Access restriction: The type BASE64Encoder is not accessible due to restrict

       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 files\java\jre6\lib\rt.jar
      Access restriction : The constructor BASE64Decoder() is not accessible due to restriction on required library C:\Program files\java\jre6\lib\rt.jar
      搞不懂是为什么，最后在http://forums.dzone.com/eclipse/384-access-restriction- problems.html找到答案，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。。。
原文见：http://zhoushuyan.cn/java/the-type-base64decoder-is-not-accessible-due-to-restriction-on-required-library/