在结果集中重复检索将返回null [英] Repeated retrieval in result set returns null
问题描述
我在ResultSet.getString
时遇到问题,当我第一次调用它时,它返回值,但是第二次返回null(这没有关闭结果集或sql连接)
I'm having an issue with ResultSet.getString
where when I call it the first time it returns the value but the second time returns null (this without closing the resultset or the sql connection)
if (rs.getString(i) == null) {
properties.setProperty(metaData.getColumnLabel(i), "");
} else {
properties.setProperty(metaData.getColumnLabel(i), rs.getString(i));
}
当然,该代码很容易修复,从而避免了第二次调用rs.getString()
的问题,但是我想理解的是为什么如果第二次调用rs.getString()
时第二次返回null,则返回null
of course the code is easily fixable to avoid the second call to rs.getString()
but what I want to understand is why the second return null if the first one didn't
推荐答案
来自 Javadoc :
为了获得最大的可移植性,应按从左到右的顺序读取每一行中的结果集列,并且每一列只能读取一次.
For maximum portability, result set columns within each row should be read in left-to-right order, and each column should be read only once.
在后台,您正在消耗TCP流中的字节,因此不能两次检索相同的列值也就不足为奇了.
Under the hood, you are consuming bytes from a TCP stream, so it isn't too surprising that you can't retrieve the same column value twice.
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