How to do it...

1. Create a new package folder named entity under the src/main/java/com/example/bookpub directory from the root of our project.
2. In this newly created package, create a new class named Book with the following content:

@Entity 
public class Book { 
  @Id 
  @GeneratedValue 
  private Long id; 
  private String isbn; 
  private String title; 
  private String description; 
 
  @ManyToOne 
  private Author author; 
  @ManyToOne 
  private Publisher publisher; 
 
  @ManyToMany 
  private List<Reviewers> reviewers; 
 
  protected Book() {} 
 
  public Book(String isbn, String title, Author author, 
       Publisher publisher) { 
    this.isbn = isbn; 
    this.title = title; 
    this.author = author; 
    this.publisher = publisher; 
  } 
  //Skipping getters and setters to save space, but we do need them 
} 

1. As any book should have an author and a publisher, and ideally some reviewers, we need to create these entity objects as well. Let's start by creating an Author entity class, under the same directory as our Book, as follows:

@Entity 
public class Author { 
  @Id 
  @GeneratedValue 
  private Long id; 
  private String firstName; 
  private String lastName; 
  @OneToMany(mappedBy = "author") 
  private List<Book> books; 
 
  protected Author() {} 
 
  public Author(String firstName, String lastName) {...} 
    //Skipping implementation to save space, but we do need 
       it all 
} 

1. Similarly, we will create the Publisher and Reviewer classes, as shown in the following code:

@Entity 
public class Publisher { 
  @Id 
  @GeneratedValue 
  private Long id; 
  private String name; 
  @OneToMany(mappedBy = "publisher") 
  private List<Book> books; 
 
  protected Publisher() {} 
 
  public Publisher(String name) {...} 
} 
 
@Entity 
public class Reviewer { 
  @Id 
  @GeneratedValue 
  private Long id; 
  private String firstName; 
  private String lastName; 
 
  protected Reviewer() {} 
 
  public Reviewer(String firstName, String lastName) 
     {...}
} 

1. Now we will create our BookRepository interface by extending Spring's CrudRepository interface under the src/main/java/com/example/bookpub/repository package, as follows:

@Repository 
public interface BookRepository 
       extends CrudRepository<Book, Long> { 
   public Book findBookByIsbn(String isbn); 
} 

1. Finally, let's modify our StartupRunner class in order to print the number of books in our collection, instead of some random datasource string, by autowiring a newly created BookRepository and printing the result of a .count() call to the log, as follows:

public class StartupRunner implements CommandLineRunner { 
  @Autowired private BookRepository bookRepository; 
 
  public void run(String... args) throws Exception { 
    logger.info("Number of books: " + 
       bookRepository.count()); 
  } 
}