sun.misc.BASE64Encoder找不到jar包的解决方法

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
解决方法：
1、在project build path中先移除JRE System Library，再添加库JRE System Library，重新编译
2、Windows -> Preferences -> Java -> Compiler -> Errors/Warnings -> Deprecated and trstricted API -> Forbidden reference (access rules): -> change to warning