What is the ORM mechanism of Java Hibernate framework?

Hibernate is a Java ORM framework used to establish mapping between Java objects and relational databases. Its ORM mechanism includes the following steps: Annotation/Configuration: The object class is marked with annotations or XML files, specifying its mapped database tables and columns. Session factory: manages the connection between Hibernate and the database. Session: Represents an active connection to the database and is used to perform query and update operations. Persistence: Save data to the database through the save() or update() method. Query: Use Criteria and HQL to define complex queries to retrieve data.

Java Hibernate Framework ORM Mechanism

Introduction

Hibernate is a popular Java Object Relational Mapping (ORM) framework that allows seamless mapping between Java objects and relational databases. The role of an ORM is to manage the process of converting object state into database records and vice versa, simplifying the development and maintenance of data access code.

ORM mechanism

Hibernate uses Java annotations or XML configuration files to define the mapping of objects to database tables. It implements ORM through the following key steps:

• Annotation/Configuration: Object classes use Hibernate annotations (such as @Entity, @Table) or XML configuration file to specify which table and column in the database it maps to.
• Session Factory: The session factory manages the connection between Hibernate and the database.
• Session: A session represents an active connection to the database and is used to perform query and update operations.
• Persistence: To save data to the database, the object must be persisted through the save() or update() method.
• Query: Criteria and HQL (Hibernate query language) are used to define complex queries and retrieve data.

Practical case

We create a Java class Employee to represent the employee table in the database:

@Entity
@Table(name = "employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;
    private int salary;

    // 省略getter、setter和构造方法
}

In the main() method, we use the Hibernate configuration file (hibernate.cfg.xml) to create a session factory, open a session, and save an Employee object , and retrieve all employees:

public static void main(String[] args) {
    // 创建会话工厂
    SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();

    // 打开会话
    Session session = sessionFactory.openSession();
    session.beginTransaction();

    // 保存一个Employee对象
    Employee employee = new Employee();
    employee.setName("John Doe");
    employee.setSalary(10000);
    session.save(employee);

    // 查询所有Employee对象
    List<Employee> employees = session.createQuery("from Employee", Employee.class).list();

    // 打印员工信息
    for (Employee emp : employees) {
        System.out.println(emp.getName() + " - " + emp.getSalary());
    }

    // 提交事务并关闭会话
    session.getTransaction().commit();
    session.close();
}

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