Hibernate Polymorphic Mapping: In-depth Analysis

Introduction
In object-oriented programming, It is common to encounter situations where you need to map inherited classes into the database. Hibernate can easily achieve this requirement through polymorphic mapping functionality.

Polymorphic mapping types
Hibernate provides several polymorphic mapping types for handling the relationship between inherited classes:

• joined-subclass: Create a separate table for each subclass and contain all columns from the parent class table.
• table-per-class: Create a separate table for each subclass, containing only subclass-specific columns.
• union-subclass: Similar to joined-subclass, but unions all subclass columns in the parent class table.

Practical case
Suppose we have an abstract class Person and two subclasses Student and Teacher . The Person class has id and name fields, the Student class has the grade field, Teacher The class has salary fields.

Hibernate Mapping Configuration
For Person Class:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Person {
  @Id
  private Long id;
  private String name;
  // ... 省略其他字段
}

For Student Class:

@Entity
public class Student extends Person {
  private Integer grade;
  // ... 省略其他字段
}

For Teacher class:

@Entity
public class Teacher extends Person {
  private Integer salary;
  // ... 省略其他字段
}

Query example
To query all people, you can use the following code:

Session session = sessionFactory.getCurrentSession();
List<Person> persons = session.createQuery("from Person", Person.class).list();

Hibernate will query based on The inheritance strategy automatically loads subclass data.

Advantages

• Simplify the mapping of inherited classes.
• Supports polymorphic queries to retrieve all subcategory data in a consistent manner.
• According to the inheritance strategy, the table structure can be optimized.

Note

• It is important to choose an appropriate inheritance strategy, which affects performance and availability features.
• When using polymorphic mapping, you should ensure that the structure of the database table matches the mapping configuration.

The above is the detailed content of How does Hibernate implement polymorphic mapping?. For more information, please follow other related articles on the PHP Chinese website!