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DAO 和 JDBC 的关系？

发布于 2024-11-29 17:05:14 字数 94 浏览 1 评论 0原文

我知道Hibernate实现了ORM（对象关系映射），那么JDBC实现了什么类型的映射？它实现了 DAO 吗？我不完全理解 DAO 如何/是否与 JDBC 相关......？

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§普罗旺斯的薰衣草 2024-12-06 17:05:14

DAO 不是映射。 DAO 代表数据访问对象。它看起来像这样：

public interface UserDAO {

    public User find(Long id) throws DAOException;

    public void save(User user) throws DAOException;

    public void delete(User user) throws DAOException;

    // ...
}

对于 DAO，JDBC 只是一个实现细节。

public class UserDAOJDBC implements UserDAO {

    public User find(Long id) throws DAOException {
        // Write JDBC code here to return User by id.
    }

    // ...
}

Hibernate 可能是另一种。

public class UserDAOHibernate implements UserDAO {

    public User find(Long id) throws DAOException {
        // Write Hibernate code here to return User by id.
    }

    // ...
}

JPA可能是另一种（如果您要将现有的遗留应用程序迁移到 JPA；对于新应用程序，这会有点奇怪，因为 JPA 本身实际上是 DAO，例如 Hibernate 和 EclipseLink作为可用的实现）。

public class UserDAOJPA implements UserDAO {

    public User find(Long id) throws DAOException {
        // Write JPA code here to return User by id.
    }

    // ...
}

它允许您切换 UserDAO 实现，而无需更改使用 DAO 的业务代码（当然，前提是您针对接口正确编码）。

对于 JDBC，您只需要编写很多行来查找/保存/删除所需的信息，而使用 Hibernate 只需要几行。 Hiberenate 作为一个 ORM，完全从您手中接走了令人讨厌的 JDBC 工作，无论您是否使用 DAO。

另请参阅：

DAO isn't a mapping. DAO stands for Data Access Object. It look something like this:

public interface UserDAO {

    public User find(Long id) throws DAOException;

    public void save(User user) throws DAOException;

    public void delete(User user) throws DAOException;

    // ...
}

For DAO, JDBC is just an implementation detail.

public class UserDAOJDBC implements UserDAO {

    public User find(Long id) throws DAOException {
        // Write JDBC code here to return User by id.
    }

    // ...
}

Hibernate could be another one.

public class UserDAOHibernate implements UserDAO {

    public User find(Long id) throws DAOException {
        // Write Hibernate code here to return User by id.
    }

    // ...
}

JPA could be another one (in case you're migrating an existing legacy app to JPA; for new apps, it would be a bit weird as JPA is by itself actually the DAO, with e.g. Hibernate and EclipseLink as available implementations).

public class UserDAOJPA implements UserDAO {

    public User find(Long id) throws DAOException {
        // Write JPA code here to return User by id.
    }

    // ...
}

It allows you for switching of UserDAO implementation without changing the business code which is using the DAO (only if you're properly coding against the interface, of course).

For JDBC you'll only need to write a lot of lines to find/save/delete the desired information while with Hibernate it's a matter of only a few lines. Hiberenate as being an ORM takes exactly that nasty JDBC work from your hands, regardless of whether you're using a DAO or not.