Mybatis Mapper.xml继承机制

谁践踏了优雅 2022-05-31 12:19 344阅读 0赞

# 文章来源：https://segmentfault.com/a/1190000012470056 #

# Mapper.xml继承机制 #

[github地址][github]

Mybatis实际上隐藏了一个功能：Mapper.xml可以继承，这个在官方文档中并没有提到过，不过在这个[issue][] ([commit][])里提到过。

## Statement覆盖 ##

利用Mapper.xml的继承机制，我们可以做到ChildMapper覆盖ParentMapper中`select`、`insert`、`delete`、`update`。下面举例说明：

Interface：

@MybatisMapper
    public interface ParentMapper {
    
      String selectFoo();
    
      String selectBar();
    }
    
    @MybatisMapper
    public interface ChildMapper extends ParentMapper {
    
      String selectLoo();
    
    }

Mapper.xml：

<mapper namespace="me.chanjar.mybatislearn.inherit.statement.ParentMapper">
    
      <select id="selectFoo" resultType="String">
        SELECT 'Foo From Parent'
        FROM dual
      </select>
    
      <select id="selectBar" resultType="String">
        SELECT 'Bar From Parent'
        FROM dual
      </select>
    
    </mapper>
    
    <mapper namespace="me.chanjar.mybatislearn.inherit.statement.ChildMapper">
    
      
      <select id="selectFoo" resultType="String">
        SELECT 'Foo From Child'
        FROM dual
      </select>
    
      
    
      
      <select id="selectLoo" resultType="String">
        SELECT 'Loo From Child'
        FROM dual
      </select>
    
    </mapper>

规律可以总结为：

1.  ParentMapper.xml中有，ChildMapper.xml中没有，ChildMapper沿用ParentMapper.xml中的定义
2.  ParentMapper.xml中有，ChildMapper.xml中也有，ChildMapper使用ChildMapper.xml中的定义
3.  ParentMapper.xml中没有，ChildMapper.xml中有，ChildMapper使用ChildMapper.xml中的定义

相关代码：[Java代码][Java]、[测试代码][Link 1]、[配置文件][Link 2]

## ResultMap覆盖 ##

Mapper.xml继承机制只针对statement有效，对于`sql`、`resultMap`是无效的。  
如果要在ChildMapper.xml中覆盖这些，必须要先覆盖ParentMapper.xml中的statement，然后让这些statement使用新的`sql`、`resultMap`等。

下面举例一个给ITEM表添加字段，但是不修改原来的ItemMapper的例子：

Model:

public class Item {
    
      private Integer id;
      private String title;
      // setter and getter ...
    }
    
    public class ItemEx extends Item {
    
      private String name;
      // setter and getter ...
    
    }

Interface:

@MybatisMapper
    public interface ItemMapper {
    
      Item getById(@Param("id") Long id);
    
    }
    @MybatisMapper
    public interface ItemExMapper extends ItemMapper {
    
    }

Mapper.xml:

<mapper namespace="me.chanjar.mybatislearn.inherit.resultmap.ItemMapper">
    
      <select id="getById" resultMap="Item">
        select
          *
        from item where id = #{id}
      </select>
    
      <resultMap id="Item" type="me.chanjar.mybatislearn.inherit.resultmap.Item" autoMapping="true">
        <id property="id" column="id" />
      </resultMap>
    
    </mapper>
    
    <mapper namespace="me.chanjar.mybatislearn.inherit.resultmap.ItemExMapper">
    
      
      <select id="getById" resultMap="Item">
        select
        *
        from item where id = #{id}
      </select>
    
      <resultMap id="Item" type="me.chanjar.mybatislearn.inherit.resultmap.ItemEx" autoMapping="true">
        <id property="id" column="id" />
      </resultMap>
    
    </mapper>

相关代码： [Java代码][Java 1] 、 [测试代码][Link 3] 、 [配置文件][Link 4]

[github]: https://github.com/chanjarster/mybatis-learn/blob/master/mapper-xml-inherit.md
[issue]: https://github.com/mybatis/mybatis-3/issues/35
[commit]: https://github.com/mybatis/mybatis-3/commit/4b465eb36f499607a490c8f784504be108a26cd3
[Java]: https://github.com/chanjarster/mybatis-learn/tree/master/src/main/java/me/chanjar/mybatislearn/inherit/statement
[Link 1]: https://github.com/chanjarster/mybatis-learn/tree/master/src/test/java/me/chanjar/mybatislearn/inherit/statement
[Link 2]: https://github.com/chanjarster/mybatis-learn/tree/master/src/main/resources/mappers/inherit/statement
[Java 1]: https://github.com/chanjarster/mybatis-learn/tree/master/src/main/java/me/chanjar/mybatislearn/inherit/resultmap
[Link 3]: https://github.com/chanjarster/mybatis-learn/tree/master/src/test/java/me/chanjar/mybatislearn/inherit/resultmap
[Link 4]: https://github.com/chanjarster/mybatis-learn/tree/master/src/main/resources/mappers/inherit/resultmap