JdbcTemplate - DAO design pattern - join queries

Question

I have a Java + Spring + MySQL web application which stores and displays a list of books. There are three SQL tables - Authors, Books and Book_Authors.

------------
Authors
------------
author_id
firstname
surname
------------

------------
Books
------------
book_id
title
------------

------------
Book_Authors
------------
author_id
book_id
------------

Note that the relation between books and authors is M:N, i.e. it is possible for a single person to be an author of more than one book and at the same time a single book can be authored by more than one person.

I have got the following DAO classes.

@Service("bookDao")
public class BookDao
{
    @Resource(name="jdbcTemplate")
    private JdbcTemplate jdbcTemplate;

    private RowMapper<Book> mapper = new RowMapper<Book>() {
        ...
    };

    public List<Book> selectAll() {
        ...
    }

    ...
}

@Service("authorDao")
public class AuthorDao
{
    @Resource(name="jdbcTemplate")
    private JdbcTemplate jdbcTemplate;

    private RowMapper<Author> mapper = new RowMapper<Author>() {
        ...
    };

    public List<Author> selectAll() {
        ...
    }

}

For some of my views I need to load (a subset of) all books, including the set of authors of each of them. So, my data class and controller should look like this.

public class Book {
    private List<Author> authors = ...;
    ...
}

@Controller
public class Controller 
{
    @Resource(name = "authorDao")
    private AuthorDao authorDao;

    @Resource(name = "bookDao")
    private BookDao bookDao;

    @RequestMapping(value = "/books.html")
    public String books()
    {
        List<Book> books = ???;
        ...
        return "books";
    }
}

The question is, how do I design the operation to load books including authors (using a single JOIN query)? Should it be the responsibility of the BookDao class to load the books including authors? If so then how should the RowMapper look like? Or should I create a third Dao called say BookAuthorsDao for that purpose? Or another way?

Answer 1

In this case, since your primary attribute is the book(due to my undestanding) if I was you I would put the join query in the BookDao. It's not a mistake to create another dao but I think it's not necessary.

One recommendation: Annotate daos with @Repository and put em in service classes annotated with @Service and then use the service classes in the controller. Also put business logic in the Service classes. You can read more here What's the difference between @Component, @Repository & @Service annotations in Spring?