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。
解决方案1（推荐）：
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。
解决方案2：
Windows -> Preferences -> Java -> Compiler -> Errors/Warnings ->
Deprecated and trstricted API -> Forbidden reference (access rules): -> change to warning