适用于所有课程的Java DAO

Question

I'm new to Java and I must make some DAO for my app. However, I don't want to make a DAO for each class (with interface) et override methods. Is it possible to make a DAO extended by all the others, with methods working with all kind of Class ? For example, a DAO that could handle class MyClass and class Foo with a single mehtod getList(). Thank you !

Answer 1

Not that gooed idea, in general, but...

If it is about low-level JDBC (no framework like Hibernate, Spring, etc.), then:

You can make an AbstractDAO class, then your other DAO-classes (UserDAO, ProductDAO, etc.), then you can make a CommonService class that has all those DAO-classes and provides the functions you need.

Example:

abstract class AbstractDAO {
    private DataSource dataSource;

    protected getDataSource() { // Inject it or hard-coded dataSource
        return dataSource;
    }
}

---

public class UserDAO extends AbstractDAO {

    public User read(long id) {
        // blablabla
        return user;
    }

    public List<User> findAll() {
        // blablabla
        return users;
    }
    // and so on...
}

---

public class ProductDAO extends AbstractDAO {

    public Product read(long id) {
        // blablabla
        return product;
    }

    public List<Product> findAll() {
        // blablabla
        return products;
    }
    // and so on...
}

Then other repositories, and then:

public class CommonService {
    private final UserDAO userDAO = new UserDAO();
    private final ProductDAO productDAO = new ProductDAO();
    // other repositories

    public User readUser(long id) {
       return userDAO.read(id);
    }

    public Product readProduct(long id) {
       return productDAO.read(id);
    }

    public List<User> findAllUsers() {
        return userDAO.findAll();
    }

    public List<Product> findAllProducts() {
        return productDAO.findAll();
    }
}

Answer 2

And if you mean you want to make a generic repository (DAO), again not that good idea, because Spring has already made it in a quite good way (it calls JpaRepository , eg interface MyRepository extends JpaRepository<User, Long> { } ): Source: https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repositories.multiple-modules

But if you want, you can make such a mechanism too, based on something like this (but it will be a cumbersome work to make it work like it does in Spring, for instance; because they are a team of experts who worked day and night to realize such a tremendous project):

public abstract class Repo<T, K> {
    public abstract T read(K id);
    public abstract List<T> findAll();
}

or

public interface Repo<T, K> {
    T read(K id);
    List<T> findAll();
}