解决mapper.xml中resultType映射类型的问题

<select id="queryAreaBy" parameterType="String" resultType="String">
       select SUM(area) as area from tb_forward_area where
                    server_time = #{serverTime}  and
                    region_code like concat('%',#{regionCode},'%')
                    and del_flag='0'
    </select>

<select id="queryAreaBy" parameterType="String" resultType="java.lang.String" >
        select SUM(area) as area from tb_forward_area where
                    server_time = #{serverTime}  and
                    region_code like concat('%',#{regionCode},'%')
                    and del_flag='0'
    </select>

/**
 * 代理工具方法，可以为任意接口生成任意实例代理，该方式使用在了mybatis的Mapper接口映射Mapper.xml中，具体在     MapperProxyFactory
 * @param tClass
 * @param invocationHandler
 * @param <T>
 * @return
 */
public <T> T test4(Class<T> tClass, InvocationHandler invocationHandler) {
   T t = (T) Proxy.newProxyInstance(tClass.getClassLoader(), new Class[] {tClass},invocationHandler);
   return t;
}
@Test
public void test6() {
   UserMapper userMapper = test4(UserMapper.class, new InvocationHandler() {
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
         System.out.println("查询所有");
         return null;
      }
   });
   userMapper.getAll();
}