报错：
Access restriction: The method decodeBuffer(String) from the type
CharacterDecoder is not accessible due to restriction on required library
Access restriction: The constructor BASE64Decoder() is not accessible due
to restriction on required library
Access restriction: The type
BASE64Decoder is not accessible due to restriction on required library
解决方法：
这是eclipse设置问题，eclipse默认把这些受访问限制的API设成了ERROR，只要把
Window-Preferences-Java
Complicer-Errors/Warnings
里面的Deprecated and restricted
API中的Forbidden references(access rules)设置为Warning即可。