How Hibernate Generates SQL

Direct Answer

Hibernate builds mapping metadata from entities, XML/programmatic mappings, and configuration when the SessionFactory/EntityManagerFactory is created. At runtime it translates entity operations, HQL/JPQL, criteria queries, and flush-time changes into SQL suited to the configured database dialect and mappings. The exact SQL, number of statements, and timing depend on fetch plans, batching, id generation, flush mode, and provider version.

Code Example

import jakarta.persistence.*;
import java.util.*;

@Entity
@Table(name = "customers")
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "full_name", nullable = false)
    private String fullName;

    @Column(name = "email", unique = true)
    private String email;

    @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL)
    private List<Order> orders = new ArrayList<>();
}
// Hibernate reads these annotations at startup and generates, for PostgreSQL:
//   CREATE TABLE customers (id BIGSERIAL PRIMARY KEY,
//     full_name VARCHAR(255) NOT NULL, email VARCHAR(255) UNIQUE);
//   persist(c)        -> INSERT INTO customers (full_name, email) VALUES (?, ?);
//   findById(1L)      -> SELECT id, full_name, email FROM customers WHERE id = ?;

Common Mistakes & Pitfalls

• Assuming runtime parsing — Hibernate reads annotations once at startup into a static model; change an annotation and you must restart.
• Ignoring the dialect — without the right dialect, Hibernate may emit incompatible SQL (e.g. LONGTEXT on PostgreSQL, which expects TEXT).
• Using ddl-auto=update in production — it can cause data loss or locking; use migration tools like Flyway instead.