Access restriction: The type 'FtpProtocolException' is not API (restriction on required library 'C:\Program Files\Java\jdk1.8.0_171\jre\lib\rt.jar')

报错： 
Access restriction: The type 'FtpProtocolException' is not API (restriction on required library 'C:\Program Files\Java\jdk1.8.0_171\jre\lib\rt.jar')

解决方法： 
Project -> Properties -> libraries， 
先remove掉JRE System Library，然后再Add Library重新加入。

具体移除、加入：