MyBatis 是如何将 sql 执行结果转换为目标对象并返回的？有哪些映射形式？

冷不防 2022-12-13 01:26 109阅读 0赞

*  方式一、<select> 标签使用 resultType 参数，传递 Java 类，sql 中 select 的字段名保持与 Java 类属性名称一致
 *  方式二、使用 <resultMap> 标签，定义数据库列名和对象属性名之间的映射关系
 *  方式三、使用注解 select 的字段名保持与接口方法返回的 Java 类或集合的元素类的属性名称一致

方式一
    <select id="selectUser" resultType="constxiong.po.User" parameterType="constxiong.po.User">
        select * from user where id = #{id}
    </select>
    
    
    方式二
    <select id="selectUserByResultMap" resultMap="userMap" parameterType="constxiong.po.User">
        select * from user where id = #{id}
    </select>
    <resultMap id="userMap" type="constxiong.po.User">
        <id property="id" column="id" />
        <result property="mc" column="name"/>
    </resultMap>
    
    
    方式三
    @Select("select * from user")
    List<User>  selectAllUsers();

根据解析得到 ResultMap 结合 sql 执行结果，通过反射创建对象，根据映射关系反射填充返回对象的属性

源码体现在 DefaultResultSetHandler 的 handleResultSets 方法

public List<Object> handleResultSets(Statement stmt) throws SQLException {
        ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    
        final List<Object> multipleResults = new ArrayList<>();
    
        int resultSetCount = 0;
        ResultSetWrapper rsw = getFirstResultSet(stmt);
    
        List<ResultMap> resultMaps = mappedStatement.getResultMaps();
        int resultMapCount = resultMaps.size();
        validateResultMapsCount(rsw, resultMapCount);
        while (rsw != null && resultMapCount > resultSetCount) {
          ResultMap resultMap = resultMaps.get(resultSetCount);
          handleResultSet(rsw, resultMap, multipleResults, null);
          rsw = getNextResultSet(stmt);
          cleanUpAfterHandlingResultSet();
          resultSetCount++;
        }
    
        String[] resultSets = mappedStatement.getResultSets();
        if (resultSets != null) {
          while (rsw != null && resultSetCount < resultSets.length) {
            ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
            if (parentMapping != null) {
              String nestedResultMapId = parentMapping.getNestedResultMapId();
              ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
              handleResultSet(rsw, resultMap, null, parentMapping);
            }
            rsw = getNextResultSet(stmt);
            cleanUpAfterHandlingResultSet();
            resultSetCount++;
          }
        }
    
        return collapseSingleResultList(multipleResults);
      }

--------------------

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