MyBatis discriminator标签原理实例解析

<resultMap id="userMapForTestDiscriminator" type="user" autoMapping="false">
    <!--关闭自动映射，那么没有指定的列名不会出现在结果集中-->
    <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">
    <!--关闭自动映射，那么没有指定的列名不会出现在结果集中-->
    <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">
    <!--关闭自动映射，那么没有指定的列名不会出现在结果集中-->
    <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. 解析配置文件的标签创建为Discriminator对象
    Discriminator discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
    // 2. 构建ResultMapResolver对象，再调用resolve方法创建ResultMap对象，Discriminator就是在resolve方法中被ResultMap对象中的
    // 说白了是构建ResultMap的辅助类。
    ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
    return resultMapResolver.resolve();
}

private Discriminator processDiscriminatorElement(XNode context, Class<?> resultType, List<ResultMapping> resultMappings) {
  // 通过 column javaType jdbcType typeHandler javaTypeClass typeHandlerClass 
  // 来构造Discriminator对象中的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<>();
  // 遍历discriminator标签，把case节点的信息封装为Discriminator对象中的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);
  }
  // 调用辅助类整整的构建Discriminator对象
  return builderAssistant.buildDiscriminator(resultType, column, javaTypeClass, jdbcTypeEnum, typeHandlerClass, discriminatorMap);
}

public ResultMap resolveDiscriminatedResultMap(ResultSet rs, ResultMap resultMap, String columnPrefix) throws SQLException {
  // 1 通过resultMap获取鉴别器对象Discriminator
  Discriminator discriminator = resultMap.getDiscriminator();
  while (discriminator != null) {
    // 2. 从鉴别器中获取结果集中对应的需要被鉴别的值。拿文章开头的业务举例，在此就是获取结果集中的age列的值，第一行age的值是18。也就是value的值为18。第二行age的值为22
    final Object value = getDiscriminatorValue(rs, discriminator, columnPrefix);
    final String discriminatedMapId = discriminator.getMapIdFor(String.valueOf(value));
    // 3. 判断configuration对象中是否有指定的resultMap对象，后面会根据这个resultMap进行映射结果集
    if (configuration.hasResultMap(discriminatedMapId)) {
      resultMap = configuration.getResultMap(discriminatedMapId);
    }
  }
  return resultMap;
}