前台页面图片无法显示

通过使用mybatis的xml配置文件来进行数据库查询

xml中的图片查询语句：

<select id="selectPaintingImg" resultType="com.imooc.gallery.entity.Oil_painting">
    select `smallImg` from `oil-painting` where id = #{id}
</select>

DAO中获取图片的方法：

public static Oil_painting getPaintingImg(Long id) {
    Oil_painting painting = null;
    try {
        sqlSession = MybatisUtils.getSqlSession();
        painting = sqlSession.selectOne("selectPaintingImg",new Oil_painting(id));
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        sqlSession.close();
    }
    return painting;
}

service中代码：

if ("/getImg.jpg".equals(request.getServletPath())){
    //获取油画的图片id
    String idStr = request.getParameter("id");
    //获得指定id的油画信息
    Oil_painting painting = HomeService.getPaintingImg(Long.valueOf(idStr));

    InputStream input = null;
    ServletOutputStream out = null;
    try {
        response.setContentType("multipart/form-data");
        if (null != painting && null != painting.getSmallImg()){
            System.out.println(painting.getSmallImg().length);
            input = new ByteArrayInputStream(painting.getSmallImg());
            out = response.getOutputStream();
            byte[] b = new byte[1024];
            int length = input.read(b);
            while(length != -1){
                out.write(b);
                length = input.read(b);
            }
            out.flush();
            response.flushBuffer();
        }
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        out.close();
        input.close();
    }
}

某些时候可以加载所有图片：

当刷新页面后有些图片就无法显示：

根据调试的结果显示是没有从数据库中获得相应id的油画对象而抛出的空指针异常。对DAO层的查询方法调试的时候当执行查询语句时抛出异常：Executor was closed

org.apache.ibatis.exceptions.PersistenceException: 
### Error querying database.  Cause: java.lang.NullPointerException
### The error may exist in mapper/PaintingMapper.xml
### The error may involve com.imooc.gallery.entity.Oil_painting.selectPaintingImg
### The error occurred while executing a query
### Cause: java.lang.NullPointerException