面试官问：Mybatis中的TypeHandler你用过吗？

前言

环境配置

什么是TypeHandler？

public interface TypeHandler<T> {
  //设置参数，java类型转换为jdbc类型
  void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;
  //将查询的结果转换为java类型
  T getResult(ResultSet rs, String columnName) throws SQLException;

  T getResult(ResultSet rs, int columnIndex) throws SQLException;

  T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}

如何自定义并使用TypeHandler？

如何自定义？

@MappedJdbcTypes(JdbcType.INTEGER)
@MappedTypes(String.class)
public class GenderTypeHandler extends BaseTypeHandler {

    //设置参数，这里将Java的String类型转换为JDBC的Integer类型
    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, Object parameter, JdbcType jdbcType) throws SQLException {
        ps.setInt(i, StringUtils.equals(parameter.toString(),"男")?1:2);
    }

    //以下三个参数都是将查询的结果转换
    @Override
    public Object getNullableResult(ResultSet rs, String columnName) throws SQLException {
        return rs.getInt(columnName)==1?"男":"女";
    }

    @Override
    public Object getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        return rs.getInt(columnIndex)==1?"男":"女";
    }

    @Override
    public Object getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        return cs.getInt(columnIndex)==1?"男":"女";
    }
}

如何将其添加到Mybatis中？

## 设置自定义的Typehandler所在的包，启动的时候会自动扫描配置到Mybatis中
mybatis.type-handlers-package=cn.cb.demo.typehandler

@Bean("sqlSessionFactory")
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
        SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
        sqlSessionFactoryBean.setDataSource(dataSource);
        sqlSessionFactoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources(MAPPER_LOCATOIN));
        org.apache.ibatis.session.Configuration configuration = new org.apache.ibatis.session.Configuration();
        // 自动将数据库中的下划线转换为驼峰格式
        configuration.setMapUnderscoreToCamelCase(true);
        configuration.setDefaultFetchSize(100);
        configuration.setDefaultStatementTimeout(30);
        sqlSessionFactoryBean.setConfiguration(configuration);
        //将typehandler注册到mybatis
        GenderTypeHandler genderTypeHandler = new GenderTypeHandler();
        TypeHandler[] typeHandlers=new TypeHandler[]{genderTypeHandler};
        sqlSessionFactoryBean.setTypeHandlers(typeHandlers);
        return sqlSessionFactoryBean.getObject();
    }

XML文件中如何指定TypeHandler？

<insert id="insertUser">
        insert into user_info(user_id,his_id,name,gender,password,create_time)
        values(#{userId,jdbcType=VARCHAR},#{hisId,jdbcType=VARCHAR},#{name,jdbcType=VARCHAR},
        #{gender,jdbcType=INTEGER,typeHandler=cn.cb.demo.typehandler.GenderTypeHandler},#{password,jdbcType=VARCHAR},now())
    </insert>

<resultMap id="userResultMap" type="cn.cb.demo.domain.UserInfo">
        <id column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="his_id" property="hisId"/>
            <!-- 指定typeHandler属性为全类名-->
        <result column="gender" property="gender" typeHandler="cn.cb.demo.typehandler.GenderTypeHandler"/>
        <result column="name" property="name"/>
        <result column="password" property="password"/>
    </resultMap>

    <select id="selectList" resultMap="userResultMap">
        select * from user_info where status=1
        and user_id in
        <foreach collection="userIds" item="item" open="(" separator="," close=")" >
            #{item}
        </foreach>
    </select>

源码中如何执行TypeHandler？

入参如何转换？

 @Override
  public void parameterize(Statement statement) throws SQLException {
    //实际调用的是DefaultParameterHandler
    parameterHandler.setParameters((PreparedStatement) statement);
  }

public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    //获取参数映射
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    //遍历参数映射，一一设置
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          //获取类型处理器，如果不存在，使用默认的
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          //JdbcType
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
            //调用类型处理器中的方法设置参数，将Java类型转换为JDBC类型
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          } catch (SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }

结果如何转换？

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    //执行SQL
    ps.execute();
    //处理结果
    return resultSetHandler.handleResultSets(ps);
  }

private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
      throws SQLException {
    if (propertyMapping.getNestedQueryId() != null) {
      return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);
    } else if (propertyMapping.getResultSet() != null) {
      addPendingChildRelation(rs, metaResultObject, propertyMapping);   // TODO is that OK?
      return DEFERRED;
    } else {
      final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
      final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
      //执行typeHandler中的方法获取结果并且转换为对应的Java类型
      return typeHandler.getResult(rs, column);
    }
  }

总结

总结