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。