Spring Boot 4 Microservices & REST APIs · レッスン

エンティティとリポジトリの作成

データモデルを表すJPAエンティティを定義し、データアクセスにSpring Dataリポジトリを使用します。

「エンティティとリポジトリの作成」はCoddyKit上の無料Spring Boot 4 Microservices & REST APIsレッスンです。これはレッスン3/6です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはSpring Boot 4 Microservices & REST APIs学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Spring Boot 4 Microservices & REST APIsコースには全6レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

What are JPA Entities?

Welcome to creating your data model! In Spring Boot with JPA, an Entity is a plain Java object that represents a table in your database.

Think of it as a blueprint for the rows in your table. Each instance of an entity class corresponds to a single row in the database table.

Marking a Class as an Entity

To tell Spring Boot that a Java class is an entity, we use the @Entity annotation. This annotation comes from the Java Persistence API (JPA).

When Spring sees @Entity, it knows to map this class to a database table. By default, the table name will be the same as the class name (e.g., a Product class maps to a Product table).

Defining Primary Keys with @Id

Every database table needs a primary key to uniquely identify each record. In JPA, we mark a field as the primary key using the @Id annotation.

Often, we want the database to automatically generate this ID for us. For this, we use the @GeneratedValue annotation, typically with a strategy like IDENTITY for auto-incrementing numbers.

Our First Product Entity

Let's create a simple Product entity. It will have an ID, a name, and a price. Notice the annotations @Entity, @Id, and @GeneratedValue.

Remember to include a default constructor and getters/setters for JPA to work correctly.

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;

@Entity
public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private double price;

    public Product() {}

    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }

    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 double getPrice() { return price; }
    public void setPrice(double price) { this.price = price; }

    @Override
    public String toString() {
        return "Product{" +
               "id=" + id +
               ", name='" + name + '\'' +
               ", price=" + price +
               '}';
    }
}

Introducing Spring Data Repositories

Now that we have an entity, how do we interact with the database to save, retrieve, update, or delete products? This is where Repositories come in!

Spring Data JPA provides a powerful abstraction that simplifies data access. Instead of writing complex SQL queries, you define simple interfaces.

The JpaRepository Interface

The core of Spring Data JPA is the JpaRepository interface. When your repository interface extends JpaRepository, Spring automatically provides standard CRUD (Create, Read, Update, Delete) methods for your entity.

You just need to specify the entity type and the type of its primary key, like this: JpaRepository<YourEntity, ID_Type>.

Defining Our Product Repository

Let's create a repository for our Product entity. It's just an interface that extends JpaRepository, telling Spring to generate the necessary code for us.

We specify Product as the entity type and Long as the primary key type.

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
    // Spring Data JPA automatically provides methods like:
    // save(), findById(), findAll(), deleteById(), etc.
}

Using Repositories in Spring

In a Spring Boot application, you would typically inject your repository into a service or controller using @Autowired. Then, you can call the built-in methods.

repository.save(product): Saves a new product or updates an existing one.
repository.findById(id): Finds a product by its ID.
repository.findAll(): Retrieves all products.

Entity & Repository in Action

Here's a conceptual example showing how an entity is created and how a repository would save it. We're simulating the repository behavior to make it runnable without a full Spring Boot setup.

This demonstrates creating a Product object and using a 'save' method.

// --- Simulated Product Entity --- //
class Product {
    private Long id;
    private String name;
    private double price;

    public Product(Long id, String name, double price) {
        this.id = id;
        this.name = name;
        this.price = price;
    }
    // Getters and Setters (simplified for brevity)
    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 double getPrice() { return price; }
    public void setPrice(double price) { this.price = price; }

    @Override
    public String toString() {
        return "Product{id=" + id + ", name='" + name + "', price=" + price + "}";
    }
}

// --- Simulated Product Repository Interface --- //
interface ProductRepository {
    Product save(Product product);
}

// --- Simulated Product Repository Implementation --- //
class SimpleProductRepository implements ProductRepository {
    private static Long nextId = 1L;
    @Override
    public Product save(Product product) {
        if (product.getId() == null) {
            product.setId(nextId++); // Assign a new ID
        }
        System.out.println("Simulating save: " + product);
        return product;
    }
}

public class Main {
    public static void main(String[] args) {
        // 1. Create a Product entity instance
        Product newProduct = new Product(null, "Wireless Earbuds", 99.99);
        System.out.println("Created product: " + newProduct);

        // 2. Simulate saving it using our repository
        ProductRepository repository = new SimpleProductRepository();
        Product savedProduct = repository.save(newProduct);

        System.out.println("Product after simulated save: " + savedProduct);
    }
}

Quick Check: Core Concepts

Which of the following annotations is used to mark a class as a JPA entity that maps to a database table?

Recap: Entities & Repositories

You've learned the fundamentals of defining your data model with Spring Data JPA!

Entities are Java objects representing database tables, marked with @Entity.
Primary keys are defined with @Id and often auto-generated using @GeneratedValue.
Repositories are interfaces that extend JpaRepository, providing powerful, ready-to-use methods for interacting with your database.

These two concepts are the cornerstone of data persistence in Spring Boot.

よくある質問

「エンティティとリポジトリの作成」レッスンは無料ですか？

はい。「エンティティとリポジトリの作成」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Spring Boot 4 Microservices & REST APIsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Spring Boot 4 Microservices & REST APIsコースには全6レッスンが含まれています。

「エンティティとリポジトリの作成」で何を学びますか？

データモデルを表すJPAエンティティを定義し、データアクセスにSpring Dataリポジトリを使用します。ブラウザで直接実行するハンズオンコードでSpring Boot 4 Microservices & REST APIsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

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事前経験は必要ありません。CoddyKitのSpring Boot 4 Microservices & REST APIsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン3/6です。

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