The constructor BASE64Encoder() is not accessible due to restriction on required library

在Eclipse中编写Java代码时，用到了BASE64Decoder，import sun.misc.BASE64Decoder;可是Eclipse提示：
Access restriction : The type BASE64Decoder is not accessible due to restriction on required library C:Program
filesjavajre6lib t.jar
Access restriction : The constructor BASE64Decoder() is not accessible due to restriction on required library C:Program filesjavajre6lib t.jar
搞不懂是为什么，只需要在project build path中先移除JRE System Library，再添加库JRE System Library，重新编译后就一切正常了。但是我仍然很疑惑是为什么。。。

1. Open project properties.
2. Select Java Build Path node.
3. Select Libraries tab.
4. Remove JRE System Library.
5. Add Library JRE System Library.