应该是返回null的时候表示结束了，在官方文档中有的Returns the next line of text available from this reader. A line is represented by zero or more characters followed by '\n', '\r', "\r\n" or the end of the reader. The string does not include the newline sequence.Returns: the contents of the line or null if no characters were read before the end of the reader has been reached.
the contents of the line or null if no characters were read before the end of the reader has been reached.