Book.java
```java package com.attacomsian.jpa.domains; import javax.persistence.*; @Entity public class Book { @Id @GeneratedValue(strategy = GenerationType.AUTO) private Long id; private String title; @Column(unique = true) private String isbn; public Book() { } public Book(String title, String isbn) { this.title = title; this.isbn = isbn; } // getters and setters removed for brevity } ``` The above `Book` class is very similar to what we defined in the previous article. The class is annotated with `@Entity`, indicating that it is a JPA entity. The `id` attribute is annotated with both `@Id` and `@GeneratedValue` annotations. This means that it is the object's auto-generated primary key. The `@Column` annotation is used to specify the mapped column for a persistent property or field in the database. We set the `unique` property to `true` to indicate that this column is a unique key. In short, the `isbn` property must be unique for all books. ## Create a Repository Let us create a repository interface for our `Book` entity:BookRepository.java
```java package com.attacomsian.jpa.repositories; import com.attacomsian.jpa.domains.Book; import org.springframework.data.repository.CrudRepository; import java.util.List; public interface BookRepository extends CrudRepositoryApplication.java
```java package com.attacomsian.jpa; import com.attacomsian.jpa.domains.Book; import com.attacomsian.jpa.repositories.BookRepository; import org.springframework.boot.CommandLineRunner; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.context.annotation.Bean; @SpringBootApplication public class Application { public static void main(String[] args) { SpringApplication.run(Application.class, args); } @Bean public CommandLineRunner bookDemo(BookRepository bookRepository) { return (args) -> { // create books bookRepository.save(new Book("Thinking in Java", "0136597238")); bookRepository.save(new Book("Beginning Java 2", "1861002238")); bookRepository.save(new Book("Java Gently", "0201342979")); bookRepository.save(new Book("Java 2 Platform Unleashed", "0672316315")); // fetch all books System.out.println("Books found with findAll():"); System.out.println("---------------------------"); for (Book book : bookRepository.findAll()) { System.out.println(book.toString()); } System.out.println(); // fetch book by id Book book = bookRepository.findById(1L).get(); System.out.println("Book found with findById(1L):"); System.out.println("-----------------------------"); System.out.println(book.toString()); System.out.println(); // fetch book by ISBN Book bookWithIBSN = bookRepository.findByIsbn("0201342979"); System.out.println("Book found with findByEmail('0201342979'):"); System.out.println("------------------------------------------"); System.out.println(bookWithIBSN.toString()); System.out.println(); // fetch all books that contain the keyword `java` System.out.println("Books that contain keyword 'java':"); System.out.println("----------------------------------"); for (Book b : bookRepository.findByTitleContaining("java")) { System.out.println(b.toString()); } System.out.println(); // update book title Book bookUpdate = bookRepository.findById(2L).get(); bookUpdate.setTitle("Java Gently 2nd Edition"); bookRepository.save(bookUpdate); System.out.println("Update book title:"); System.out.println("------------------"); System.out.println(bookUpdate.toString()); System.out.println(); // total books in DB System.out.println("Total books in DB:"); System.out.println("------------------"); System.out.println(bookRepository.count()); System.out.println(); // delete all books bookRepository.deleteAll(); }; } } ``` The above main application class demonstrates how to use the `BookRepository` interface for creating, accessing, updating, and deleting `Book` entities. ## Run the Application You can easily run the application from the command line by using Gradle or Maven build tools. For Gradle, run the application by executing the following command: ```bash $ ./gradlew bootRun ``` For Maven, you can run the application by typing: ```bash $ ./mvnw spring-boot:run ``` Once the application is started, you should see the following output printed on the console: ```plaintext Books found with findAll(): --------------------------- Book{id=1, title='Thinking in Java', isbn='0136597238'} Book{id=2, title='Beginning Java 2', isbn='1861002238'} Book{id=3, title='Java Gently', isbn='0201342979'} Book{id=4, title='Java 2 Platform Unleashed', isbn='0672316315'} Book found with findById(1L): ----------------------------- Book{id=1, title='Thinking in Java', isbn='0136597238'} Book found with findByEmail('0201342979'): ------------------------------------------ Book{id=3, title='Java Gently', isbn='0201342979'} Books that contain the keyword 'java': ---------------------------------- Update book title: ------------------ Book{id=2, title='Java Gently 2nd Edition', isbn='1861002238'} Total books in DB: ------------------ 4 ``` > **Source code:** Download the complete source code from [GitHub](https://github.com/attacomsian/code-examples/tree/master/spring-data-jpa/jpa-h2) available under MIT license. ## Conclusion That's all folks for today. In this article, you have learned how to use Spring Data JPA with H2 in-memory database for storing and retrieving books in a Spring Boot application. Check out the [getting started with Spring Data JPA](https://attacomsian.com/blog/getting-started-spring-data-jpa) guide to learn more about its core features and how to configure different data sources. If you are using MySQL with Spring Data JPA, check out [Accessing Data with Spring Data JPA and MySQL](https://attacomsian.com/blog/accessing-data-spring-data-jpa-mysql) guide.