Product.java

package com.example.demo.entity;

import jakarta.persistence.*;

import java.math.BigDecimal;
import java.time.Instant;

/**
 * JPA entity representing a product in the database.
 * <p>
 * Mapped to the {@code products} table. Uses {@code IDENTITY} for primary key generation
 * (database auto-increment). {@code @PrePersist} sets {@code createdAt} before the first persist.
 */
@Entity
@Table(name = "products")
public class Product {

    /** Primary key; auto-generated by the database (IDENTITY strategy). */
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    /** Product name; required, max 255 characters. */
    @Column(nullable = false, length = 255)
    private String name;

    /** Price; required, stored with precision 19 and scale 2 (e.g. for currency). */
    @Column(nullable = false, precision = 19, scale = 2)
    private BigDecimal price;

    /** Stock quantity; non-negative. */
    private int stock;

    /** Timestamp when the product was created; set once and not updated. */
    @Column(name = "created_at", updatable = false)
    private Instant createdAt;

    /**
     * JPA lifecycle callback: set {@code createdAt} to current time before the entity is first persisted.
     */
    @PrePersist
    protected void onCreate() {
        createdAt = Instant.now();
    }

    /** No-argument constructor; required by JPA. */
    public Product() {}

    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public BigDecimal getPrice() { return price; }
    public void setPrice(BigDecimal price) { this.price = price; }
    public int getStock() { return stock; }
    public void setStock(int stock) { this.stock = stock; }
    public Instant getCreatedAt() { return createdAt; }
    public void setCreatedAt(Instant createdAt) { this.createdAt = createdAt; }
}