<resultMap id="userMapForTestDiscriminator" type="user" autoMapping="false">
    <!--關(guān)閉自動(dòng)映射，那么沒(méi)有指定的列名不會(huì)出現(xiàn)在結(jié)果集中-->
    <id property="id" column="id"/>
    <result property="username" column="username"/>
    <result property="age" column="age"/>
    <discriminator javaType="int" column="age">
        <case value="18" resultType="user">
            <result property="birthday" column="birthday"/>
        </case>
    </discriminator>
</resultMap>
<select id="selectDiscriminator" resultMap="userMapForTestDiscriminator">
    select * from user limit 2
</select>

<resultMap id="userMapForTestDiscriminator" type="user" autoMapping="false">
    <!--關(guān)閉自動(dòng)映射，那么沒(méi)有指定的列名不會(huì)出現(xiàn)在結(jié)果集中-->
    <id property="id" column="id"/>
    <result property="username" column="username"/>
    <result property="age" column="age"/>
    <result property="birthday" column="birthday"/>
</resultMap>

<resultMap id="userMapForTestDiscriminator" type="user" autoMapping="false">
    <!--關(guān)閉自動(dòng)映射，那么沒(méi)有指定的列名不會(huì)出現(xiàn)在結(jié)果集中-->
    <id property="id" column="id"/>
    <result property="username" column="username"/>
    <result property="age" column="age"/>
</resultMap>

public interface UserMapper {
  List<User> selectDiscriminator();
}

@Test
public void test1() throws Exception {
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    List<User> users = userMapper.selectDiscriminator();
    for (Object user : users) {
        System.out.println(user);
    }
}

public class Discriminator {
  private ResultMapping resultMapping;
  private Map<String, String> discriminatorMap;
}

private ResultMap resultMapElement(XNode resultMapNode, List<ResultMapping> additionalResultMappings, Class<?> enclosingType) {
    // 1. 解析配置文件的標(biāo)簽創(chuàng)建為Discriminator對(duì)象
    Discriminator discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
    // 2. 構(gòu)建ResultMapResolver對(duì)象，再調(diào)用resolve方法創(chuàng)建ResultMap對(duì)象，Discriminator就是在resolve方法中被ResultMap對(duì)象中的
    // 說(shuō)白了是構(gòu)建ResultMap的輔助類。
    ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
    return resultMapResolver.resolve();
}

private Discriminator processDiscriminatorElement(XNode context, Class<?> resultType, List<ResultMapping> resultMappings) {
  // 通過(guò) column javaType jdbcType typeHandler javaTypeClass typeHandlerClass 
  // 來(lái)構(gòu)造Discriminator對(duì)象中的resultMapping字段
  String column = context.getStringAttribute("column");
  String javaType = context.getStringAttribute("javaType");
  String jdbcType = context.getStringAttribute("jdbcType");
  String typeHandler = context.getStringAttribute("typeHandler");
  Class<?> javaTypeClass = resolveClass(javaType);
  Class<? extends TypeHandler<?>> typeHandlerClass = resolveClass(typeHandler);
  JdbcType jdbcTypeEnum = resolveJdbcType(jdbcType);
  Map<String, String> discriminatorMap = new HashMap<>();
  // 遍歷discriminator標(biāo)簽，把case節(jié)點(diǎn)的信息封裝為Discriminator對(duì)象中的discriminatorMap字段
  for (XNode caseChild : context.getChildren()) {
    String value = caseChild.getStringAttribute("value");
    String resultMap = caseChild.getStringAttribute("resultMap", processNestedResultMappings(caseChild, resultMappings, resultType));
    discriminatorMap.put(value, resultMap);
  }
  // 調(diào)用輔助類整整的構(gòu)建Discriminator對(duì)象
  return builderAssistant.buildDiscriminator(resultType, column, javaTypeClass, jdbcTypeEnum, typeHandlerClass, discriminatorMap);
}

public ResultMap resolveDiscriminatedResultMap(ResultSet rs, ResultMap resultMap, String columnPrefix) throws SQLException {
  // 1 通過(guò)resultMap獲取鑒別器對(duì)象Discriminator
  Discriminator discriminator = resultMap.getDiscriminator();
  while (discriminator != null) {
    // 2. 從鑒別器中獲取結(jié)果集中對(duì)應(yīng)的需要被鑒別的值。拿文章開(kāi)頭的業(yè)務(wù)舉例，在此就是獲取結(jié)果集中的age列的值，第一行age的值是18。也就是value的值為18。第二行age的值為22
    final Object value = getDiscriminatorValue(rs, discriminator, columnPrefix);
    final String discriminatedMapId = discriminator.getMapIdFor(String.valueOf(value));
    // 3. 判斷configuration對(duì)象中是否有指定的resultMap對(duì)象，后面會(huì)根據(jù)這個(gè)resultMap進(jìn)行映射結(jié)果集
    if (configuration.hasResultMap(discriminatedMapId)) {
      resultMap = configuration.getResultMap(discriminatedMapId);
    }
  }
  return resultMap;
}